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From:   Josh Don <joshdon@google.com>
Date:   Wed, 13 Apr 2022 17:08:36 -0700
Message-ID: <CABk29Nu9QTUKHP3cyx3qpiGcNM-PbaSxCvfhvBvOs++67KeXCw@mail.gmail.com>
Subject: Re: [RFC v2 2/2] sched/fair: introduce sched-idle balance
To:     Abel Wu <wuyun.abel@bytedance.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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> >>   /*
> >>    * Use locality-friendly rq->overloaded to cache the status of the rq
> >>    * to minimize the heavy cost on LLC shared data.
> >> @@ -7837,6 +7867,22 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
> >>          if (kthread_is_per_cpu(p))
> >>                  return 0;
> >>
> >> +       if (unlikely(task_h_idle(p))) {
> >> +               /*
> >> +                * Disregard hierarchically idle tasks during sched-idle
> >> +                * load balancing.
> >> +                */
> >> +               if (env->idle == CPU_SCHED_IDLE)
> >> +                       return 0;
> >> +       } else if (!static_branch_unlikely(&sched_asym_cpucapacity)) {
> >> +               /*
> >> +                * It's not gonna help if stacking non-idle tasks on one
> >> +                * cpu while leaving some idle.
> >> +                */
> >> +               if (cfs_rq_busy(env->src_rq) && !need_pull_cfs_task(env->dst_rq))
> >> +                       return 0;
> >
> > These checks don't involve the task at all, so this kind of check
> > should be pushed into the more general load balance function. But, I'm
> > not totally clear on the motivation here. If we have cpu A with 1
> > non-idle task and 100 idle tasks, and cpu B with 1 non-idle task, we
> > should definitely try to load balance some of the idle tasks from A to
> > B. idle tasks _do_ get time to run (although little), and this can add
> > up and cause antagonism to the non-idle task if there are a lot of
> > idle threads.
>
> CPU_SCHED_IDLE means triggered by sched_idle_balance() in which pulls
> a non-idle task for the unoccupied cpu from the overloaded ones, so
> idle tasks are not the target and should be skipped.
>
> The second part is: if we have cpu A with 1 non-idle task and 100 idle
> tasks, and B with >=1 non-idle task, we don't migrate the last non-idle
> task on A to B.

It could be possible that we do want to migrate the last non-idle task
from A to B, if the weight sum of idle tasks on A is very high (easily
possible with affinity restrictions). So I think we should leave
regular load balance alone here if it really wants to move the
non-idle task, and wrap this entire block in an if (env->idle ==
CPU_SCHED_IDLE).

Thanks,
Josh