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SubjectRe: [PATCH v3] sched/rt: fix bad task migration for rt tasks
On Sat, 09 Jul 2022 02:19:42 +0800
Schspa Shi <schspa@gmail.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

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