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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Tue, 19 Jan 2021 15:19:30 +0100
Message-ID: <CAKfTPtCRPViF7B1iqeNVDi31X2sgOWKDJSGCBSzbj0c_Zb=mzA@mail.gmail.com>
Subject: Re: [PATCH] sched/eas: Don't update misfit status if the task is pinned
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     Qais Yousef <qais.yousef@arm.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>
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On Tue, 19 Jan 2021 at 14:54, Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
> On 19/01/21 14:34, Vincent Guittot wrote:
> > On Tue, 19 Jan 2021 at 13:08, Qais Yousef <qais.yousef@arm.com> wrote:
> >>
> >> If the task is pinned to a cpu, setting the misfit status means that
> >> we'll unnecessarily continuously attempt to migrate the task but fail.
> >>
> >> This continuous failure will cause the balance_interval to increase to
> >> a high value, and eventually cause unnecessary significant delays in
> >> balancing the system when real imbalance happens.
> >>
> >> Caught while testing uclamp where rt-app calibration loop was pinned to
> >> cpu 0, shortly after which we spawn another task with high util_clamp
> >> value. The task was failing to migrate after over 40ms of runtime due to
> >> balance_interval unnecessary expanded to a very high value from the
> >> calibration loop.
> >>
> >> Not done here, but it could be useful to extend the check for pinning to
> >> verify that the affinity of the task has a cpu that fits. We could end
> >> up in a similar situation otherwise.
> >>
> >> Fixes: 3b1baa6496e6 ("sched/fair: Add 'group_misfit_task' load-balance type")
> >> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> >> ---
> >>  kernel/sched/fair.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index 197a51473e0c..9379a481dd8c 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -4060,7 +4060,7 @@ static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
> >>         if (!static_branch_unlikely(&sched_asym_cpucapacity))
> >>                 return;
> >>
> >> -       if (!p) {
> >> +       if (!p || p->nr_cpus_allowed == 1) {
> >
> > Side question: What happens if there is 2 misfit tasks and the current
> > one is pinned but not the other waiting one
> >
>
> update_misfit_status() is called either on the current task (at tick) or
> on the task picked by pick_next_task_fair() - i.e. CFS current or
> about-to-be-current.
>
> So if you have 2 CPU hogs enqueued on a single LITTLE, and one of them
> is pinned, the other one will be moved away either via regular load

This doesn't seem reliable because it uses load or nr_running

> balance, or via misfit balance sometime after it's picked as the next
> task to run.
>
> Admittedly that second case suffers from unfortunate timing mostly
> related to the load balance interval. There was an old patch in the
> Android stack that would reduce the balance interval upon detecting a

Shouldn't we keep track of enqueue misfit tasks instead ?

> misfit task to "accelerate" its upmigration; this might need to be
> revisited...
>
> >>                 rq->misfit_task_load = 0;
> >>                 return;
> >>         }
> >> --
> >> 2.25.1
> >>