linux-kernel - Re: [RFC PATCH 0/3] newidle_balance() latency mitigation

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Message-ID: <jhjwo5xcc1e.mognet@arm.com>
Date:   Thu, 30 Apr 2020 14:56:29 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Scott Wood <swood@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Rik van Riel <riel@...riel.com>,
        Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [RFC PATCH 0/3] newidle_balance() latency mitigation


On 30/04/20 13:42, Vincent Guittot wrote:
>> >> Random thought that just occurred to me; in the grand scheme of things,
>> >> with something in the same spirit as task-stealing (i.e. don't bother with
>> >> a full fledged balance at newidle, just pick one spare task somewhere),
>> >> none of this would be required.
>> >
>> > newly idle load balance already stops after picking 1 task
>>
>> Mph, I had already forgotten your changes there. Is that really always the
>> case for newidle? In e.g. the busiest->group_type == group_fully_busy case,
>> I think we can pull more than one task.
>
> for newly_idle load balance, detach_tasks stops after finding 1 suitable task
>

Right, I hadn't noticed

  7e96fa5875d4 ("sched: pull only one task during NEWIDLE balancing to limit critical section")

>>
>> > Now if your proposal is to pick one random task on one random cpu, I'm
>> > clearly not sure that's a good idea
>> >
>>
>> IIRC Steve's implementation was to "simply" pull one task from any CPU
>> within the LLC domain that had > 1 runnable tasks. I quite like this since
>> picking any one task is almost always better than switching to the idle
>> task, but it wasn't a complete newidle_balance() replacement just yet.
>>
>> >
>> >>
>> >> Sadly I don't think anyone has been looking at it any recently.