Messages in this thread | | | Date | Tue, 15 Apr 2014 14:44:42 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 29/38] tick-sched: remove wrapper around __tick_nohz_task_switch() |
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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
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