From: Vincent Guittot <vincent.guittot@linaro.org>
To: Abel Wu <wuyun.abel@bytedance.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ben Segall <bsegall@google.com>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>, Mel Gorman <mgorman@suse.de>,
Steven Rostedt <rostedt@goodmis.org>,
linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 0/5] introduce sched-idle balancing
Date: Fri, 25 Feb 2022 14:15:04 +0100 [thread overview]
Message-ID: <CAKfTPtB9Dhg+4sQnFBu3qXiV3vwnfAjf-R2_4qvKXGAGS1pW-Q@mail.gmail.com> (raw)
In-Reply-To: <d4557587-b52c-049d-a0c8-e48aaa8a1c1e@bytedance.com>
On Fri, 25 Feb 2022 at 11:46, Abel Wu <wuyun.abel@bytedance.com> wrote:
>
> On 2/25/22 4:29 PM, Vincent Guittot Wrote:
> > On Fri, 25 Feb 2022 at 07:46, Abel Wu <wuyun.abel@bytedance.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.
I was referring to your sentence " The two balancers are triggered on
the rqs that have no tasks on them". When there is only sched-idle
tasks on a rq, the load_balance behave like the Idle Load Balance when
there is no task i.e. as often
>
> >
> >> 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
but we could easily change this like if !sched_idle_cpus then LB can
migrate only cfs tasks otherwise can migrate sched_idle task as well.
Instead of creating another side channel
>
> 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
next prev parent reply other threads:[~2022-02-25 13:15 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-17 15:43 [RFC PATCH 0/5] introduce sched-idle balancing Abel Wu
2022-02-17 15:43 ` [RFC PATCH 1/5] sched/fair: record overloaded cpus Abel Wu
2022-02-24 7:10 ` Gautham R. Shenoy
2022-02-24 14:36 ` Abel Wu
2022-02-27 8:08 ` Aubrey Li
2022-02-17 15:43 ` [RFC PATCH 2/5] sched/fair: introduce sched-idle balance Abel Wu
2022-02-17 15:43 ` [RFC PATCH 3/5] sched/fair: add stats for sched-idle balancing Abel Wu
2022-02-17 15:44 ` [RFC PATCH 4/5] sched/fair: filter out overloaded cpus in sis Abel Wu
2022-02-17 15:44 ` [RFC PATCH 5/5] sched/fair: favor cpu capacity for idle tasks Abel Wu
2022-02-24 3:19 ` [RFC PATCH 0/5] introduce sched-idle balancing Abel Wu
2022-02-24 15:20 ` Peter Zijlstra
2022-02-24 15:29 ` Vincent Guittot
2022-02-25 6:51 ` Abel Wu
2022-02-25 6:46 ` Abel Wu
2022-02-25 8:29 ` Vincent Guittot
2022-02-25 10:46 ` Abel Wu
2022-02-25 13:15 ` Vincent Guittot [this message]
2022-02-24 16:47 ` Mel Gorman
2022-02-25 8:15 ` Abel Wu
2022-02-25 10:16 ` Mel Gorman
2022-02-25 13:20 ` Abel Wu
2022-03-02 0:41 ` Josh Don
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