linux-kernel - Re: [PATCH] sched/idle: Make idle poll dynamic per-cpu

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Date:   Mon, 16 Jan 2023 10:02:02 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Daniel Bristot de Oliveira <bristot@...nel.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Joe Mario <jmario@...hat.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH] sched/idle: Make idle poll dynamic per-cpu


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Jan 12, 2023 at 05:24:26PM +0100, Daniel Bristot de Oliveira wrote:
> > idle=poll is frequently used on ultra-low-latency systems. Examples of
> > such systems are high-performance trading and 5G NVRAM. The performance
> > gain is given by avoiding the idle driver machinery and by keeping the
> > CPU is always in an active state - avoiding (odd) hardware heuristics that
> > are out of the control of the OS.
> > 
> > Currently, idle=poll is an all-or-nothing static option defined at
> > boot time. The motivation for creating this option dynamic and per-cpu
> > are two:
> > 
> >   1) Reduce the power usage/heat by allowing only selected CPUs to
> >      do idle polling;
> >   2) Allow multi-tenant systems (e.g., Kubernetes) to enable idle
> >      polling only when ultra-low-latency applications are present
> >      on specific CPUs.
> > 
> > Joe Mario did some experiments with this option enabled, and the results
> > were significant. For example, by using dynamic idle polling on
> > selected CPUs, cyclictest performance is optimal (like when using
> > idle=poll), but cpu power consumption drops from 381 to 233 watts.
> > 
> > Also, limiting idle=poll to the set of CPUs that benefits from
> > it allows other CPUs to benefit from frequency boosts. Joe also
> > shows that the results can be in the order of 80nsec round trip
> > improvement when system-wide idle=poll was not used.
> > 
> > The user can enable idle polling with this command:
> >   # echo 1 > /sys/devices/system/cpu/cpu{CPU_ID}/idle_poll
> > 
> > And disable it via:
> >   # echo 0 > /sys/devices/system/cpu/cpu{CPU_ID}/idle_poll
> > 
> > By default, all CPUs have idle polling disabled (the current behavior).
> > A static key avoids the CPU mask check overhead when no idle polling
> > is enabled.
> 
> Urgh, can we please make this a cpuidle governor thing or so? So that we
> don't need to invent new interfaces and such.

I think the desired property here would be to make this interface on top of 
pretty much any governor. Ie. have a governor, but also a way to drop any 
CPU into idle-poll, overriding that.

Thanks,

	Ingo