linux-kernel - Re: [PATCH] avoid second smp_processor_id() call in __touch

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Date:	Wed, 22 Sep 2010 18:47:05 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Don Zickus <dzickus@...hat.com>,
	Cyrill Gorcunov <gorcunov@...il.com>
Subject: Re: [PATCH] avoid second smp_processor_id() call in
	__touch_watchdog

On Wed, Sep 22, 2010 at 06:39:19PM +0200, Peter Zijlstra wrote:
> On Wed, 2010-09-22 at 18:27 +0200, Frederic Weisbecker wrote:
> > 
> > I'm not sure we want this. This is called by the watchdog internally,
> > from the timer or the cpu bound thread, so we probably should better
> > keep __get_cpu_var() because it checks that we are not in a preemptable
> > section. 
> 
> The smp_processor_id() right at the start already does that.
> 
> That said, I doubt it really matter one way or the other, compilers have
> been known to do CSE for quite a while.


I don't mind personally. We indeed have this smp_processor_id() that
does the check already. But that's also for readability: reviewers
that are used to deal with per cpu datas are also used to see
per_cpu() for remote percpu data access and get_cpu_var() for local
percpu.

Plus some archs may override their __my_cpu_offset implementation
to provide a faster access.

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