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SubjectRe: [PATCH v3] sched/rt: fix bad task migration for rt tasks
On Fri,  8 Jul 2022 00:50:14 +0800
Schspa Shi <schspa@gmail.com> wrote:

> Please refer to the following scenarios.

I'm not sure this is what is happening. Do you have a trace to back this up?

>
> CPU0 CPU1
> ------------------------------------------------------------------
> push_rt_task
> check is_migration_disabled(next_task)
> task not running and
> migration_disabled == 0
> find_lock_lowest_rq(next_task, rq);
> _double_lock_balance(this_rq, busiest);
> raw_spin_rq_unlock(this_rq);
> double_rq_lock(this_rq, busiest);
> <<wait for busiest rq>>
> <wakeup>

Here's the problem I have. next_task is queued on CPU0, (otherwise CPU0
would not be pushing it). As CPU0 is currently running push_rt_task, how
did next_task start running to set its migrate_disable flag?

Even if it was woken up on another CPU and ran there, by setting
migrate_disable, it would not be put back to CPU0, because its
migrate_disable flag is set (if it is, then there's the bug).

After releasing the rq lock and retaking it, we check that the next_task is
still the next task on CPU0 to push.


> task become running
> migrate_disable();
> <context out>
> deactivate_task(rq, next_task, 0);
> set_task_cpu(next_task, lowest_rq->cpu);
> WARN_ON_ONCE(is_migration_disabled(p));
> ---------OOPS-------------

I don't see how this can happen.

-- Steve

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