linux-kernel - Re: [External] Re: [PATCH] sched/fair: favor non-idle group in tick preemption

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Date:   Fri, 4 Nov 2022 11:49:16 +0800
From:   Hao Jia <jiahao.os@...edance.com>
To:     Josh Don <joshdon@...gle.com>, Chuyi Zhou <zhouchuyi@...edance.com>
Cc:     peterz@...radead.org, juri.lelli@...hat.com, mingo@...hat.com,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        Abel Wu <wuyun.abel@...edance.com>
Subject: Re: [External] Re: [PATCH] sched/fair: favor non-idle group in tick
 preemption



On 2022/11/2 Josh Don wrote:
>>> Some weirdness about this change though, is that if there is a
>>> non-idle current entity, and the two next entities on the cfs_rq are
>>> idle and non-idle respectively, we'll now take longer to preempt the
>>> on-cpu non-idle entity, because the non-idle entity on the cfs_rq is
>>> 'hidden' by the idle 'first' entity. Wakeup preemption is different
>>> because we're always directly comparing the current entity with the
>>> newly woken entity.
>>>
>> You are right, this can happen with high probability.
>> This patch just compared the curr with the first entity in
>> the tick, and it seems hard to consider all the other entity
>> in cfs_rq.
>>
>> So, what specific negative effects this situation would cause?
>> For example, the "hidden" non-idle entity's latency will be worse
>> than before?
> 
> As Abel points out in his email, it can push out the time it'll take
> to switch to the other non-idle entity. The change might boost some
> benchmarks numbers, but I don't think it is conclusive enough to say
> it is a generically beneficial improvement that should be integrated.
> 
> By the way, I'm curious if you modified any of the sched_idle_cpu()
> and related load balancing around idle entities given that you've made
> it so that idle entities can have arbitrary weight (since, as I
> described in my prior email, this can otherwise cause issues there).

If we want to make it easier for non-idle tasks to preempt idle tasks in 
tick, maybe we can consider lowering sysctl_sched_idle_min_granularity. 
Of course this may not ensure that non-idle tasks successfully preempt 
idle tasks every time, but it seems to be more beneficial for non-idle 
tasks.

IMHO, even if it is allowed to increase the weight of non-idle, it seems 
that we can make it easier for non-idle tasks to preempt idle tasks by 
lowering sysctl_sched_idle_min_granularity.

Thanks,
Hao