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:	Tue, 20 Nov 2012 02:52:18 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Hakan Akkan <hakanakkan@...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

2012/11/20 Steven Rostedt <rostedt@...dmis.org>:
> 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.

If you want isolation on your full dynticks CPU it's right. Now you
could have lower requirements, a different policy that rather enforce
energy saving.

But I realize we can integrate such granularity later if users request
it and take the behaviour you both describe as the default for now. So
let's take that direction.

Thanks.
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