linux-kernel - Re: [PATCH 29/38] tick-sched: remove wrapper around __tick_nohz_task

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Date:	Tue, 15 Apr 2014 15:23:37 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arvind Chauhan <Arvind.Chauhan@....com>,
	Linaro Networking <linaro-networking@...aro.org>
Subject: Re: [PATCH 29/38] tick-sched: remove wrapper around __tick_nohz_task_switch()

On 15 April 2014 14:43, Frederic Weisbecker <fweisbec@...il.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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