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Date: Tue, 20 Nov 2012 02:52:18 +0100
Message-ID: <CAFTL4hw6vWsHa5gqThUpWhC7MQUueXpze=Fyuvoyzy2t=zYrhA@mail.gmail.com>
Subject: Re: [PATCH] nohz/cpuset: Make a CPU stick with do_timer() duty in the
 presence of nohz cpusets
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Hakan Akkan <hakanakkan@gmail.com>, LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>
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2012/11/20 Steven Rostedt <rostedt@goodmis.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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