lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  linux-cve-announce  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <CAKfTPtAHTTt7mJeBBU4wxrAWrzngKTfp9yXBiwhncaHKXA1yrw@mail.gmail.com>
Date: Thu, 16 Jan 2025 12:26:34 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Hao Jia <jiahao.kernel@...il.com>
Cc: mingo@...hat.com, peterz@...radead.org, mingo@...nel.org, 
	juri.lelli@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	linux-kernel@...r.kernel.org, Hao Jia <jiahao1@...iang.com>
Subject: Re: [PATCH v2] sched/core: Prioritize migrating eligible tasks in sched_balance_rq()

On Wed, 15 Jan 2025 at 12:55, Hao Jia <jiahao.kernel@...il.com> wrote:
>
>
>
> On 2025/1/15 17:28, Vincent Guittot wrote:
> > On Wed, 15 Jan 2025 at 09:55, Hao Jia <jiahao.kernel@...il.com> wrote:
> >>
> >>
> >>
> >> On 2025/1/14 16:07, Vincent Guittot wrote:
> >>> On Tue, 14 Jan 2025 at 04:18, Hao Jia <jiahao.kernel@...il.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 2025/1/14 00:40, Vincent Guittot wrote:
> >>>>> On Mon, 13 Jan 2025 at 10:21, Hao Jia <jiahao.kernel@...il.com> wrote:
> >>>>>>
> >>>>>> Friendly ping...
> >>>>>>
> >>>>>>
> >>>>>> On 2024/12/23 17:14, Hao Jia wrote:
> >>>>>>> From: Hao Jia <jiahao1@...iang.com>
> >>>>>>>
> >>>>>>> When the PLACE_LAG scheduling feature is enabled and
> >>>>>>> dst_cfs_rq->nr_queued is greater than 1, if a task is
> >>>>>>> ineligible (lag < 0) on the source cpu runqueue, it will
> >>>>>>> also be ineligible when it is migrated to the destination
> >>>>>>> cpu runqueue. Because we will keep the original equivalent
> >>>>>>> lag of the task in place_entity(). So if the task was
> >>>>>>> ineligible before, it will still be ineligible after
> >>>>>>> migration.
> >>>>>>>
> >>>>>>> So in sched_balance_rq(), we prioritize migrating eligible
> >>>>>>> tasks, and we soft-limit ineligible tasks, allowing them
> >>>>>>> to migrate only when nr_balance_failed is non-zero to
> >>>>>>> avoid load-balancing trying very hard to balance the load.
> >>>>>
> >>>>> Could you explain why you think it's better to balance eligible tasks
> >>>>> in priority and potentially skip a load balance ?
> >>>>
> >>>> In place_entity(), we maintain the task's original equivalent lag, even
> >>>> if we migrate the task to dst_rq, this does not change its eligibility
> >>>> attribute.
> >>>
> >>> Yes, but you don't answer the question why it's better to select an
> >>> eligible task vs a non eligible task.
> >>>
> >>>>
> >>>> When there are multiple tasks on src_rq, and the dst_cpu has some
> >>>> runnable tasks, migrating ineligible tasks to dst_rq will not allow them
> >>>> to run. Therefore, such task migration is inefficient. We should
> >>>
> >>> Why is it inefficient ? load balance is about evenly balancing the
> >>> number of tasks or the load between CPUs, it never says that the newly
> >>> migrated task should run immediately
> >>
> >>
> >> My initial thought is that when we need to migrate some tasks during
> >> load balancing, at the current point in time, migrating ineligible tasks
> >> to dst_cpu means they definitely cannot run there. Therefore, I prefer
> >> to keep them on src_cpu to reduce the overhead of dequeueing and
> >> enqueueing ineligible tasks.
> >
> > Sorry but I still don't get why it's important and would make a
> > difference. They are all runnable but ineligible tasks got more
> > runtime than other at that point in time so there is no real
> > difference
>
>
> I adopt a lazy strategy for ineligible tasks. At the current point in
> time, even if we migrate ineligible tasks to the dst CPU, they still
> have to wait on the dst CPU until they become eligible. We do not see
> clear benefits from migrating ineligible tasks, but their dequeueing and
> enqueueing would instead incur overhead.

But your explanation doesn't make sense.
Not migrating an ineligible task only make sense for delayed_dequeue
tasks because they don't really want to run but only exhaust their lag
but this is already taken into account by
61b82dfb6b7e ("sched/fair: Do not try to migrate delayed dequeue task")

Did you run your benchmark on top of this change ?

>
> Let them wait on the src CPU until they become eligible before migrating
> them. this can reduce the number of task migrations.
>
>
>
> Thanks,
> Hao
>
>
> >
> >>
> >> Migrating eligible tasks to dst_cpu does not guarantee that they will
> >> run earlier than on src_cpu. it depends on too many factors.
> >>
> >>
> >>
> >>>
> >>>> prioritize migrating tasks that can run on dst_rq.
> >>>>
> >>>> In other words, migrating ineligible tasks is merely moving them to
> >>>> another runqueue to wait until they become eligible.
> >>>
> >>> But I don't get why it's a problem. Migrating an eligible task might
> >>> delay its scheduling because of its deadline vs other tasks already
> >>> eligible on the dst_rq. Eligible and non eligible tasks are all
> >>> runnable, it's just how much they have already run. In addition,
> >>> migrating an eligible task will clear its positive vlag with
> >>> DELAY_ZERO which is unfair IMO
> >>
> >>
> >> Sorry, I'd like to ask you a question that confuses me: Why does
> >> migrating eligible task will clear the positive vlag?
> >
> > sorry I mess up everything that only for delayed dequeue task
> >
> >>
> >> In detach_task(), the ENQUEUE_DELAYED and DEQUEUE_SLEEP flags are not
> >> set, and in dequeue_entity(), eligible tasks will not set sched_delayed,
> >> so they will be dequeued normally with se->on_rq being 0.
> >>
> >> Similarly, attach_task() does not set the ENQUEUE_DELAYED and
> >> DEQUEUE_SLEEP flags, and since se->on_rq is 0, it will not call
> >> requeue_delayed_entity().
> >>
> >>
> >> Thanks,
> >> Hao
>

Powered by blists - more mailing lists

Powered by Openwall GNU/*/Linux Powered by OpenVZ