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Date:   Mon, 16 Jan 2023 11:11:44 +0100
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Subject: Re: [PATCH] sched/idle: Make idle poll dynamic per-cpu
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To:     Vincent Guittot <vincent.guittot@linaro.org>,
        Ingo Molnar <mingo@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Joe Mario <jmario@redhat.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>
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From:   Daniel Bristot de Oliveira <bristot@kernel.org>
In-Reply-To: <CAKfTPtCBn20ECopZoT5iC0by8SgpRN_xi4Q0Q8Uc8VQ6BR=u=Q@mail.gmail.com>
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On 1/16/23 10:51, Vincent Guittot wrote:
> On Mon, 16 Jan 2023 at 10:28, Ingo Molnar <mingo@kernel.org> wrote:
>>
>>
>> * Ingo Molnar <mingo@kernel.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