On 03/19/2014 03:39 PM, Viresh Kumar wrote:
On 19 March 2014 15:20, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
No, its not about burden. Its about the elegance of the design. We should not be overly "smart" in the cpufreq core. Hiding the synchronization inside the cpufreq core only encourages people to write buggy code in their drivers.
What kind of buggy code can be there? They are supposed to call notifiers in the order mentioned and so it shouldn't be a problem at all.. Don't know..
Why don't we go with what Rafael suggested? We can have dedicated begin_transition() and end_transition() calls to demarcate the frequency transitions. That way, it makes it very clear how the synchronization is done. Of course, these functions would be provided (exported) by the cpufreq core, by implementing them using locks/counters/whatever.
Basically what I'm arguing against, is the idea of having the cpufreq core figure out what the driver _intended_ to do, from inside the cpufreq_notify_transition() call.
What I would prefer instead is to have the cpufreq driver do something like this:
cpufreq_freq_transition_begin();
cpufreq_notify_transition(CPUFREQ_PRECHANGE);
Why do we need two routines then? What about doing notification from inside cpufreq_freq_transition_begin()?
Hmm, that's a good idea. I thought about ways to simplify the synchronization and this is what I came up with. Its completely untested though. Let me know what you think!
--------------------------------------------------------------------------- From: Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com Subject: [PATCH v3] cpufreq: Make sure frequency transitions are serialized
Whenever we change the frequency of a CPU, we call the PRECHANGE and POSTCHANGE notifiers. They must be serialized, i.e. PRECHANGE and POSTCHANGE notifiers should strictly alternate, thereby preventing two different sets of PRECHANGE or POSTCHANGE notifiers from interleaving arbitrarily.
The following examples illustrate why this is important:
Scenario 1: ----------- A thread reading the value of cpuinfo_cur_freq, will call __cpufreq_cpu_get()->cpufreq_out_of_sync()->cpufreq_notify_transition()
The ondemand governor can decide to change the frequency of the CPU at the same time and hence it can end up sending the notifications via ->target().
If the notifiers are not serialized, the following sequence can occur: - PRECHANGE Notification for freq A (from cpuinfo_cur_freq) - PRECHANGE Notification for freq B (from target()) - Freq changed by target() to B - POSTCHANGE Notification for freq B - POSTCHANGE Notification for freq A
We can see from the above that the last POSTCHANGE Notification happens for freq A but the hardware is set to run at freq B.
Where would we break then?: adjust_jiffies() in cpufreq.c & cpufreq_callback() in arch/arm/kernel/smp.c (which also adjusts the jiffies). All the loops_per_jiffy calculations will get messed up.
Scenario 2: ----------- The governor calls __cpufreq_driver_target() to change the frequency. At the same time, if we change scaling_{min|max}_freq from sysfs, it will end up calling the governor's CPUFREQ_GOV_LIMITS notification, which will also call __cpufreq_driver_target(). And hence we end up issuing concurrent calls to ->target().
Typically, platforms have the following logic in their ->target() routines: (Eg: cpufreq-cpu0, omap, exynos, etc)
A. If new freq is more than old: Increase voltage B. Change freq C. If new freq is less than old: decrease voltage
Now, if the two concurrent calls to ->target() are X and Y, where X is trying to increase the freq and Y is trying to decrease it, we get the following race condition:
X.A: voltage gets increased for larger freq Y.A: nothing happens Y.B: freq gets decreased Y.C: voltage gets decreased X.B: freq gets increased X.C: nothing happens
Thus we can end up setting a freq which is not supported by the voltage we have set. That will probably make the clock to the CPU unstable and the system might not work properly anymore.
This patch introduces a set of synchronization primitives to serialize frequency transitions, which are to be used as shown below:
cpufreq_freq_transition_begin();
//Perform the frequency change
cpufreq_freq_transition_end();
The _begin() call sends the PRECHANGE notification whereas the _end() call sends the POSTCHANGE notification. Also, all the necessary synchronization is handled within these calls. In particular, even drivers which set the ASYNC_NOTIFICATION flag can also use these APIs for performing frequency transitions (ie., you can call _begin() from one task, and call the corresponding _end() from a different task).
The actual synchronization underneath is not that complicated:
The key challenge is to allow drivers to begin the transition from one thread and end it in a completely different thread (this is to enable drivers that do asynchronous POSTCHANGE notification from bottom-halves, to also use the same interface).
To achieve this, a 'transition_ongoing' flag, a 'transition_lock' mutex and a wait-queue are added per-policy. The flag and the wait-queue are used in conjunction to create an "uninterrupted flow" from _begin() to _end(). The mutex-lock is used to ensure that only one such "flow" is in flight at any given time. Put together, this provides us all the necessary synchronization.
Based-on-patch-by: Viresh Kumar viresh.kumar@linaro.org Signed-off-by: Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com ---
drivers/cpufreq/cpufreq.c | 34 ++++++++++++++++++++++++++++++++++ include/linux/cpufreq.h | 5 +++++ 2 files changed, 39 insertions(+)
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c index 199b52b..e90388f 100644 --- a/drivers/cpufreq/cpufreq.c +++ b/drivers/cpufreq/cpufreq.c @@ -349,6 +349,38 @@ void cpufreq_notify_post_transition(struct cpufreq_policy *policy, EXPORT_SYMBOL_GPL(cpufreq_notify_post_transition);
+void cpufreq_freq_transition_begin(struct cpufreq_policy *policy, + struct cpufreq_freqs *freqs, unsigned int state) +{ +wait: + wait_event(&policy->transition_wait, !policy->transition_ongoing); + + if (!mutex_trylock(&policy->transition_lock)) + goto wait; + + policy->transition_ongoing++; + + cpufreq_notify_transition(policy, freqs, CPUFREQ_PRECHANGE); + + mutex_unlock(&policy->transition_lock); +} + +void cpufreq_freq_transition_end(struct cpufreq_policy *policy, + struct cpufreq_freqs *freqs, unsigned int state) +{ + cpufreq_notify_transition(policy, freqs, CPUFREQ_POSTCHANGE); + + /* + * We don't need to take any locks for this update, since only + * one POSTCHANGE notification can be pending at any time, and + * at the moment, that's us :-) + */ + policy->transition_ongoing = false; + + wake_up(&policy->transition_wait); +} + + /********************************************************************* * SYSFS INTERFACE * *********************************************************************/ @@ -968,6 +1000,8 @@ static struct cpufreq_policy *cpufreq_policy_alloc(void)
INIT_LIST_HEAD(&policy->policy_list); init_rwsem(&policy->rwsem); + mutex_init(&policy->transition_lock); + init_waitqueue_head(&policy->transition_wait);
return policy;
diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h index 4d89e0e..8bded24 100644 --- a/include/linux/cpufreq.h +++ b/include/linux/cpufreq.h @@ -101,6 +101,11 @@ struct cpufreq_policy { * __cpufreq_governor(data, CPUFREQ_GOV_POLICY_EXIT); */ struct rw_semaphore rwsem; + + /* Synchronization for frequency transitions */ + bool transition_ongoing; /* Tracks transition status */ + struct mutex transition_lock; + wait_queue_head_t transition_wait; };
/* Only for ACPI */