linux-kernel - Re: [PATCH] specific do_timer

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Message-ID: <20120215143710.GA10543@sgi.com>
Date:	Wed, 15 Feb 2012 08:37:10 -0600
From:	Dimitri Sivanich <sivanich@....com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] specific do_timer_cpu value for nohz off mode

On Wed, Feb 15, 2012 at 03:16:34PM +0100, Thomas Gleixner wrote:
> On Tue, 8 Nov 2011, Dimitri Sivanich wrote:
> > 
> > Allow manual override of the tick_do_timer_cpu.
> > 
> > While not necessarily harmful, doing jiffies updates on an application cpu
> > does cause some extra overhead that HPC benchmarking people notice.  They
> > prefer to have OS activity isolated to certain cpus.  They like reproducibility
> > of results, and having jiffies updates bouncing around introduces variability.
> 
> I really wonder about this changelog. The only case where jiffies
> updates bounces around is the NOHZ case. In all other modes (periodic
> or highres) the boot cpu gets the do_timer() duty and it's never
> assigned to any other cpu.
> 
> So what's the point of this exercise? Moving it away from CPU0 for
> acedemic reasons or what?
>
I wasn't specifically trying to move it away from CPU0 (having jiffies updates
on CPU0 was and would be just fine for the nohz=off case).  The issue was
that the tick_do_timer_cpu could be any cpu even in the nohz=off case (maybe
something has changed that since?).  After the point of assignment it is
static, but you never know which cpu it is.

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