On 15 April 2014 14:43, Frederic Weisbecker <fweis...@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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
