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

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Date:   Fri, 25 Feb 2022 18:46:26 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        linux-kernel@...r.kernel.org, Abel Wu <wuyun.abel@...edance.com>
Subject: Re: [RFC PATCH 0/5] introduce sched-idle balancing

On 2/25/22 4:29 PM, Vincent Guittot Wrote:
> On Fri, 25 Feb 2022 at 07:46, Abel Wu <wuyun.abel@...edance.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

I'm afraid I don't quite follow you, since sched-idle balancer doesn't
touch the ILB part, can you elaborate on this? Thanks.

> 
>> 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

This feature might not that friendly to the situation that only
sched-idle tasks are running in the system. And this situation
can last more than half a day in our co-location systems in which
the training/batch tasks are placed under idle groups or directly
assigned to SCHED_IDLE.

> 
>> 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