Re: sched: hang in migrate_swap

[Sasha Levin]

On 02/24/2014 07:12 AM, Peter Zijlstra wrote:

Anyway, the below seems to work; it avoids playing tricks with the idle
thread and instead uses a magic constant.

The comparison should be faster too; seeing how we avoid dereferencing
p->sched_class.

---
Subject: sched: Guarantee task priority in pick_next_task()
From: Peter Zijlstra<pet...@infradead.org>
Date: Fri Feb 14 12:25:08 CET 2014

Michael spotted that the idle_balance() push down created a task
priority problem.

Previously, when we called idle_balance() before pick_next_task() it
wasn't a problem when -- because of the rq->lock droppage -- an rt/dl
task slipped in.

Similarly for pre_schedule(), rt pre-schedule could have a dl task
slip in.

But by pulling it into the pick_next_task() loop, we'll not try a
higher task priority again.

Cure this by creating a re-start condition in pick_next_task(); and
triggering this from pick_next_task_{rt,fair}().

Fixes: 38033c37faab ("sched: Push down pre_schedule() and idle_balance()")
Cc: Juri Lelli<juri.le...@gmail.com>
Cc: Ingo Molnar<mi...@kernel.org>
Cc: Steven Rostedt<rost...@goodmis.org>
Reported-by: Michael Wang<wang...@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra<pet...@infradead.org>

Sign me up to the fan club of this patch, I love it.




Thanks,
Sasha

After running it for a bit I see that all the hangs I've seen before are gone, even those that I didn't assume were sched related.

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