Re: [PATCH 4/5] sched/fair: Remove double_lock_balance() from active_load_balance_cpu_stop()

[Kirill Tkhai <ktkhai@parallels.com>]

В Чт, 24/07/2014 в 17:04 -0700, Tim Chen пишет:
> On Tue, 2014-07-22 at 15:30 +0400, Kirill Tkhai wrote:
> > Bad situation:
> > 
> > double_lock_balance() drops busiest_rq lock. The busiest_rq is *busiest*,
> > and a lot of tasks and context switches there. We are dropping the lock
> > and waiting for it again.
> > 
> > Let's just detach the task and once finally unlock it!
> > 
> > Warning: this admits unlocked using of can_migrate_task(), throttled_lb_pair(),
> > and task_hot(). I added comments about that.
> > 
> 
> Wonder if we should also consider removing double_lock_balance usage
> from rt.c and deadline.c? Then those two schedulers will also not
> lock both the source and destination queues at the same time
> for load balancing.

rt.c and deadline.c are similar, so we are able to discuss about one of them.

There are two places with double_lock_balance() in rt.c:

1)push_rt_task()->find_lock_lowest_rq()

We can't detach a task before we are locked lowest_rq. It's unknown whether
it will still be suitable to be attached to lowest_rq after we are locked it,
because the highest prioriry of lowest_rq may change. We have the race there.

2)pull_rt_task()

The same with here. The situation may change. We must keep both locks locked
to be sure the priorities won't change. For example, somebody may wake a high
priority task on src_rq, or somebody can pull a task there.

RT balancing is stricter than fair's..

Regards,
	Kirill