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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Thu, 30 Apr 2020 14:42:50 +0200
Message-ID: <CAKfTPtA_duZpnvMx+czAosCikVL=cESKhPQcRrQUdKWKobZBaA@mail.gmail.com>
Subject: Re: [RFC PATCH 0/3] newidle_balance() latency mitigation
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     Scott Wood <swood@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Rik van Riel <riel@surriel.com>,
        Mel Gorman <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-rt-users <linux-rt-users@vger.kernel.org>
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On Thu, 30 Apr 2020 at 12:14, Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
>
> On 30/04/20 08:44, Vincent Guittot wrote:
> > On Thu, 30 Apr 2020 at 01:13, Valentin Schneider
> > <valentin.schneider@arm.com> wrote:
> >>
> >>
> >> On 28/04/20 06:02, Scott Wood wrote:
> >> > These patches mitigate latency caused by newidle_balance() on large
> >> > systems, by enabling interrupts when the lock is dropped, and exiting
> >> > early at various points if an RT task is runnable on the current CPU.
> >> >
> >> > When applied to an RT kernel on a 72-core machine (2 threads per core), I
> >> > saw significant reductions in latency as reported by rteval -- from
> >> > over 500us to around 160us with hyperthreading disabled, and from
> >> > over 1400us to around 380us with hyperthreading enabled.
> >> >
> >> > This isn't the first time something like this has been tried:
> >> > https://lore.kernel.org/lkml/20121222003019.433916240@goodmis.org/
> >> > That attempt ended up being reverted:
> >> > https://lore.kernel.org/lkml/5122CD9C.9070702@oracle.com/
> >> >
> >> > The problem in that case was the failure to keep BH disabled, and the
> >> > difficulty of fixing that when called from the post_schedule() hook.
> >> > This patchset uses finish_task_switch() to call newidle_balance(), which
> >> > enters in non-atomic context so we have full control over what we disable
> >> > and when.
> >> >
> >> > There was a note at the end about wanting further discussion on the matter --
> >> > does anyone remember if that ever happened and what the conclusion was?
> >> > Are there any other issues with enabling interrupts here and/or moving
> >> > the newidle_balance() call?
> >> >
> >>
> >> 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

>
> > 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.