On Tue, Apr 15, 2014 at 03:23:37PM +0530, Viresh Kumar wrote:
On 15 April 2014 14:43, Frederic Weisbecker fweisbec@gmail.com wrote:
Yeah. But not just that.
Using an inline saves a function call and reduce the offline case to a simple condition check. But there is also the jump label that reduce the condition check to an unconditional jump in the off case.
To summarize, here's how calling tick_nohz_task_switch() maps to final C code:
finish_task_switch() { //do things before calling tick_nohz_task_switch()... // call tick_nohz_task_switch goto offcase; if (tick_nohz_full_enabled()) __tick_nohz_task_switch(tsk); offcase: //end of call to tick_nohz_task_switch //do things before calling tick_nohz_task_switch()... }
In the offcase, the code is like above. We don't even do the check, thanks to the jump label code we unconditionally jump to what's next in finish_task_switch() (there is actually nothing afterward but that's for the picture).
Now if there is at least a CPU that is full dynticks on boot, it is enabled with context_tracking_cpu_set(). Then the jump label code patches the code in finish_task_switch() to turn the goto offcase into a nop. Then the condition is actually verified on every call to finish_task_switch().
So it goes beyond than just saving a function call.
Sorry, but my poor mind still couldn't understand what you are trying to tell me :(
Welcome to the club of the daily confused people. I'm happy to hear I'm not alone :)
So lets clarify things one by one :)
- What do you mean by offcase? CONFIG_NO_HZ_FULL not configured
into the kernel or it is configured but none of the CPUs is running in that mode?
So by offcase I mean CONFIG_NO_HZ_FULL=y but the nohz_full boot parameter is empty, or simply not passed at all. And of course CONFIG_NO_HZ_FULL_ALL=n
This config is now likely on some distros because we want to make full dynticks available for users who want it. But if it's not used (which is 99.999% of the usecases), we want to minimize as much as possible its overhead.
Lets call that dynamic off-case.
- Also what does it correspond to in code: goto offcase; ? There is no labels
or goto statements in code that I can see.. This is how the code looks to me.
finish_task_switch() { //do things before calling tick_nohz_task_switch()... // call tick_nohz_task_switch if (tick_nohz_full_enabled()) __tick_nohz_task_switch(tsk); }
Sure but check out the static_key_false() in the implementation of tick_nohz_full_enabled(). That's where the magic hides.
__tick_nohz_task_switch() may or maynot be available at all depending on CONFIG_NO_HZ_FULL is enabled into the kernel or not. But that was the case with tick_nohz_task_switch() as well in my patch. So shouldn't make a difference..
Again, sorry for not understanding what you are trying to explain here. I want to understand this once and for all and probably add a comment here as well :)
No problem, the jump label/static key code is quite tricky. And its use can be easily missed, as in here.
Also its unfamous API naming (static_key_true/static_key_true) that is anything but intuitive.
-- viresh