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

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Date:   Mon, 16 Jan 2023 09:43:38 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     Daniel Bristot de Oliveira <bristot@...nel.org>
CC:     <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
        "Peter Zijlstra" <peterz@...radead.org>,
        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>
Subject: Re: [PATCH] sched/idle: Make idle poll dynamic per-cpu

Hi Daniel,
On 2023-01-12 at 17:24:26 +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
>
Maybe I understood it incorrectly, is above command intended to
put specific CPU only in poll mode? Can the c-state sysfs
do this?

grep . /sys/devices/system/cpu/cpu0/cpuidle/state*/name 
/sys/devices/system/cpu/cpu0/cpuidle/state0/name:POLL
/sys/devices/system/cpu/cpu0/cpuidle/state1/name:C1
/sys/devices/system/cpu/cpu0/cpuidle/state2/name:C1E
/sys/devices/system/cpu/cpu0/cpuidle/state3/name:C6

grep . /sys/devices/system/cpu/cpu0/cpuidle/state*/disable
/sys/devices/system/cpu/cpu0/cpuidle/state0/disable:0
/sys/devices/system/cpu/cpu0/cpuidle/state1/disable:1
/sys/devices/system/cpu/cpu0/cpuidle/state2/disable:1
/sys/devices/system/cpu/cpu0/cpuidle/state3/disable:1
 
thanks,
Chenyu