Messages in this thread | | | Date | Tue, 15 Apr 2014 15:23:37 +0530 | Subject | Re: [PATCH 29/38] tick-sched: remove wrapper around __tick_nohz_task_switch() | From | Viresh Kumar <> |
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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 :(
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?
- 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); > }
__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 :)
-- viresh
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