Re: [RFC][PATCH 17/18] sched: Move the second half of ttwu() tothe remote cpu

[Oleg Nesterov]

On 01/18, Peter Zijlstra wrote:
>
> On Fri, 2011-01-07 at 16:22 +0100, Oleg Nesterov wrote:
>
> > Why sched_fork() does set_task_cpu() ? Just curious, it seems
> > that wake_up_new_task() does all we need.
>
> The only reason I can come up with is to properly initialize the
> data-structures before make the thing visible, by the time
> wake_up_new_task() comes along, its already fully visible.

Ah, thanks, this makes sense.

> > Doesn't __migrate_task() need pi_lock? Consider:
> >
> > ...
>
> Drad, yes I think you're right, now you've got me worried about the
> other migration paths too.. however did you come up with that
> scenario? :-)

I believe this is the only problem with migration... I mean, I failed
to find something else ;)

> A simple fix would be to keep ->pi_lock locked over the call to
> stop_one_cpu() from set_cpus_allowed_ptr().

I don't think this can work. Suppose that the target CPU spins waiting
for this ->pi_lock.

> I think the sched_fair.c load-balance code paths are ok because we only
> find a task to migrate after we've obtained both runqueue locks, so even
> if we migrate current, it cannot schedule (step 3).
>
> I'm not at all sure about the sched_rt load-balance paths, will need to
> twist my head around that..

I _think_ that sched_fair/rt are fine. At least, at first glance.

The new rules are simple, afaics. set_task_cpu/etc is protected by rq->lock
if the task was not deactivated, otherwise you need task->pi_lock. That is
why I think the things like pull_task() are fine, they are working with
the active tasks and thus they only need the src_rq->lock.

And that is why set_cpus_allowed_ptr()->__migrate_task() "call" looks wrong
in general. The first check in need_migrate_task() is fine, but then we drop
rq->lock. By the time __migrate_task() takes this lock again we can't trust
it, and thus we can't trust "if (p->on_rq)".

Oleg.

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