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

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Date:   Sat, 09 Jul 2022 03:14:44 +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 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;
	}

	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?

> 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?

> 			/*
> 			 * 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