[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <m2v8s7nzr6.fsf@gmail.com>
Date: Sat, 09 Jul 2022 03:55:40 +0800
From: Schspa Shi <schspa@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
vincent.guittot@...aro.org, dietmar.eggemann@....com,
bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
vschneid@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] sched/rt: fix bad task migration for rt tasks
Steven Rostedt <rostedt@...dmis.org> writes:
> On Sat, 09 Jul 2022 03:14:44 +0800
> Schspa Shi <schspa@...il.com> wrote:
>
>> Steven Rostedt <rostedt@...dmis.org> writes:
>>
>> > On Sat, 09 Jul 2022 02:19:42 +0800
>> > Schspa Shi <schspa@...il.com> wrote:
>> >
>> >> Yes, it's what I did in the V1 patch.
>> >> Link: https://lore.kernel.org/all/20220623182932.58589-1-schspa@gmail.com/
>> >>
>> >> But I think it's not the best solution for this problem.
>> >> In these scenarios, we still have a chance to make the task run faster
>> >> by retrying to retry to push the currently running task on this CPU away.
>> >>
>> >> There is more details on V2 patch's replay message.
>> >> Link: https://lore.kernel.org/all/CAMA88TrZ-o4W81Yfw9Wcs3ghoxwpeAKtFejtMTt78GNB0tKaSA@mail.gmail.com/#t
>> >
>> > The thing is, this situation can only happen if we release the rq lock in
>> > find_lock_lowest_rq(), and we should not be checking for it in the other
>> > cases.
>> >
>>
>> If we haven't unlock the rq in find_lock_lowest_rq(), it will return
>> NULL. It won't call this code added.
>>
>> if (unlikely(is_migration_disabled(next_task))) {
>> put_task_struct(next_task);
>> goto retry;
>> }
>
> Because it doesn't need to. If it did not unlock the run queue, there's no
> way that next_task could have run, because we hold the rq lock for
> next_task. Which means that its "migrate_disable" state would not have
> changed from the first time we checked.
>
OK, I get it.
>>
>> deactivate_task(rq, next_task, 0);
>> set_task_cpu(next_task, lowest_rq->cpu);
>>
>> Beside, find_lock_lowest_rq() return NULL doesn't means rq is rleased,
>> We need to add a _find_lock_lowest_rq to get the correct rq released
>> flags?
>
> It it returns NULL it either means that the rq lock was released or that it
> did not find a rq to push to. Which means there's nothing more to do anyway.
>
>>
>> > Perhaps add the check in find_lock_lowest_rq() and also in the !lowest_rq
>> > case do:
>> >
>> > task = pick_next_pushable_task(rq);
>> > if (task == next_task) {
>> > + /*
>> > + * If next task has now disabled migrating, see if we
>> > + * can push the current task.
>> > + */
>> > + if (unlikely(is_migrate_disabled(task)))
>> > + goto retry;
>>
>> Ahh, It can be added, And do we need this to be a separate PATCH?
>
> Sure.
>
> The "fix" to the crash you see should be in the find_lock_lowest_rq() as I
> suggested. And then you can add this as an optimization.
OK, I will make a V4 patch for this, Please review it then.
>
> -- Steve
>
>>
>> > /*
>> > * The task hasn't migrated, and is still the next
>> > * eligible task, but we failed to find a run-queue
>> > * to push it to. Do not retry in this case, since
>> > * other CPUs will pull from us when ready.
>> > */
>> > goto out;
>> > }
>> >
>> > -- Steve
>>
--
BRs
Schspa Shi
Powered by blists - more mailing lists