linux-kernel - Re: [PATCH] specific do_timer

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Message-ID: <20120215153438.GB11343@sgi.com>
Date:	Wed, 15 Feb 2012 09:34:38 -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 03:52:06PM +0100, Thomas Gleixner wrote:
> On Wed, 15 Feb 2012, Dimitri Sivanich wrote:
> 
> > 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.
> 
> It's always the boot cpu and that has been there from day one of that code.
> 
> tick_setup_device()
> {
> 	/*
> 	 * First device setup ?
> 	 */
> 	if (!td->evtdev) {
> 		/*
> 		 * If no cpu took the do_timer update, assign it to
> 		 * this cpu:
> 		 */
> 		if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {
> 			tick_do_timer_cpu = cpu;
> 
> So the first CPU which registers a clock event device takes it. That's
> the boot CPU, no matter what.
>
Both kernel tracing and the original patch that I proposed for this
showed plainly (at the time) that the tick_do_timer_cpu was not always cpu 0
prior to modifying it for nohz=off.  Maybe that is no longer the case?
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