Re: [RFC][PATCH] cpufreq: Do not hold driver module references for additional policy CPUs
On 1 August 2013 13:41, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
On 08/01/2013 05:38 AM, Rafael J. Wysocki wrote:
From: Rafael J. Wysocki rafael.j.wysocki@intel.com
The cpufreq core is a little inconsistent in the way it uses the driver module refcount.
Namely, if __cpufreq_add_dev() is called for a CPU without siblings or generally a CPU for which a new policy object has to be created, it grabs a reference to the driver module to start with, but drops that reference before returning. As a result, the driver module refcount is then equal to 0 after __cpufreq_add_dev() has returned.
On the other hand, if the given CPU is a sibling of some other CPU already having a policy, cpufreq_add_policy_cpu() is called to link the new CPU to the existing policy. In that case, cpufreq_cpu_get() is called to obtain that policy and grabs a reference to the driver module, but that reference is not released and the module refcount will be different from 0 after __cpufreq_add_dev() returns (unless there is an error). That prevents the driver module from being unloaded until __cpufreq_remove_dev() is called for all the CPUs that cpufreq_add_policy_cpu() was called for previously.
To remove that inconsistency make cpufreq_add_policy_cpu() execute cpufreq_cpu_put() for the given policy before returning, which decrements the driver module refcount so that it will be 0 after __cpufreq_add_dev() returns,
Removing the inconsistency is a good thing, but I think we should make it consistent the other way around - make a CPU-online increment the driver module refcount and decrement it only on CPU-offline.
I took time to review to this mail as I was looking at the problem yesterday. I am sorry to say, but I have completely different views as compared to You and Rafael both :)
First of all, Rafael's patch is incomplete as it hasn't fixed the issue completely. When we have multiple CPUs per policy and cpufreq_add_dev() is called for the first one, it call cpufreq_get_cpu() for all cpus of this policy(), so count is == x (no. of CPUs in this policy) + 1 (This is the initial value of .owner).
And so we still have module count getting incremented for other cpus :)
Now few lines about My point of view to this whole thing. I believe we should get rid of .owner field from struct cpufreq_driver completely. It doesn't make sense to me in doing all this management at all. Surprised? Shocked? Laughing at me? :)
Well I may be wrong but this is what I think: - It looks stupid to me that I can't do this from userspace in one go: $ insmod cpufreq_driver.ko $ rmmod cpufreq_driver.ko
What the hell changed in between that isn't visible to user? It looked completely stupid in that way..
Something like this sure makes sense: $ insmod ondemand-governor.ko $ change governor to ondemand for few CPUs $ rmmod ondemand-governor.ko
as we have deliberately add few users of governor. And so without second step, rmmod should really work smoothly. And it does.
Now, why shouldn't there be a problem with this approach? I will write that inline to the problems you just described.
The reason is, one should not be able to unload the back-end cpufreq driver module when some CPUs are still being managed. Nasty things will result if we allow that. For example, if we unload the module, and then try to do a CPU offline, then the cpufreq hotplug notifier won't even be called (because cpufreq_unregister_driver also unregisters the hotplug notifier). And that might be troublesome.
So what? Its simply equivalent to we have booted our system, haven't inserted cpufreq module and taken out a cpu.
Even worse, if we unload a cpufreq driver module and load a new one and *then* try to offline the CPU, then the cpufreq_driver->exit() function that we call during CPU offline will end up calling the corresponding function of an entirely different driver! So the ->init() and ->exit() calls won't match.
That's not true. When we unload the module, it must call cpufreq_unregister_driver() which should call cpufreq_remove_cpu() for all cpus and so exit() is already called for last module.
If we get something new now, it should simply work.
What do you think gentlemen?
-- viresh
On 08/01/2013 08:14 PM, Viresh Kumar wrote:
On 1 August 2013 13:41, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
On 08/01/2013 05:38 AM, Rafael J. Wysocki wrote:
From: Rafael J. Wysocki rafael.j.wysocki@intel.com
The cpufreq core is a little inconsistent in the way it uses the driver module refcount.
Namely, if __cpufreq_add_dev() is called for a CPU without siblings or generally a CPU for which a new policy object has to be created, it grabs a reference to the driver module to start with, but drops that reference before returning. As a result, the driver module refcount is then equal to 0 after __cpufreq_add_dev() has returned.
On the other hand, if the given CPU is a sibling of some other CPU already having a policy, cpufreq_add_policy_cpu() is called to link the new CPU to the existing policy. In that case, cpufreq_cpu_get() is called to obtain that policy and grabs a reference to the driver module, but that reference is not released and the module refcount will be different from 0 after __cpufreq_add_dev() returns (unless there is an error). That prevents the driver module from being unloaded until __cpufreq_remove_dev() is called for all the CPUs that cpufreq_add_policy_cpu() was called for previously.
To remove that inconsistency make cpufreq_add_policy_cpu() execute cpufreq_cpu_put() for the given policy before returning, which decrements the driver module refcount so that it will be 0 after __cpufreq_add_dev() returns,
Removing the inconsistency is a good thing, but I think we should make it consistent the other way around - make a CPU-online increment the driver module refcount and decrement it only on CPU-offline.
I took time to review to this mail as I was looking at the problem yesterday. I am sorry to say, but I have completely different views as compared to You and Rafael both :)
First of all, Rafael's patch is incomplete as it hasn't fixed the issue completely. When we have multiple CPUs per policy and cpufreq_add_dev() is called for the first one, it call cpufreq_get_cpu() for all cpus of this policy(), so count is == x (no. of CPUs in this policy)
- 1 (This is the initial value of .owner).
And so we still have module count getting incremented for other cpus :)
Good catch!
Now few lines about My point of view to this whole thing. I believe we should get rid of .owner field from struct cpufreq_driver completely. It doesn't make sense to me in doing all this management at all. Surprised? Shocked? Laughing at me? :)
Well I may be wrong but this is what I think:
- It looks stupid to me that I can't do this from userspace in one go: $ insmod cpufreq_driver.ko $ rmmod cpufreq_driver.ko
What the hell changed in between that isn't visible to user? It looked completely stupid in that way..
Something like this sure makes sense: $ insmod ondemand-governor.ko $ change governor to ondemand for few CPUs $ rmmod ondemand-governor.ko
as we have deliberately add few users of governor. And so without second step, rmmod should really work smoothly. And it does.
Now, why shouldn't there be a problem with this approach? I will write that inline to the problems you just described.
The reason is, one should not be able to unload the back-end cpufreq driver module when some CPUs are still being managed. Nasty things will result if we allow that. For example, if we unload the module, and then try to do a CPU offline, then the cpufreq hotplug notifier won't even be called (because cpufreq_unregister_driver also unregisters the hotplug notifier). And that might be troublesome.
So what? Its simply equivalent to we have booted our system, haven't inserted cpufreq module and taken out a cpu.
Even worse, if we unload a cpufreq driver module and load a new one and *then* try to offline the CPU, then the cpufreq_driver->exit() function that we call during CPU offline will end up calling the corresponding function of an entirely different driver! So the ->init() and ->exit() calls won't match.
That's not true. When we unload the module, it must call cpufreq_unregister_driver() which should call cpufreq_remove_cpu() for all cpus and so exit() is already called for last module.
Sorry, I missed this one.
If we get something new now, it should simply work.
Yeah, I now see your point. It won't create any problems by unloading the module and loading a new one.
What do you think gentlemen?
Well, I now agree that we don't have to keep the module refcount non-zero as long as CPUs are being managed (that was just my misunderstanding, sorry for the noise). However, I think the _get() and _put() used in the existing code is for synchronization: that is, to avoid races between trying to unload the cpufreq driver module and running a hotplug notifier (and calling the driver module's ->init() or ->exit() function).
With that being the case, I think we can retain the module refcounts and use them only for synchronization. And naturally the refcount should drop to zero after the critical section; no point keeping it incremented until the CPU is taken offline.
Or, am I confused again?
Regards, Srivatsa S. Bhat
On 1 August 2013 20:54, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
Well, I now agree that we don't have to keep the module refcount non-zero as long as CPUs are being managed (that was just my misunderstanding, sorry for the noise). However, I think the _get() and _put() used in the existing code is for synchronization: that is, to avoid races between trying to unload the cpufreq driver module and running a hotplug notifier (and calling the driver module's ->init() or ->exit() function).
With that being the case, I think we can retain the module refcounts and use them only for synchronization. And naturally the refcount should drop to zero after the critical section; no point keeping it incremented until the CPU is taken offline.
Or, am I confused again?
No, you aren't.
But, for synchronization we need some blocking stuff, so that when user tries to rmmod the module, it should wait until other critical sections are over and then unload the module. But here, we are simply returning from rmmod, saying that we are busy :)
So, for that kind of synchronization we better use locks available inside cpufreq core of if required another variable which can be used for refcount. But now this.
On Thursday, August 01, 2013 10:54:08 PM Viresh Kumar wrote:
On 1 August 2013 20:54, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
Well, I now agree that we don't have to keep the module refcount non-zero as long as CPUs are being managed (that was just my misunderstanding, sorry for the noise). However, I think the _get() and _put() used in the existing code is for synchronization: that is, to avoid races between trying to unload the cpufreq driver module and running a hotplug notifier (and calling the driver module's ->init() or ->exit() function).
With that being the case, I think we can retain the module refcounts and use them only for synchronization. And naturally the refcount should drop to zero after the critical section; no point keeping it incremented until the CPU is taken offline.
Or, am I confused again?
No, you aren't.
But, for synchronization we need some blocking stuff, so that when user tries to rmmod the module, it should wait until other critical sections are over and then unload the module. But here, we are simply returning from rmmod, saying that we are busy :)
So, for that kind of synchronization we better use locks available inside cpufreq core of if required another variable which can be used for refcount. But now this.
I suppose you wanted to say "But not this"?
I agree. That said the situation now is that for acpi-cpufreq the module usage count is 0 (as of linux-next from today), so you can actually rmmod it.
I don't see why it should be different in the case when there are multiple CPUs for the same policy and hence my patch. [I don't see the problem with it as I said to my reply to Srivatsa, so care to explain it to me?.]
Now it seems to me that the code *is* racy (I haven't verified that, though), so I agree that we should use some other synchronization framework to be able to rmmod and modprobe cpufreq drivers safely. Using the driver module usage counter for that is not quite correct in my view.
Thanks, Rafael
On Thursday, August 01, 2013 08:54:59 PM Srivatsa S. Bhat wrote:
On 08/01/2013 08:14 PM, Viresh Kumar wrote:
On 1 August 2013 13:41, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
On 08/01/2013 05:38 AM, Rafael J. Wysocki wrote:
From: Rafael J. Wysocki rafael.j.wysocki@intel.com
The cpufreq core is a little inconsistent in the way it uses the driver module refcount.
Namely, if __cpufreq_add_dev() is called for a CPU without siblings or generally a CPU for which a new policy object has to be created, it grabs a reference to the driver module to start with, but drops that reference before returning. As a result, the driver module refcount is then equal to 0 after __cpufreq_add_dev() has returned.
On the other hand, if the given CPU is a sibling of some other CPU already having a policy, cpufreq_add_policy_cpu() is called to link the new CPU to the existing policy. In that case, cpufreq_cpu_get() is called to obtain that policy and grabs a reference to the driver module, but that reference is not released and the module refcount will be different from 0 after __cpufreq_add_dev() returns (unless there is an error). That prevents the driver module from being unloaded until __cpufreq_remove_dev() is called for all the CPUs that cpufreq_add_policy_cpu() was called for previously.
To remove that inconsistency make cpufreq_add_policy_cpu() execute cpufreq_cpu_put() for the given policy before returning, which decrements the driver module refcount so that it will be 0 after __cpufreq_add_dev() returns,
Removing the inconsistency is a good thing, but I think we should make it consistent the other way around - make a CPU-online increment the driver module refcount and decrement it only on CPU-offline.
I took time to review to this mail as I was looking at the problem yesterday. I am sorry to say, but I have completely different views as compared to You and Rafael both :)
First of all, Rafael's patch is incomplete as it hasn't fixed the issue completely. When we have multiple CPUs per policy and cpufreq_add_dev() is called for the first one, it call cpufreq_get_cpu() for all cpus of this policy(), so count is == x (no. of CPUs in this policy)
- 1 (This is the initial value of .owner).
And so we still have module count getting incremented for other cpus :)
Good catch!
Sorry, I don't see how this happens.
__cpufreq_add_dev() only directly calls cpufreq_cpu_get() at the beginning to check if the policy is there and then immediately calls cpufreq_cpu_put() in that case (for that policy).
Next, cpufreq_add_policy_cpu() calls cpufreq_cpu_get(), but that's what my patch changes.
I don't see where else cpufreq_cpu_get() is called by __cpufreq_add_dev() whether directly or indirectly.
Moreover, if I'm completely wrong and it is called there in an invisible hush-hush way, it has to be explained why the module usage count as printed by lsmod for acpi-cpufreq is 0 (in current linux-next).
Now few lines about My point of view to this whole thing. I believe we should get rid of .owner field from struct cpufreq_driver completely. It doesn't make sense to me in doing all this management at all. Surprised? Shocked? Laughing at me? :)
Well I may be wrong but this is what I think:
- It looks stupid to me that I can't do this from userspace in one go: $ insmod cpufreq_driver.ko $ rmmod cpufreq_driver.ko
What the hell changed in between that isn't visible to user? It looked completely stupid in that way..
Something like this sure makes sense: $ insmod ondemand-governor.ko $ change governor to ondemand for few CPUs $ rmmod ondemand-governor.ko
as we have deliberately add few users of governor. And so without second step, rmmod should really work smoothly. And it does.
Now, why shouldn't there be a problem with this approach? I will write that inline to the problems you just described.
The reason is, one should not be able to unload the back-end cpufreq driver module when some CPUs are still being managed. Nasty things will result if we allow that. For example, if we unload the module, and then try to do a CPU offline, then the cpufreq hotplug notifier won't even be called (because cpufreq_unregister_driver also unregisters the hotplug notifier). And that might be troublesome.
So what? Its simply equivalent to we have booted our system, haven't inserted cpufreq module and taken out a cpu.
I'd put that differently.
With the current code as is it may cause problems to happen, but there are two ways to change that in general:
(1) Disallow the removal of the cpufreq driver while there are any users, but for that we only need the driver to be refcounted *once* when a new policy is created (and not as many times as there are CPUs using that policy). Then, the reference can be dropped while removing the policy object.
(2) Allow the removal of the cpufreq driver, but harden the code against that. [Maybe it doesn't have to be hardened any more as is, I haven't checked that.]
I agree with Viresh that (1) is kind of weird from the usability perspective, because if we did that, it wouldn't be practically possible to remove cpufreq driver modules after loading them (cpuidle currently has that problem).
Even worse, if we unload a cpufreq driver module and load a new one and *then* try to offline the CPU, then the cpufreq_driver->exit() function that we call during CPU offline will end up calling the corresponding function of an entirely different driver! So the ->init() and ->exit() calls won't match.
That's not true. When we unload the module, it must call cpufreq_unregister_driver() which should call cpufreq_remove_cpu() for all cpus and so exit() is already called for last module.
Sorry, I missed this one.
If we get something new now, it should simply work.
Yeah, I now see your point. It won't create any problems by unloading the module and loading a new one.
What do you think gentlemen?
Well, I now agree that we don't have to keep the module refcount non-zero as long as CPUs are being managed (that was just my misunderstanding, sorry for the noise). However, I think the _get() and _put() used in the existing code is for synchronization: that is, to avoid races between trying to unload the cpufreq driver module and running a hotplug notifier (and calling the driver module's ->init() or ->exit() function).
With that being the case, I think we can retain the module refcounts and use them only for synchronization. And naturally the refcount should drop to zero after the critical section; no point keeping it incremented until the CPU is taken offline.
Or, am I confused again?
No, I don't think so.
In fact, the point of my patch was to make the module refcount stay 0 beyond critical sections, but it looks like I overlooked something. What is that?
Rafael
On 08/01/2013 11:34 PM, Rafael J. Wysocki wrote:
On Thursday, August 01, 2013 08:54:59 PM Srivatsa S. Bhat wrote:
On 08/01/2013 08:14 PM, Viresh Kumar wrote:
On 1 August 2013 13:41, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
On 08/01/2013 05:38 AM, Rafael J. Wysocki wrote:
From: Rafael J. Wysocki rafael.j.wysocki@intel.com
The cpufreq core is a little inconsistent in the way it uses the driver module refcount.
Namely, if __cpufreq_add_dev() is called for a CPU without siblings or generally a CPU for which a new policy object has to be created, it grabs a reference to the driver module to start with, but drops that reference before returning. As a result, the driver module refcount is then equal to 0 after __cpufreq_add_dev() has returned.
On the other hand, if the given CPU is a sibling of some other CPU already having a policy, cpufreq_add_policy_cpu() is called to link the new CPU to the existing policy. In that case, cpufreq_cpu_get() is called to obtain that policy and grabs a reference to the driver module, but that reference is not released and the module refcount will be different from 0 after __cpufreq_add_dev() returns (unless there is an error). That prevents the driver module from being unloaded until __cpufreq_remove_dev() is called for all the CPUs that cpufreq_add_policy_cpu() was called for previously.
To remove that inconsistency make cpufreq_add_policy_cpu() execute cpufreq_cpu_put() for the given policy before returning, which decrements the driver module refcount so that it will be 0 after __cpufreq_add_dev() returns,
Removing the inconsistency is a good thing, but I think we should make it consistent the other way around - make a CPU-online increment the driver module refcount and decrement it only on CPU-offline.
I took time to review to this mail as I was looking at the problem yesterday. I am sorry to say, but I have completely different views as compared to You and Rafael both :)
First of all, Rafael's patch is incomplete as it hasn't fixed the issue completely. When we have multiple CPUs per policy and cpufreq_add_dev() is called for the first one, it call cpufreq_get_cpu() for all cpus of this policy(), so count is == x (no. of CPUs in this policy)
- 1 (This is the initial value of .owner).
And so we still have module count getting incremented for other cpus :)
Good catch!
Sorry, I don't see how this happens.
__cpufreq_add_dev() only directly calls cpufreq_cpu_get() at the beginning to check if the policy is there and then immediately calls cpufreq_cpu_put() in that case (for that policy).
Next, cpufreq_add_policy_cpu() calls cpufreq_cpu_get(), but that's what my patch changes.
I don't see where else cpufreq_cpu_get() is called by __cpufreq_add_dev() whether directly or indirectly.
__cpufreq_add_dev()->cpufreq_add_dev_interface()->cpufreq_add_dev_symlink().
The last one does:
815 for_each_cpu(j, policy->cpus) { 816 struct cpufreq_policy *managed_policy; 817 struct device *cpu_dev; 818 819 if (j == cpu) 820 continue; 821 822 pr_debug("CPU %u already managed, adding link\n", j); 823 managed_policy = cpufreq_cpu_get(cpu); 824 cpu_dev = get_cpu_device(j); 825 ret = sysfs_create_link(&cpu_dev->kobj, &policy->kobj, 826 "cpufreq"); ... }
Moreover, if I'm completely wrong and it is called there in an invisible hush-hush way, it has to be explained why the module usage count as printed by lsmod for acpi-cpufreq is 0 (in current linux-next).
Perhaps because none of your policies have more than one CPU associated with it? I think related_cpus should be able to tell us that..
But yes, it is a little hidden and moreover, we don't take the refcount if there is only one CPU in the mask. Which is a little inconsistent, IMHO.
Now few lines about My point of view to this whole thing. I believe we should get rid of .owner field from struct cpufreq_driver completely. It doesn't make sense to me in doing all this management at all. Surprised? Shocked? Laughing at me? :)
Well I may be wrong but this is what I think:
- It looks stupid to me that I can't do this from userspace in one go: $ insmod cpufreq_driver.ko $ rmmod cpufreq_driver.ko
What the hell changed in between that isn't visible to user? It looked completely stupid in that way..
Something like this sure makes sense: $ insmod ondemand-governor.ko $ change governor to ondemand for few CPUs $ rmmod ondemand-governor.ko
as we have deliberately add few users of governor. And so without second step, rmmod should really work smoothly. And it does.
Now, why shouldn't there be a problem with this approach? I will write that inline to the problems you just described.
The reason is, one should not be able to unload the back-end cpufreq driver module when some CPUs are still being managed. Nasty things will result if we allow that. For example, if we unload the module, and then try to do a CPU offline, then the cpufreq hotplug notifier won't even be called (because cpufreq_unregister_driver also unregisters the hotplug notifier). And that might be troublesome.
So what? Its simply equivalent to we have booted our system, haven't inserted cpufreq module and taken out a cpu.
I'd put that differently.
With the current code as is it may cause problems to happen, but there are two ways to change that in general:
(1) Disallow the removal of the cpufreq driver while there are any users, but for that we only need the driver to be refcounted *once* when a new policy is created (and not as many times as there are CPUs using that policy). Then, the reference can be dropped while removing the policy object.
(2) Allow the removal of the cpufreq driver, but harden the code against that. [Maybe it doesn't have to be hardened any more as is, I haven't checked that.]
I agree with Viresh that (1) is kind of weird from the usability perspective, because if we did that, it wouldn't be practically possible to remove cpufreq driver modules after loading them (cpuidle currently has that problem).
Yes, I think we can go with Viresh's approach and use plain locking to synchronize things. Returning -EBUSY isn't really beneficial, since the critical sections are small and finite - its not like the user has to wait a long time to rmmod the module if we use locking.
Even worse, if we unload a cpufreq driver module and load a new one and *then* try to offline the CPU, then the cpufreq_driver->exit() function that we call during CPU offline will end up calling the corresponding function of an entirely different driver! So the ->init() and ->exit() calls won't match.
That's not true. When we unload the module, it must call cpufreq_unregister_driver() which should call cpufreq_remove_cpu() for all cpus and so exit() is already called for last module.
Sorry, I missed this one.
If we get something new now, it should simply work.
Yeah, I now see your point. It won't create any problems by unloading the module and loading a new one.
What do you think gentlemen?
Well, I now agree that we don't have to keep the module refcount non-zero as long as CPUs are being managed (that was just my misunderstanding, sorry for the noise). However, I think the _get() and _put() used in the existing code is for synchronization: that is, to avoid races between trying to unload the cpufreq driver module and running a hotplug notifier (and calling the driver module's ->init() or ->exit() function).
With that being the case, I think we can retain the module refcounts and use them only for synchronization. And naturally the refcount should drop to zero after the critical section; no point keeping it incremented until the CPU is taken offline.
Or, am I confused again?
No, I don't think so.
In fact, the point of my patch was to make the module refcount stay 0 beyond critical sections, but it looks like I overlooked something. What is that?
Its the cpufreq_cpu_get() hidden away in cpufreq_add_dev_symlink(). With that taken care of, everything should be OK. Then we can change the synchronization part to avoid using refcounts.
Regards, Srivatsa S. Bhat
On Thursday, August 01, 2013 11:36:49 PM Srivatsa S. Bhat wrote:
On 08/01/2013 11:34 PM, Rafael J. Wysocki wrote:
On Thursday, August 01, 2013 08:54:59 PM Srivatsa S. Bhat wrote:
On 08/01/2013 08:14 PM, Viresh Kumar wrote:
On 1 August 2013 13:41, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
On 08/01/2013 05:38 AM, Rafael J. Wysocki wrote:
From: Rafael J. Wysocki rafael.j.wysocki@intel.com
The cpufreq core is a little inconsistent in the way it uses the driver module refcount.
Namely, if __cpufreq_add_dev() is called for a CPU without siblings or generally a CPU for which a new policy object has to be created, it grabs a reference to the driver module to start with, but drops that reference before returning. As a result, the driver module refcount is then equal to 0 after __cpufreq_add_dev() has returned.
On the other hand, if the given CPU is a sibling of some other CPU already having a policy, cpufreq_add_policy_cpu() is called to link the new CPU to the existing policy. In that case, cpufreq_cpu_get() is called to obtain that policy and grabs a reference to the driver module, but that reference is not released and the module refcount will be different from 0 after __cpufreq_add_dev() returns (unless there is an error). That prevents the driver module from being unloaded until __cpufreq_remove_dev() is called for all the CPUs that cpufreq_add_policy_cpu() was called for previously.
To remove that inconsistency make cpufreq_add_policy_cpu() execute cpufreq_cpu_put() for the given policy before returning, which decrements the driver module refcount so that it will be 0 after __cpufreq_add_dev() returns,
Removing the inconsistency is a good thing, but I think we should make it consistent the other way around - make a CPU-online increment the driver module refcount and decrement it only on CPU-offline.
I took time to review to this mail as I was looking at the problem yesterday. I am sorry to say, but I have completely different views as compared to You and Rafael both :)
First of all, Rafael's patch is incomplete as it hasn't fixed the issue completely. When we have multiple CPUs per policy and cpufreq_add_dev() is called for the first one, it call cpufreq_get_cpu() for all cpus of this policy(), so count is == x (no. of CPUs in this policy)
- 1 (This is the initial value of .owner).
And so we still have module count getting incremented for other cpus :)
Good catch!
Sorry, I don't see how this happens.
__cpufreq_add_dev() only directly calls cpufreq_cpu_get() at the beginning to check if the policy is there and then immediately calls cpufreq_cpu_put() in that case (for that policy).
Next, cpufreq_add_policy_cpu() calls cpufreq_cpu_get(), but that's what my patch changes.
I don't see where else cpufreq_cpu_get() is called by __cpufreq_add_dev() whether directly or indirectly.
__cpufreq_add_dev()->cpufreq_add_dev_interface()->cpufreq_add_dev_symlink().
Ah, OK, thanks!
The last one does:
815 for_each_cpu(j, policy->cpus) { 816 struct cpufreq_policy *managed_policy; 817 struct device *cpu_dev; 818 819 if (j == cpu) 820 continue; 821 822 pr_debug("CPU %u already managed, adding link\n", j); 823 managed_policy = cpufreq_cpu_get(cpu); 824 cpu_dev = get_cpu_device(j); 825 ret = sysfs_create_link(&cpu_dev->kobj, &policy->kobj, 826 "cpufreq"); ... }
And when do we drop this one?
Moreover, if I'm completely wrong and it is called there in an invisible hush-hush way, it has to be explained why the module usage count as printed by lsmod for acpi-cpufreq is 0 (in current linux-next).
Perhaps because none of your policies have more than one CPU associated with it? I think related_cpus should be able to tell us that..
Yes, that's the case.
But yes, it is a little hidden and moreover, we don't take the refcount if there is only one CPU in the mask. Which is a little inconsistent, IMHO.
Well, I obviously agree.
Now few lines about My point of view to this whole thing. I believe we should get rid of .owner field from struct cpufreq_driver completely. It doesn't make sense to me in doing all this management at all. Surprised? Shocked? Laughing at me? :)
Well I may be wrong but this is what I think:
- It looks stupid to me that I can't do this from userspace in one go: $ insmod cpufreq_driver.ko $ rmmod cpufreq_driver.ko
What the hell changed in between that isn't visible to user? It looked completely stupid in that way..
Something like this sure makes sense: $ insmod ondemand-governor.ko $ change governor to ondemand for few CPUs $ rmmod ondemand-governor.ko
as we have deliberately add few users of governor. And so without second step, rmmod should really work smoothly. And it does.
Now, why shouldn't there be a problem with this approach? I will write that inline to the problems you just described.
The reason is, one should not be able to unload the back-end cpufreq driver module when some CPUs are still being managed. Nasty things will result if we allow that. For example, if we unload the module, and then try to do a CPU offline, then the cpufreq hotplug notifier won't even be called (because cpufreq_unregister_driver also unregisters the hotplug notifier). And that might be troublesome.
So what? Its simply equivalent to we have booted our system, haven't inserted cpufreq module and taken out a cpu.
I'd put that differently.
With the current code as is it may cause problems to happen, but there are two ways to change that in general:
(1) Disallow the removal of the cpufreq driver while there are any users, but for that we only need the driver to be refcounted *once* when a new policy is created (and not as many times as there are CPUs using that policy). Then, the reference can be dropped while removing the policy object.
(2) Allow the removal of the cpufreq driver, but harden the code against that. [Maybe it doesn't have to be hardened any more as is, I haven't checked that.]
I agree with Viresh that (1) is kind of weird from the usability perspective, because if we did that, it wouldn't be practically possible to remove cpufreq driver modules after loading them (cpuidle currently has that problem).
Yes, I think we can go with Viresh's approach and use plain locking to synchronize things. Returning -EBUSY isn't really beneficial, since the critical sections are small and finite - its not like the user has to wait a long time to rmmod the module if we use locking.
Agreed.
Even worse, if we unload a cpufreq driver module and load a new one and *then* try to offline the CPU, then the cpufreq_driver->exit() function that we call during CPU offline will end up calling the corresponding function of an entirely different driver! So the ->init() and ->exit() calls won't match.
That's not true. When we unload the module, it must call cpufreq_unregister_driver() which should call cpufreq_remove_cpu() for all cpus and so exit() is already called for last module.
Sorry, I missed this one.
If we get something new now, it should simply work.
Yeah, I now see your point. It won't create any problems by unloading the module and loading a new one.
What do you think gentlemen?
Well, I now agree that we don't have to keep the module refcount non-zero as long as CPUs are being managed (that was just my misunderstanding, sorry for the noise). However, I think the _get() and _put() used in the existing code is for synchronization: that is, to avoid races between trying to unload the cpufreq driver module and running a hotplug notifier (and calling the driver module's ->init() or ->exit() function).
With that being the case, I think we can retain the module refcounts and use them only for synchronization. And naturally the refcount should drop to zero after the critical section; no point keeping it incremented until the CPU is taken offline.
Or, am I confused again?
No, I don't think so.
In fact, the point of my patch was to make the module refcount stay 0 beyond critical sections, but it looks like I overlooked something. What is that?
Its the cpufreq_cpu_get() hidden away in cpufreq_add_dev_symlink(). With that taken care of, everything should be OK. Then we can change the synchronization part to avoid using refcounts.
So I actually don't see why cpufreq_add_dev_symlink() needs to call cpufreq_cpu_get() at all, since the policy refcount is already 1 at the point it is called and the bumping up of the driver module refcount is pointless.
However, if I change that I also need to change the piece of code that calls the complementary cpufreq_cpu_put() and I kind of cannot find it.
Thanks, Rafael
On 08/02/2013 12:31 AM, Rafael J. Wysocki wrote:
On Thursday, August 01, 2013 11:36:49 PM Srivatsa S. Bhat wrote:
Its the cpufreq_cpu_get() hidden away in cpufreq_add_dev_symlink(). With that taken care of, everything should be OK. Then we can change the synchronization part to avoid using refcounts.
So I actually don't see why cpufreq_add_dev_symlink() needs to call cpufreq_cpu_get() at all, since the policy refcount is already 1 at the point it is called and the bumping up of the driver module refcount is pointless.
Hmm, yes, it seems so.
However, if I change that I also need to change the piece of code that calls the complementary cpufreq_cpu_put() and I kind of cannot find it.
... I guess that's because you are looking at the code with your patch applied (and your patch removed that _put()) ;-)
Its this part in __cpufreq_remove_dev():
1303 } else { 1304 1305 if (!frozen) { 1306 pr_debug("%s: removing link, cpu: %d\n", __func__, cpu); 1307 cpufreq_cpu_put(data); 1308 } 1309
Regards, Srivatsa S. Bhat
On Friday, August 02, 2013 12:31:23 AM Srivatsa S. Bhat wrote:
On 08/02/2013 12:31 AM, Rafael J. Wysocki wrote:
On Thursday, August 01, 2013 11:36:49 PM Srivatsa S. Bhat wrote:
Its the cpufreq_cpu_get() hidden away in cpufreq_add_dev_symlink(). With that taken care of, everything should be OK. Then we can change the synchronization part to avoid using refcounts.
So I actually don't see why cpufreq_add_dev_symlink() needs to call cpufreq_cpu_get() at all, since the policy refcount is already 1 at the point it is called and the bumping up of the driver module refcount is pointless.
Hmm, yes, it seems so.
However, if I change that I also need to change the piece of code that calls the complementary cpufreq_cpu_put() and I kind of cannot find it.
... I guess that's because you are looking at the code with your patch applied (and your patch removed that _put()) ;-)
No, it's not that one. That one was complementary to the cpufreq_cpu_get() done by cpufreq_add_policy_cpu() before my patch. Since my patch changes cpufreq_add_policy_cpu() to call cpufreq_cpu_put() before returning and bump up the policy refcount with kobject_get(), the one in __cpufreq_remove_dev() is changed into kobject_put() (correctly, IMO).
What gives?
Rafael
On 08/02/2013 12:51 AM, Rafael J. Wysocki wrote:
On Friday, August 02, 2013 12:31:23 AM Srivatsa S. Bhat wrote:
On 08/02/2013 12:31 AM, Rafael J. Wysocki wrote:
On Thursday, August 01, 2013 11:36:49 PM Srivatsa S. Bhat wrote:
Its the cpufreq_cpu_get() hidden away in cpufreq_add_dev_symlink(). With that taken care of, everything should be OK. Then we can change the synchronization part to avoid using refcounts.
So I actually don't see why cpufreq_add_dev_symlink() needs to call cpufreq_cpu_get() at all, since the policy refcount is already 1 at the point it is called and the bumping up of the driver module refcount is pointless.
Hmm, yes, it seems so.
However, if I change that I also need to change the piece of code that calls the complementary cpufreq_cpu_put() and I kind of cannot find it.
... I guess that's because you are looking at the code with your patch applied (and your patch removed that _put()) ;-)
No, it's not that one. That one was complementary to the cpufreq_cpu_get() done by cpufreq_add_policy_cpu() before my patch. Since my patch changes cpufreq_add_policy_cpu() to call cpufreq_cpu_put() before returning and bump up the policy refcount with kobject_get(), the one in __cpufreq_remove_dev() is changed into kobject_put() (correctly, IMO).
What gives?
Actually, it _is_ the one I pointed above. This thing is tricky, here's why:
cpufreq_add_policy_cpu() is called only if: a. The CPU being onlined has per_cpu(cpufreq_cpu_data, cpu) == NULL and b. Its is present in some CPU's related_cpus mask.
If condition (a) doesn't hold good, you get out right in the beginning of __cpufreq_add_dev().
So, cpufreq_add_policy_cpu() is called very rarely because, inside __cpufreq_add_dev we do:
1093 write_lock_irqsave(&cpufreq_driver_lock, flags); 1094 for_each_cpu(j, policy->cpus) { 1095 per_cpu(cpufreq_cpu_data, j) = policy; 1096 per_cpu(cpufreq_policy_cpu, j) = policy->cpu; 1097 } 1098 write_unlock_irqrestore(&cpufreq_driver_lock, flags);
So for all the CPUs in the above policy->cpus mask, we simply return without further ado when they are onlined. In particular, we *dont* call cpufreq_add_policy_cpu() for any of them.
And their refcounts are incremented by the cpufreq_add_dev_interface()-> cpufreq_add_dev_symlink() function.
So, ultimately, we increment the refcount for a given non-policy-owner CPU only once. *Either* in cpufreq_add_dev_symlink() *or* in cpufreq_add_policy_cpu(), but never both.
So, in the teardown path, __cpufreq_remove_dev() needs only one place to decrement it as shown below:
1303 } else { 1304 1305 if (!frozen) { 1306 pr_debug("%s: removing link, cpu: %d\n", __func__, cpu); 1307 cpufreq_cpu_put(data); 1308 }
Pretty good maze, right? ;-(
Regards, Srivatsa S. Bhat
On Friday, August 02, 2013 12:51:24 AM Srivatsa S. Bhat wrote:
On 08/02/2013 12:51 AM, Rafael J. Wysocki wrote:
On Friday, August 02, 2013 12:31:23 AM Srivatsa S. Bhat wrote:
On 08/02/2013 12:31 AM, Rafael J. Wysocki wrote:
On Thursday, August 01, 2013 11:36:49 PM Srivatsa S. Bhat wrote:
Its the cpufreq_cpu_get() hidden away in cpufreq_add_dev_symlink(). With that taken care of, everything should be OK. Then we can change the synchronization part to avoid using refcounts.
So I actually don't see why cpufreq_add_dev_symlink() needs to call cpufreq_cpu_get() at all, since the policy refcount is already 1 at the point it is called and the bumping up of the driver module refcount is pointless.
Hmm, yes, it seems so.
However, if I change that I also need to change the piece of code that calls the complementary cpufreq_cpu_put() and I kind of cannot find it.
... I guess that's because you are looking at the code with your patch applied (and your patch removed that _put()) ;-)
No, it's not that one. That one was complementary to the cpufreq_cpu_get() done by cpufreq_add_policy_cpu() before my patch. Since my patch changes cpufreq_add_policy_cpu() to call cpufreq_cpu_put() before returning and bump up the policy refcount with kobject_get(), the one in __cpufreq_remove_dev() is changed into kobject_put() (correctly, IMO).
What gives?
Actually, it _is_ the one I pointed above. This thing is tricky, here's why:
cpufreq_add_policy_cpu() is called only if: a. The CPU being onlined has per_cpu(cpufreq_cpu_data, cpu) == NULL and b. Its is present in some CPU's related_cpus mask.
If condition (a) doesn't hold good, you get out right in the beginning of __cpufreq_add_dev().
So, cpufreq_add_policy_cpu() is called very rarely because, inside __cpufreq_add_dev we do:
1093 write_lock_irqsave(&cpufreq_driver_lock, flags); 1094 for_each_cpu(j, policy->cpus) { 1095 per_cpu(cpufreq_cpu_data, j) = policy; 1096 per_cpu(cpufreq_policy_cpu, j) = policy->cpu; 1097 } 1098 write_unlock_irqrestore(&cpufreq_driver_lock, flags);
So for all the CPUs in the above policy->cpus mask, we simply return without further ado when they are onlined. In particular, we *dont* call cpufreq_add_policy_cpu() for any of them.
And their refcounts are incremented by the cpufreq_add_dev_interface()-> cpufreq_add_dev_symlink() function.
So, ultimately, we increment the refcount for a given non-policy-owner CPU only once. *Either* in cpufreq_add_dev_symlink() *or* in cpufreq_add_policy_cpu(), but never both.
So, in the teardown path, __cpufreq_remove_dev() needs only one place to decrement it as shown below:
1303 } else { 1304 1305 if (!frozen) { 1306 pr_debug("%s: removing link, cpu: %d\n", __func__, cpu); 1307 cpufreq_cpu_put(data); 1308 }
Pretty good maze, right? ;-(
Oh dear. Right.
I tgought I could change cpufreq_add_dev_symlink() to use kobject_get() to bump up the policy refcount in analogy with cpufreq_add_policy_cpu() and then it wouldn't need to call cpufreq_cpu_get() at all, but there is a bug in the error code path of cpufreq_add_dev_interface(), because if cpufreq_add_dev_symlink() fails for one of the CPUs sharing the policy, it will just fail to drop references grabbed in there. [Moreover, if it fails for the first one different from policy->cpu, kobject_put() will be called for that policy twice in a row if I'm not mistaken (first by cpufreq_add_dev_interface() and then by __cpufreq_add_dev()), but that's a different matter.]
So I think that neither cpufreq_add_dev_symlink() nor cpufreq_add_policy_cpu() should bump up the policy refcount in any way.
Which entirely boils down to something like this:
--- drivers/cpufreq/cpufreq.c | 31 +++++++------------------------ 1 file changed, 7 insertions(+), 24 deletions(-)
Index: linux-pm/drivers/cpufreq/cpufreq.c =================================================================== --- linux-pm.orig/drivers/cpufreq/cpufreq.c +++ linux-pm/drivers/cpufreq/cpufreq.c @@ -818,14 +818,11 @@ static int cpufreq_add_dev_symlink(struc continue;
pr_debug("Adding link for CPU: %u\n", j); - cpufreq_cpu_get(policy->cpu); cpu_dev = get_cpu_device(j); ret = sysfs_create_link(&cpu_dev->kobj, &policy->kobj, "cpufreq"); - if (ret) { - cpufreq_cpu_put(policy); - return ret; - } + if (ret) + break; } return ret; } @@ -908,7 +905,8 @@ static int cpufreq_add_policy_cpu(unsign unsigned long flags;
policy = cpufreq_cpu_get(sibling); - WARN_ON(!policy); + if (WARN_ON_ONCE(!policy)) + return -ENODATA;
if (has_target) __cpufreq_governor(policy, CPUFREQ_GOV_STOP); @@ -930,16 +928,10 @@ static int cpufreq_add_policy_cpu(unsign }
/* Don't touch sysfs links during light-weight init */ - if (frozen) { - /* Drop the extra refcount that we took above */ - cpufreq_cpu_put(policy); - return 0; - } - - ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq"); - if (ret) - cpufreq_cpu_put(policy); + if (!frozen) + ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");
+ cpufreq_cpu_put(policy); return ret; } #endif @@ -1117,9 +1109,6 @@ err_out_unregister: } write_unlock_irqrestore(&cpufreq_driver_lock, flags);
- kobject_put(&policy->kobj); - wait_for_completion(&policy->kobj_unregister); - err_set_policy_cpu: per_cpu(cpufreq_policy_cpu, cpu) = -1; cpufreq_policy_free(policy); @@ -1298,12 +1287,6 @@ static int __cpufreq_remove_dev(struct d if (!frozen) cpufreq_policy_free(data); } else { - - if (!frozen) { - pr_debug("%s: removing link, cpu: %d\n", __func__, cpu); - cpufreq_cpu_put(data); - } - if (cpufreq_driver->target) { __cpufreq_governor(data, CPUFREQ_GOV_START); __cpufreq_governor(data, CPUFREQ_GOV_LIMITS);
On 08/02/2013 01:34 AM, Rafael J. Wysocki wrote:
On Friday, August 02, 2013 12:51:24 AM Srivatsa S. Bhat wrote:
On 08/02/2013 12:51 AM, Rafael J. Wysocki wrote:
On Friday, August 02, 2013 12:31:23 AM Srivatsa S. Bhat wrote:
On 08/02/2013 12:31 AM, Rafael J. Wysocki wrote:
On Thursday, August 01, 2013 11:36:49 PM Srivatsa S. Bhat wrote:
Its the cpufreq_cpu_get() hidden away in cpufreq_add_dev_symlink(). With that taken care of, everything should be OK. Then we can change the synchronization part to avoid using refcounts.
So I actually don't see why cpufreq_add_dev_symlink() needs to call cpufreq_cpu_get() at all, since the policy refcount is already 1 at the point it is called and the bumping up of the driver module refcount is pointless.
Hmm, yes, it seems so.
However, if I change that I also need to change the piece of code that calls the complementary cpufreq_cpu_put() and I kind of cannot find it.
... I guess that's because you are looking at the code with your patch applied (and your patch removed that _put()) ;-)
No, it's not that one. That one was complementary to the cpufreq_cpu_get() done by cpufreq_add_policy_cpu() before my patch. Since my patch changes cpufreq_add_policy_cpu() to call cpufreq_cpu_put() before returning and bump up the policy refcount with kobject_get(), the one in __cpufreq_remove_dev() is changed into kobject_put() (correctly, IMO).
What gives?
Actually, it _is_ the one I pointed above. This thing is tricky, here's why:
cpufreq_add_policy_cpu() is called only if: a. The CPU being onlined has per_cpu(cpufreq_cpu_data, cpu) == NULL and b. Its is present in some CPU's related_cpus mask.
If condition (a) doesn't hold good, you get out right in the beginning of __cpufreq_add_dev().
So, cpufreq_add_policy_cpu() is called very rarely because, inside __cpufreq_add_dev we do:
1093 write_lock_irqsave(&cpufreq_driver_lock, flags); 1094 for_each_cpu(j, policy->cpus) { 1095 per_cpu(cpufreq_cpu_data, j) = policy; 1096 per_cpu(cpufreq_policy_cpu, j) = policy->cpu; 1097 } 1098 write_unlock_irqrestore(&cpufreq_driver_lock, flags);
So for all the CPUs in the above policy->cpus mask, we simply return without further ado when they are onlined. In particular, we *dont* call cpufreq_add_policy_cpu() for any of them.
And their refcounts are incremented by the cpufreq_add_dev_interface()-> cpufreq_add_dev_symlink() function.
So, ultimately, we increment the refcount for a given non-policy-owner CPU only once. *Either* in cpufreq_add_dev_symlink() *or* in cpufreq_add_policy_cpu(), but never both.
So, in the teardown path, __cpufreq_remove_dev() needs only one place to decrement it as shown below:
1303 } else { 1304 1305 if (!frozen) { 1306 pr_debug("%s: removing link, cpu: %d\n", __func__, cpu); 1307 cpufreq_cpu_put(data); 1308 }
Pretty good maze, right? ;-(
Oh dear. Right.
I tgought I could change cpufreq_add_dev_symlink() to use kobject_get() to bump up the policy refcount in analogy with cpufreq_add_policy_cpu() and then it wouldn't need to call cpufreq_cpu_get() at all, but there is a bug in the error code path of cpufreq_add_dev_interface(), because if cpufreq_add_dev_symlink() fails for one of the CPUs sharing the policy, it will just fail to drop references grabbed in there. [Moreover, if it fails for the first one different from policy->cpu, kobject_put() will be called for that policy twice in a row if I'm not mistaken (first by cpufreq_add_dev_interface() and then by __cpufreq_add_dev()), but that's a different matter.]
So I think that neither cpufreq_add_dev_symlink() nor cpufreq_add_policy_cpu() should bump up the policy refcount in any way.
Yeah, that greatly simplifies things, as seen in the patch below.
Which entirely boils down to something like this:
Looks good to me.
Reviewed-by: Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com
Regards, Srivatsa S. Bhat
drivers/cpufreq/cpufreq.c | 31 +++++++------------------------ 1 file changed, 7 insertions(+), 24 deletions(-)
Index: linux-pm/drivers/cpufreq/cpufreq.c
--- linux-pm.orig/drivers/cpufreq/cpufreq.c +++ linux-pm/drivers/cpufreq/cpufreq.c @@ -818,14 +818,11 @@ static int cpufreq_add_dev_symlink(struc continue;
pr_debug("Adding link for CPU: %u\n", j);
cpufreq_cpu_get(policy->cpu);if (ret) {cpufreq_cpu_put(policy);return ret;}
if (ret)break;} @@ -908,7 +905,8 @@ static int cpufreq_add_policy_cpu(unsign unsigned long flags;
policy = cpufreq_cpu_get(sibling);
- WARN_ON(!policy);
if (WARN_ON_ONCE(!policy))
return -ENODATA;if (has_target) __cpufreq_governor(policy, CPUFREQ_GOV_STOP);
@@ -930,16 +928,10 @@ static int cpufreq_add_policy_cpu(unsign }
/* Don't touch sysfs links during light-weight init */
- if (frozen) {
/* Drop the extra refcount that we took above */cpufreq_cpu_put(policy);return 0;- }
- ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");
- if (ret)
cpufreq_cpu_put(policy);
if (!frozen)
ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");cpufreq_cpu_put(policy); return ret;
} #endif @@ -1117,9 +1109,6 @@ err_out_unregister: } write_unlock_irqrestore(&cpufreq_driver_lock, flags);
- kobject_put(&policy->kobj);
- wait_for_completion(&policy->kobj_unregister);
err_set_policy_cpu: per_cpu(cpufreq_policy_cpu, cpu) = -1; cpufreq_policy_free(policy); @@ -1298,12 +1287,6 @@ static int __cpufreq_remove_dev(struct d if (!frozen) cpufreq_policy_free(data); } else {
if (!frozen) {pr_debug("%s: removing link, cpu: %d\n", __func__, cpu);cpufreq_cpu_put(data);}- if (cpufreq_driver->target) { __cpufreq_governor(data, CPUFREQ_GOV_START); __cpufreq_governor(data, CPUFREQ_GOV_LIMITS);
On Friday, August 02, 2013 01:56:21 AM Srivatsa S. Bhat wrote:
On 08/02/2013 01:34 AM, Rafael J. Wysocki wrote:
On Friday, August 02, 2013 12:51:24 AM Srivatsa S. Bhat wrote:
On 08/02/2013 12:51 AM, Rafael J. Wysocki wrote:
On Friday, August 02, 2013 12:31:23 AM Srivatsa S. Bhat wrote:
On 08/02/2013 12:31 AM, Rafael J. Wysocki wrote:
On Thursday, August 01, 2013 11:36:49 PM Srivatsa S. Bhat wrote: > Its the cpufreq_cpu_get() hidden away in cpufreq_add_dev_symlink(). With > that taken care of, everything should be OK. Then we can change the > synchronization part to avoid using refcounts.
So I actually don't see why cpufreq_add_dev_symlink() needs to call cpufreq_cpu_get() at all, since the policy refcount is already 1 at the point it is called and the bumping up of the driver module refcount is pointless.
Hmm, yes, it seems so.
However, if I change that I also need to change the piece of code that calls the complementary cpufreq_cpu_put() and I kind of cannot find it.
... I guess that's because you are looking at the code with your patch applied (and your patch removed that _put()) ;-)
No, it's not that one. That one was complementary to the cpufreq_cpu_get() done by cpufreq_add_policy_cpu() before my patch. Since my patch changes cpufreq_add_policy_cpu() to call cpufreq_cpu_put() before returning and bump up the policy refcount with kobject_get(), the one in __cpufreq_remove_dev() is changed into kobject_put() (correctly, IMO).
What gives?
Actually, it _is_ the one I pointed above. This thing is tricky, here's why:
cpufreq_add_policy_cpu() is called only if: a. The CPU being onlined has per_cpu(cpufreq_cpu_data, cpu) == NULL and b. Its is present in some CPU's related_cpus mask.
If condition (a) doesn't hold good, you get out right in the beginning of __cpufreq_add_dev().
So, cpufreq_add_policy_cpu() is called very rarely because, inside __cpufreq_add_dev we do:
1093 write_lock_irqsave(&cpufreq_driver_lock, flags); 1094 for_each_cpu(j, policy->cpus) { 1095 per_cpu(cpufreq_cpu_data, j) = policy; 1096 per_cpu(cpufreq_policy_cpu, j) = policy->cpu; 1097 } 1098 write_unlock_irqrestore(&cpufreq_driver_lock, flags);
So for all the CPUs in the above policy->cpus mask, we simply return without further ado when they are onlined. In particular, we *dont* call cpufreq_add_policy_cpu() for any of them.
And their refcounts are incremented by the cpufreq_add_dev_interface()-> cpufreq_add_dev_symlink() function.
So, ultimately, we increment the refcount for a given non-policy-owner CPU only once. *Either* in cpufreq_add_dev_symlink() *or* in cpufreq_add_policy_cpu(), but never both.
So, in the teardown path, __cpufreq_remove_dev() needs only one place to decrement it as shown below:
1303 } else { 1304 1305 if (!frozen) { 1306 pr_debug("%s: removing link, cpu: %d\n", __func__, cpu); 1307 cpufreq_cpu_put(data); 1308 }
Pretty good maze, right? ;-(
Oh dear. Right.
I tgought I could change cpufreq_add_dev_symlink() to use kobject_get() to bump up the policy refcount in analogy with cpufreq_add_policy_cpu() and then it wouldn't need to call cpufreq_cpu_get() at all, but there is a bug in the error code path of cpufreq_add_dev_interface(), because if cpufreq_add_dev_symlink() fails for one of the CPUs sharing the policy, it will just fail to drop references grabbed in there. [Moreover, if it fails for the first one different from policy->cpu, kobject_put() will be called for that policy twice in a row if I'm not mistaken (first by cpufreq_add_dev_interface() and then by __cpufreq_add_dev()), but that's a different matter.]
So I think that neither cpufreq_add_dev_symlink() nor cpufreq_add_policy_cpu() should bump up the policy refcount in any way.
Yeah, that greatly simplifies things, as seen in the patch below.
Which entirely boils down to something like this:
Looks good to me.
Reviewed-by: Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com
Thanks! :-)
I actually think that I should move the error code path bug fix ->
drivers/cpufreq/cpufreq.c | 31 +++++++------------------------ 1 file changed, 7 insertions(+), 24 deletions(-)
Index: linux-pm/drivers/cpufreq/cpufreq.c
--- linux-pm.orig/drivers/cpufreq/cpufreq.c +++ linux-pm/drivers/cpufreq/cpufreq.c @@ -818,14 +818,11 @@ static int cpufreq_add_dev_symlink(struc continue;
pr_debug("Adding link for CPU: %u\n", j);
cpufreq_cpu_get(policy->cpu);if (ret) {cpufreq_cpu_put(policy);return ret;}
if (ret)break;} @@ -908,7 +905,8 @@ static int cpufreq_add_policy_cpu(unsign unsigned long flags;
policy = cpufreq_cpu_get(sibling);
- WARN_ON(!policy);
if (WARN_ON_ONCE(!policy))
return -ENODATA;if (has_target) __cpufreq_governor(policy, CPUFREQ_GOV_STOP);
@@ -930,16 +928,10 @@ static int cpufreq_add_policy_cpu(unsign }
/* Don't touch sysfs links during light-weight init */
- if (frozen) {
/* Drop the extra refcount that we took above */cpufreq_cpu_put(policy);return 0;- }
- ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");
- if (ret)
cpufreq_cpu_put(policy);
if (!frozen)
ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");cpufreq_cpu_put(policy); return ret;
} #endif @@ -1117,9 +1109,6 @@ err_out_unregister: } write_unlock_irqrestore(&cpufreq_driver_lock, flags);
- kobject_put(&policy->kobj);
- wait_for_completion(&policy->kobj_unregister);
err_set_policy_cpu: per_cpu(cpufreq_policy_cpu, cpu) = -1; cpufreq_policy_free(policy);
-> into a separate patch, because it's not really related to the other changes made here.
Thanks, Rafael
From: Rafael J. Wysocki rafael.j.wysocki@intel.com Subject: cpufreq: Do not hold driver module references for additional policy CPUs
The cpufreq core is a little inconsistent in the way it uses the driver module refcount.
Namely, if __cpufreq_add_dev() is called for a CPU that doesn't share the policy object with any other CPUs, the driver module refcount it grabs to start with will be dropped by it before returning and will be equal to 0 afterward.
However, if the given CPU does share the policy object with other CPUs, either cpufreq_add_policy_cpu() is called to link the new CPU to the existing policy, or cpufreq_add_dev_symlink() is used to link the other CPUs sharing the policy with it to the just created policy object. In that case, because both cpufreq_add_policy_cpu() and cpufreq_add_dev_symlink() call cpufreq_cpu_get() for the given policy (the latter possibly many times) without the balancing cpufreq_cpu_put() (unless there is an error), the driver module refcount will be left by __cpufreq_add_dev() with a nonzero value.
To remove that inconsistency make cpufreq_add_policy_cpu() execute cpufreq_cpu_put() for the given policy before returning, which decrements the driver module refcount so that it will be 0 after __cpufreq_add_dev() returns. Moreover, remove the cpufreq_cpu_get() call from cpufreq_add_dev_symlink(), since both the policy refcount and the driver module refcount are nonzero when it is called and they don't need to be bumped up by it.
Accordingly, drop the cpufreq_cpu_put() from __cpufreq_remove_dev(), since it is only necessary to balance the cpufreq_cpu_get() called by cpufreq_add_policy_cpu() or cpufreq_add_dev_symlink().
Signed-off-by: Rafael J. Wysocki rafael.j.wysocki@intel.com Reviewed-by: Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com --- drivers/cpufreq/cpufreq.c | 28 +++++++--------------------- 1 file changed, 7 insertions(+), 21 deletions(-)
Index: linux-pm/drivers/cpufreq/cpufreq.c =================================================================== --- linux-pm.orig/drivers/cpufreq/cpufreq.c +++ linux-pm/drivers/cpufreq/cpufreq.c @@ -818,14 +818,11 @@ static int cpufreq_add_dev_symlink(struc continue;
pr_debug("Adding link for CPU: %u\n", j); - cpufreq_cpu_get(policy->cpu); cpu_dev = get_cpu_device(j); ret = sysfs_create_link(&cpu_dev->kobj, &policy->kobj, "cpufreq"); - if (ret) { - cpufreq_cpu_put(policy); - return ret; - } + if (ret) + break; } return ret; } @@ -908,7 +905,8 @@ static int cpufreq_add_policy_cpu(unsign unsigned long flags;
policy = cpufreq_cpu_get(sibling); - WARN_ON(!policy); + if (WARN_ON_ONCE(!policy)) + return -ENODATA;
if (has_target) __cpufreq_governor(policy, CPUFREQ_GOV_STOP); @@ -930,16 +928,10 @@ static int cpufreq_add_policy_cpu(unsign }
/* Don't touch sysfs links during light-weight init */ - if (frozen) { - /* Drop the extra refcount that we took above */ - cpufreq_cpu_put(policy); - return 0; - } - - ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq"); - if (ret) - cpufreq_cpu_put(policy); + if (!frozen) + ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");
+ cpufreq_cpu_put(policy); return ret; } #endif @@ -1298,12 +1290,6 @@ static int __cpufreq_remove_dev(struct d if (!frozen) cpufreq_policy_free(data); } else { - - if (!frozen) { - pr_debug("%s: removing link, cpu: %d\n", __func__, cpu); - cpufreq_cpu_put(data); - } - if (cpufreq_driver->target) { __cpufreq_governor(data, CPUFREQ_GOV_START); __cpufreq_governor(data, CPUFREQ_GOV_LIMITS);
Wow!! Lot of stuff happened while I was asleep..
@Srivatsa: Thanks for answering what I would have answered to Rafael :) And you should really get some sleep, I would suggest :)
On 2 August 2013 02:23, Rafael J. Wysocki rjw@sisk.pl wrote:
From: Rafael J. Wysocki rafael.j.wysocki@intel.com Subject: cpufreq: Do not hold driver module references for additional policy CPUs
I still have issues with this subject. Why don't we get rid of .owner field completely? And stop using a mix of cpufreq_cpu_get() and kobject_get()?
The cpufreq core is a little inconsistent in the way it uses the driver module refcount.
Namely, if __cpufreq_add_dev() is called for a CPU that doesn't share the policy object with any other CPUs, the driver module refcount it grabs to start with will be dropped by it before returning and will be equal to 0 afterward.
It wouldn't be zero but 1, this is what it is initialized with probably. That's what I can see in my tests.
However, if the given CPU does share the policy object with other CPUs, either cpufreq_add_policy_cpu() is called to link the new CPU to the existing policy, or cpufreq_add_dev_symlink() is used to link the other CPUs sharing the policy with it to the just created policy object. In that case, because both cpufreq_add_policy_cpu() and cpufreq_add_dev_symlink() call cpufreq_cpu_get() for the given policy (the latter possibly many times) without the balancing cpufreq_cpu_put() (unless there is an error), the driver module refcount will be left by __cpufreq_add_dev() with a nonzero value.
To remove that inconsistency make cpufreq_add_policy_cpu() execute cpufreq_cpu_put() for the given policy before returning, which decrements the driver module refcount so that it will be 0 after __cpufreq_add_dev() returns. Moreover, remove the cpufreq_cpu_get() call from cpufreq_add_dev_symlink(), since both the policy refcount and the driver module refcount are nonzero when it is called and they don't need to be bumped up by it.
Accordingly, drop the cpufreq_cpu_put() from __cpufreq_remove_dev(), since it is only necessary to balance the cpufreq_cpu_get() called by cpufreq_add_policy_cpu() or cpufreq_add_dev_symlink().
Signed-off-by: Rafael J. Wysocki rafael.j.wysocki@intel.com Reviewed-by: Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com
drivers/cpufreq/cpufreq.c | 28 +++++++--------------------- 1 file changed, 7 insertions(+), 21 deletions(-)
So, we can't rmmod the module as soon as it is inserted and so the problem stays as is. :(
Index: linux-pm/drivers/cpufreq/cpufreq.c
--- linux-pm.orig/drivers/cpufreq/cpufreq.c +++ linux-pm/drivers/cpufreq/cpufreq.c @@ -818,14 +818,11 @@ static int cpufreq_add_dev_symlink(struc continue;
pr_debug("Adding link for CPU: %u\n", j);
cpufreq_cpu_get(policy->cpu); cpu_dev = get_cpu_device(j); ret = sysfs_create_link(&cpu_dev->kobj, &policy->kobj, "cpufreq");if (ret) {cpufreq_cpu_put(policy);return ret;}
if (ret)break; } return ret;} @@ -908,7 +905,8 @@ static int cpufreq_add_policy_cpu(unsign unsigned long flags;
policy = cpufreq_cpu_get(sibling);
This can be skipped completely at this place. Caller of cpufreq_add_policy_cpu() has got the policy pointer with it and so can be passed. I haven't done it earlier as the impression was we need to call cpufreq_cpu_get()..
WARN_ON(!policy);
if (WARN_ON_ONCE(!policy))return -ENODATA; if (has_target) __cpufreq_governor(policy, CPUFREQ_GOV_STOP);@@ -930,16 +928,10 @@ static int cpufreq_add_policy_cpu(unsign }
/* Don't touch sysfs links during light-weight init */
if (frozen) {/* Drop the extra refcount that we took above */cpufreq_cpu_put(policy);return 0;}ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");if (ret)cpufreq_cpu_put(policy);
if (!frozen)ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");cpufreq_cpu_put(policy);
And so this will go away.
On 08/02/2013 10:07 AM, Viresh Kumar wrote:
Wow!! Lot of stuff happened while I was asleep..
@Srivatsa: Thanks for answering what I would have answered to Rafael :) And you should really get some sleep, I would suggest :)
No problem :-) And thank you for your concern :-)
On 2 August 2013 02:23, Rafael J. Wysocki rjw@sisk.pl wrote:
From: Rafael J. Wysocki rafael.j.wysocki@intel.com Subject: cpufreq: Do not hold driver module references for additional policy CPUs
I still have issues with this subject. Why don't we get rid of .owner field completely? And stop using a mix of cpufreq_cpu_get() and kobject_get()?
I guess Rafael's intention is to do one thing at a time - fix the inconsistency first, and then rework the synchronization on top of it. And that makes sense to me, since its the logical way of fixing all these issues.
The cpufreq core is a little inconsistent in the way it uses the driver module refcount.
Namely, if __cpufreq_add_dev() is called for a CPU that doesn't share the policy object with any other CPUs, the driver module refcount it grabs to start with will be dropped by it before returning and will be equal to 0 afterward.
It wouldn't be zero but 1, this is what it is initialized with probably. That's what I can see in my tests.
But lsmod shows 0 for the cpufreq driver right? (Note, your related_cpus should have only 1 CPU each, for you to see 0. Else, you'll see a non-zero value due to the very bug/inconsistency that Rafael is fixing in this patch).
However, if the given CPU does share the policy object with other CPUs, either cpufreq_add_policy_cpu() is called to link the new CPU to the existing policy, or cpufreq_add_dev_symlink() is used to link the other CPUs sharing the policy with it to the just created policy object. In that case, because both cpufreq_add_policy_cpu() and cpufreq_add_dev_symlink() call cpufreq_cpu_get() for the given policy (the latter possibly many times) without the balancing cpufreq_cpu_put() (unless there is an error), the driver module refcount will be left by __cpufreq_add_dev() with a nonzero value.
To remove that inconsistency make cpufreq_add_policy_cpu() execute cpufreq_cpu_put() for the given policy before returning, which decrements the driver module refcount so that it will be 0 after __cpufreq_add_dev() returns. Moreover, remove the cpufreq_cpu_get() call from cpufreq_add_dev_symlink(), since both the policy refcount and the driver module refcount are nonzero when it is called and they don't need to be bumped up by it.
Accordingly, drop the cpufreq_cpu_put() from __cpufreq_remove_dev(), since it is only necessary to balance the cpufreq_cpu_get() called by cpufreq_add_policy_cpu() or cpufreq_add_dev_symlink().
Signed-off-by: Rafael J. Wysocki rafael.j.wysocki@intel.com Reviewed-by: Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com
drivers/cpufreq/cpufreq.c | 28 +++++++--------------------- 1 file changed, 7 insertions(+), 21 deletions(-)
So, we can't rmmod the module as soon as it is inserted and so the problem stays as is. :(
No, we get one step closer to the solution, since we fix the inconsistency between refcounts. Next step would be to get rid of refcounts and use locking like you suggested. Then we can rmmod it easily. I'm assuming Rafael has the same plan.
(We could have done all this in one-shot, but that would make it difficult to track regressions etc. So good to have each improvement in a separate patch).
Index: linux-pm/drivers/cpufreq/cpufreq.c
--- linux-pm.orig/drivers/cpufreq/cpufreq.c +++ linux-pm/drivers/cpufreq/cpufreq.c @@ -818,14 +818,11 @@ static int cpufreq_add_dev_symlink(struc continue;
pr_debug("Adding link for CPU: %u\n", j);
cpufreq_cpu_get(policy->cpu); cpu_dev = get_cpu_device(j); ret = sysfs_create_link(&cpu_dev->kobj, &policy->kobj, "cpufreq");if (ret) {cpufreq_cpu_put(policy);return ret;}
if (ret)break; } return ret;} @@ -908,7 +905,8 @@ static int cpufreq_add_policy_cpu(unsign unsigned long flags;
policy = cpufreq_cpu_get(sibling);This can be skipped completely at this place. Caller of cpufreq_add_policy_cpu() has got the policy pointer with it and so can be passed. I haven't done it earlier as the impression was we need to call cpufreq_cpu_get()..
Agreed, that would be a good cleanup.
WARN_ON(!policy);
if (WARN_ON_ONCE(!policy))return -ENODATA; if (has_target) __cpufreq_governor(policy, CPUFREQ_GOV_STOP);@@ -930,16 +928,10 @@ static int cpufreq_add_policy_cpu(unsign }
/* Don't touch sysfs links during light-weight init */
if (frozen) {/* Drop the extra refcount that we took above */cpufreq_cpu_put(policy);return 0;}ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");if (ret)cpufreq_cpu_put(policy);
if (!frozen)ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");cpufreq_cpu_put(policy);And so this will go away.
Regards, Srivatsa S. Bhat
On 2 August 2013 12:19, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
But lsmod shows 0 for the cpufreq driver right? (Note, your related_cpus should have only 1 CPU each, for you to see 0. Else, you'll see a non-zero value due to the very bug/inconsistency that Rafael is fixing in this patch).
I have hacked the driver this way:
@@ -2114,10 +2114,16 @@ int cpufreq_register_driver(struct cpufreq_driver *driver_data) cpufreq_driver = driver_data; write_unlock_irqrestore(&cpufreq_driver_lock, flags);
+ printk(KERN_INFO "%s: Module refcount: %lu\n", __func__, + module_refcount(cpufreq_driver->owner)); + ret = subsys_interface_register(&cpufreq_interface); if (ret) goto err_null_driver;
And this gave me 1..
On 08/02/2013 12:29 PM, Viresh Kumar wrote:
On 2 August 2013 12:19, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
But lsmod shows 0 for the cpufreq driver right? (Note, your related_cpus should have only 1 CPU each, for you to see 0. Else, you'll see a non-zero value due to the very bug/inconsistency that Rafael is fixing in this patch).
I have hacked the driver this way:
@@ -2114,10 +2114,16 @@ int cpufreq_register_driver(struct cpufreq_driver *driver_data) cpufreq_driver = driver_data; write_unlock_irqrestore(&cpufreq_driver_lock, flags);
printk(KERN_INFO "%s: Module refcount: %lu\n", __func__,module_refcount(cpufreq_driver->owner));ret = subsys_interface_register(&cpufreq_interface); if (ret) goto err_null_driver;And this gave me 1..
Well, on my system, lsmod shows:
acpi_cpufreq 13643 0
The last column is the refcount, as printed by: kernel/module.c: print_unload_info()
913 seq_printf(m, " %lu ", module_refcount(mod));
I guess you are printing it at an odd time, when the module is still running its init function. Perhaps the core kernel module infrastructure increments the refcount around that region temporarily?
Regards, Srivatsa S. Bhat
On 2 August 2013 12:39, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
I guess you are printing it at an odd time, when the module is still running its init function. Perhaps the core kernel module infrastructure increments the refcount around that region temporarily?
If I think logically, that sounds correct. I haven't looked at the implementation details though. But yes, I understood why my refcount was incremented :)
Thanks.
On 2 August 2013 12:19, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
On 08/02/2013 10:07 AM, Viresh Kumar wrote:
So, we can't rmmod the module as soon as it is inserted and so the problem stays as is. :(
No, we get one step closer to the solution, since we fix the inconsistency between refcounts. Next step would be to get rid of refcounts and use locking like you suggested. Then we can rmmod it easily. I'm assuming Rafael has the same plan.
Not really. We are putting the reference at the end of add_dev() and so refcount would be zero when we aren't running any critical sections. And so, we can rmmod the module now and that problem is gone.
@Rafael: I will try to do generic cleanups in cpufreq in coming time and will take care to remove .owner field completely in that. Until that point your patches look fine:
For both of your patches: Acked-by: Viresh Kumar viresh.kumar@linaro.org
On 08/02/2013 03:06 PM, Viresh Kumar wrote:
On 2 August 2013 12:19, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
On 08/02/2013 10:07 AM, Viresh Kumar wrote:
So, we can't rmmod the module as soon as it is inserted and so the problem stays as is. :(
No, we get one step closer to the solution, since we fix the inconsistency between refcounts. Next step would be to get rid of refcounts and use locking like you suggested. Then we can rmmod it easily. I'm assuming Rafael has the same plan.
Not really. We are putting the reference at the end of add_dev() and so refcount would be zero when we aren't running any critical sections. And so, we can rmmod the module now and that problem is gone.
Ah, yes, you are right.
@Rafael: I will try to do generic cleanups in cpufreq in coming time and will take care to remove .owner field completely in that. Until that point your patches look fine:
For both of your patches: Acked-by: Viresh Kumar viresh.kumar@linaro.org
Regards, Srivatsa S. Bhat
On 2 August 2013 10:07, Viresh Kumar viresh.kumar@linaro.org wrote:
@@ -908,7 +905,8 @@ static int cpufreq_add_policy_cpu(unsign unsigned long flags;
policy = cpufreq_cpu_get(sibling);This can be skipped completely at this place. Caller of cpufreq_add_policy_cpu() has got the policy pointer with it and so can be passed. I haven't done it earlier as the impression was we need to call cpufreq_cpu_get()..
And here is the fixup to do this (attached too):
--- drivers/cpufreq/cpufreq.c | 18 ++++++------------ 1 file changed, 6 insertions(+), 12 deletions(-)
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c index 46e70ae..47f2a6e 100644 --- a/drivers/cpufreq/cpufreq.c +++ b/drivers/cpufreq/cpufreq.c @@ -889,21 +889,17 @@ static void cpufreq_init_policy(struct cpufreq_policy *policy) }
#ifdef CONFIG_HOTPLUG_CPU -static int cpufreq_add_policy_cpu(unsigned int cpu, unsigned int sibling, - struct device *dev, bool frozen) +static int cpufreq_add_policy_cpu(struct cpufreq_policy *policy, + unsigned int cpu, struct device *dev, + bool frozen) { - struct cpufreq_policy *policy; int ret = 0, has_target = !!cpufreq_driver->target; unsigned long flags;
- policy = cpufreq_cpu_get(sibling); - if (WARN_ON_ONCE(!policy)) - return -ENODATA; - if (has_target) __cpufreq_governor(policy, CPUFREQ_GOV_STOP);
- lock_policy_rwsem_write(sibling); + lock_policy_rwsem_write(policy->cpu);
write_lock_irqsave(&cpufreq_driver_lock, flags);
@@ -912,7 +908,7 @@ static int cpufreq_add_policy_cpu(unsigned int cpu, unsigned int sibling, per_cpu(cpufreq_cpu_data, cpu) = policy; write_unlock_irqrestore(&cpufreq_driver_lock, flags);
- unlock_policy_rwsem_write(sibling); + unlock_policy_rwsem_write(policy->cpu);
if (has_target) { __cpufreq_governor(policy, CPUFREQ_GOV_START); @@ -923,7 +919,6 @@ static int cpufreq_add_policy_cpu(unsigned int cpu, unsigned int sibling, if (!frozen) ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");
- cpufreq_cpu_put(policy); return ret; } #endif @@ -1006,8 +1001,7 @@ static int __cpufreq_add_dev(struct device *dev, struct subsys_interface *sif, struct cpufreq_policy *cp = per_cpu(cpufreq_cpu_data, sibling); if (cp && cpumask_test_cpu(cpu, cp->related_cpus)) { read_unlock_irqrestore(&cpufreq_driver_lock, flags); - return cpufreq_add_policy_cpu(cpu, sibling, dev, - frozen); + return cpufreq_add_policy_cpu(cp, cpu, dev, frozen); } } read_unlock_irqrestore(&cpufreq_driver_lock, flags);
On Friday, August 02, 2013 04:25:58 PM Viresh Kumar wrote:
On 2 August 2013 10:07, Viresh Kumar viresh.kumar@linaro.org wrote:
@@ -908,7 +905,8 @@ static int cpufreq_add_policy_cpu(unsign unsigned long flags;
policy = cpufreq_cpu_get(sibling);This can be skipped completely at this place. Caller of cpufreq_add_policy_cpu() has got the policy pointer with it and so can be passed. I haven't done it earlier as the impression was we need to call cpufreq_cpu_get()..
And here is the fixup to do this (attached too):
Care to add a changelog?
I'll apply this on top of my $subject patch, then.
Thanks, Rafael
drivers/cpufreq/cpufreq.c | 18 ++++++------------ 1 file changed, 6 insertions(+), 12 deletions(-)
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c index 46e70ae..47f2a6e 100644 --- a/drivers/cpufreq/cpufreq.c +++ b/drivers/cpufreq/cpufreq.c @@ -889,21 +889,17 @@ static void cpufreq_init_policy(struct cpufreq_policy *policy) }
#ifdef CONFIG_HOTPLUG_CPU -static int cpufreq_add_policy_cpu(unsigned int cpu, unsigned int sibling,
struct device *dev, bool frozen)+static int cpufreq_add_policy_cpu(struct cpufreq_policy *policy,
unsigned int cpu, struct device *dev,bool frozen){
struct cpufreq_policy *policy; int ret = 0, has_target = !!cpufreq_driver->target; unsigned long flags;
policy = cpufreq_cpu_get(sibling);
if (WARN_ON_ONCE(!policy))
return -ENODATA;if (has_target) __cpufreq_governor(policy, CPUFREQ_GOV_STOP);
lock_policy_rwsem_write(sibling);
lock_policy_rwsem_write(policy->cpu);
write_lock_irqsave(&cpufreq_driver_lock, flags);
@@ -912,7 +908,7 @@ static int cpufreq_add_policy_cpu(unsigned int cpu, unsigned int sibling, per_cpu(cpufreq_cpu_data, cpu) = policy; write_unlock_irqrestore(&cpufreq_driver_lock, flags);
- unlock_policy_rwsem_write(sibling);
unlock_policy_rwsem_write(policy->cpu);
if (has_target) { __cpufreq_governor(policy, CPUFREQ_GOV_START);
@@ -923,7 +919,6 @@ static int cpufreq_add_policy_cpu(unsigned int cpu, unsigned int sibling, if (!frozen) ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");
- cpufreq_cpu_put(policy); return ret;
} #endif @@ -1006,8 +1001,7 @@ static int __cpufreq_add_dev(struct device *dev, struct subsys_interface *sif, struct cpufreq_policy *cp = per_cpu(cpufreq_cpu_data, sibling); if (cp && cpumask_test_cpu(cpu, cp->related_cpus)) { read_unlock_irqrestore(&cpufreq_driver_lock, flags);
return cpufreq_add_policy_cpu(cpu, sibling, dev,frozen);
return cpufreq_add_policy_cpu(cp, cpu, dev, frozen);
On 2 August 2013 19:01, Rafael J. Wysocki rjw@sisk.pl wrote:
Care to add a changelog?
I'll apply this on top of my $subject patch, then.
I thought you will merge this one with your patch, but there is no harm in getting one more on my name :)
Changelog is (patch attached):
From: Viresh Kumar viresh.kumar@linaro.org Date: Fri, 2 Aug 2013 16:17:33 +0530 Subject: [PATCH] cpufreq: Pass policy to cpufreq_add_policy_cpu()
Caller of cpufreq_add_policy_cpu() already has pointer to policy structure and so there is no need to find it again in cpufreq_add_policy_cpu(). Lets pass it directly.
Signed-off-by: Viresh Kumar viresh.kumar@linaro.org --- drivers/cpufreq/cpufreq.c | 18 ++++++------------ 1 file changed, 6 insertions(+), 12 deletions(-)
On 08/02/2013 08:08 PM, Viresh Kumar wrote:
On 2 August 2013 19:01, Rafael J. Wysocki rjw@sisk.pl wrote:
Care to add a changelog?
I'll apply this on top of my $subject patch, then.
I thought you will merge this one with your patch, but there is no harm in getting one more on my name :)
Changelog is (patch attached):
From: Viresh Kumar viresh.kumar@linaro.org Date: Fri, 2 Aug 2013 16:17:33 +0530 Subject: [PATCH] cpufreq: Pass policy to cpufreq_add_policy_cpu()
Caller of cpufreq_add_policy_cpu() already has pointer to policy structure and so there is no need to find it again in cpufreq_add_policy_cpu(). Lets pass it directly.
Signed-off-by: Viresh Kumar viresh.kumar@linaro.org
The patch looks good.
Reviewed-by: Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com
Regards, Srivatsa S. Bhat
drivers/cpufreq/cpufreq.c | 18 ++++++------------ 1 file changed, 6 insertions(+), 12 deletions(-)
On 2 August 2013 02:23, Rafael J. Wysocki rjw@sisk.pl wrote:
To remove that inconsistency make cpufreq_add_policy_cpu() execute cpufreq_cpu_put() for the given policy before returning, which decrements the driver module refcount so that it will be 0 after __cpufreq_add_dev() returns. Moreover, remove the cpufreq_cpu_get() call from cpufreq_add_dev_symlink(), since both the policy refcount and the driver module refcount are nonzero when it is called and they don't need to be bumped up by it.
Sorry for creating so many problems but my concerns with this patch aren't yet over :(
Should we increment policy refcount or kobj refcount for every cpu it is used on? I think yes, that's probably the right way of doing it.
And so we simply can't remove calls to cpufreq_cpu_get() from cpufreq_add_dev_symlink() routine and also from cpufreq_add_policy_cpu()..
On 08/02/2013 04:00 PM, Viresh Kumar wrote:
On 2 August 2013 02:23, Rafael J. Wysocki rjw@sisk.pl wrote:
To remove that inconsistency make cpufreq_add_policy_cpu() execute cpufreq_cpu_put() for the given policy before returning, which decrements the driver module refcount so that it will be 0 after __cpufreq_add_dev() returns. Moreover, remove the cpufreq_cpu_get() call from cpufreq_add_dev_symlink(), since both the policy refcount and the driver module refcount are nonzero when it is called and they don't need to be bumped up by it.
Sorry for creating so many problems but my concerns with this patch aren't yet over :(
Should we increment policy refcount or kobj refcount for every cpu it is used on? I think yes, that's probably the right way of doing it.
It depends on how you look at it. The number of CPUs in the policy (cpumask_weight(policy)) itself serves as a refcount. We don't actually need yet another refcount to manage things. Besides, not bumping up the policy refcount for every CPU actually seems to simplify the code and make it easier to understand, so why not do it? :-)
And so we simply can't remove calls to cpufreq_cpu_get() from cpufreq_add_dev_symlink() routine and also from cpufreq_add_policy_cpu()..
Regards, Srivatsa S. Bhat
linaro-kernel@lists.linaro.org
-
Rafael J. Wysocki -
Srivatsa S. Bhat -
Viresh Kumar