linux-kernel - Re: [PATCH] nohz/cpuset: Make a CPU stick with do_timer() duty in the presence of nohz cpusets

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Date:	Mon, 19 Nov 2012 20:17:06 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Hakan Akkan <hakanakkan@...il.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] nohz/cpuset: Make a CPU stick with do_timer() duty in
 the presence of nohz cpusets

On Mon, 2012-11-19 at 17:27 -0700, Hakan Akkan wrote:

> >
> > I suggest to rather define a tunable timekeeping duty CPU affinity in
> > a cpumask file at /sys/devices/system/cpu/timekeeping and a toggle at
> > /sys/devices/system/cpu/cpuX/timekeeping (like the online file). This
> > way the user can decide whether adaptive nohz CPU can handle
> > timekeeping or this must be forced to other CPUs in order to enforce
> > isolation.
> 
> Well, users want tickless CPUs because they don't want timekeeping
> (or any other kernel activity for that matter) to run in there. So, I believe
> having that "timekeeping affinity" stay in the regular CPUs is good enough.
> Please let me know how users could utilize these control files to do anything
> other than keeping the timekeeping out of adaptive nohz CPUs.

I agree. If we already have some /sys cpumask that denotes which CPUs
will be adaptive NO_HZ (or simply isolated) then just keep the tick from
ever going on those CPUs. If all but one CPU is set for nohz, and that
one CPU goes idle, it should still be the one doing the tick.

-- Steve


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