linux-kernel - Re: [PATCH v3] sched/rt: fix bad task migration for rt tasks

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Date:   Fri, 8 Jul 2022 15:41:11 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Schspa Shi <schspa@...il.com>
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

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.

> 
> 	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.

-- 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  
>