Date: Wed, 15 Feb 2012 08:37:10 -0600
From: Dimitri Sivanich <sivanich@sgi.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] specific do_timer_cpu value for nohz off mode
Message-ID: <20120215143710.GA10543@sgi.com>
References: <20111108191149.GA7236@sgi.com>
 <alpine.LFD.2.02.1202151512420.2794@ionos>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <alpine.LFD.2.02.1202151512420.2794@ionos>
User-Agent: Mutt/1.5.20 (2009-06-14)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1431
Lines: 29

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.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/