Re: [PATCH v2 2/5] sched: Teach scheduler to understand ONRQ_MIGRATING state

From: Kirill Tkhai
Date: Wed Jul 30 2014 - 04:05:36 EST

Ð ÐÑ, 29/07/2014 Ð 18:19 +0200, Oleg Nesterov ÐÐÑÐÑ:
> On 07/29, Kirill Tkhai wrote:
> >
> > How about this? Everything is inside task_rq_lock() now. The patch
> > became much less.
> And with this change task_migrating() is not possible under
> task_rq_lock() or __task_rq_lock(). This means that 1/5 can be simplified
> too.

It seems to me it won't be useless anyway. In every place we underline
that a task is exactly queued or dequeued, so it's not necessary to remember
whether it is migrating or not. This is a cleanup, though it's big.

> __migrate_swap_task() is probably the notable exception...
> Off-topic, but it takes 2 ->pi_lock's. This means it can deadlock with
> try_to_wake_up_local() (if a 3rd process does ttwu() and waits for
> ->on_cpu == 0). But I guess __migrate_swap_task() should not play with
> PF_WQ_WORKER threads.

Hmm.. I'm surprised, PF_WQ_WORKER threads may be unbound. But it seems
we still can't pass them to try_to_wake_up_local.


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