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

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Date:   Fri, 8 Jul 2022 15:06:14 -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 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.

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;
			/*
			 * 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