linux-kernel - Re: [RFC PATCH 0/5] introduce sched-idle balancing

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Date:   Fri, 25 Feb 2022 14:46:15 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/5] introduce sched-idle balancing

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
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
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().

Best Regards,
	Abel