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

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Message-ID: <073cf067-908f-0bdd-e3b9-e6c5a5e3c910@kernel.org>
Date:   Mon, 16 Jan 2023 11:11:44 +0100
From:   Daniel Bristot de Oliveira <bristot@...nel.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        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

On 1/16/23 10:51, Vincent Guittot wrote:
> On Mon, 16 Jan 2023 at 10:28, Ingo Molnar <mingo@...nel.org> wrote:
>>
>>
>> * Ingo Molnar <mingo@...nel.org> wrote:
>>
>>>> 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.
>>
>> ... with the goal of having the best governor for power efficiency by
>> default - but also the ability to drop a handful of CPUs into the highest
>> performance / lowest latency idle mode.
>>
>> It's a special kind of nested policy, for workload exceptions.
> 
> User can set per cpu latency constraint with
> /sys/devices/system/cpu/cpu*/power/pm_qos_resume_latency_us
> Which is then used by cpuidle governor when selecting an idle state.
> The cpuidle governor should then select the idle state that matches
> with the wakeup latency for those CPUs but select the most power
> efficient for others. Setting a low value should filter all idle
> states except the polling one

Yep, that is a possibility, but it does not always work as expected. For example,
on virtual machines the vCPU gets scheduled out, even with this option set :-/.

-- Daniel

> Regards
> Vincent
>>
>> Thanks,
>>
>>         Ingo