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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Fri, 25 Feb 2022 09:29:41 +0100
Message-ID: <CAKfTPtD7U7=C8MTLLMtUrGxJFCjpxtU7a_S=HaBhCsZ6SBbVFA@mail.gmail.com>
Subject: Re: [RFC PATCH 0/5] introduce sched-idle balancing
To:     Abel Wu <wuyun.abel@bytedance.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ben Segall <bsegall@google.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Mel Gorman <mgorman@suse.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-kernel@vger.kernel.org
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On Fri, 25 Feb 2022 at 07:46, Abel Wu <wuyun.abel@bytedance.com> wrote:
>
> Hi Peter,
>
> On 2/24/22 11:20 PM, Peter Zijlstra Wrote:
> > On Thu, Feb 17, 2022 at 11:43:56PM +0800, Abel Wu wrote:
> >> Current load balancing is mainly based on cpu capacity
> >> and task util, which makes sense in the POV of overall
> >> throughput. While there still might be some improvement
> >> can be done by reducing number of overloaded cfs rqs if
> >> sched-idle or idle rq exists.
> >
> > I'm much confused, there is an explicit new-idle balancer and a periodic
> > idle balancer already there.
>
> The two balancers are triggered on the rqs that have no tasks on them,
> and load_balance() seems don't show a preference for non-idle tasks so

The load balance will happen at the idle pace if a sched_idle task is
running on the cpu so you will have an ILB on each cpu that run a
sched-idle task

> there might be possibility that only idle tasks are pulled during load
> balance while overloaded rqs (rq->cfs.h_nr_running > 1) exist. As a

There is a LB_MIN feature (disable by default) that filters task with
very low load ( < 16) which includes sched-idle task which has a max
load of 3

> result the normal tasks, mostly latency-critical ones in our case, on
> that overloaded rq still suffer waiting for each other. I observed this
> through perf sched.
>
> IOW the main difference from the POV of load_balance() between the
> latency-critical tasks and the idle ones is load.
>
> The sched-idle balancer is triggered on the sched-idle rqs periodically
> and the newly-idle ones. It does a 'fast' pull of non-idle tasks from
> the overloaded rqs to the sched-idle/idle ones to let the non-idle tasks
> make full use of cpu resources.
>
> The sched-idle balancer only focuses on non-idle tasks' performance, so
> it can introduce overall load imbalance, and that's why I put it before
> load_balance().

According to the very low weight of a sched-idle task, I don't expect
much imbalance because of sched-idle tasks. But this also depends of
the number of sched-idle task.


>
> Best Regards,
>         Abel