linux-kernel - [PATCH 0/9] sched: balance callbacks

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Message-Id: <20150603132903.203333087@infradead.org>
Date:	Wed, 03 Jun 2015 15:29:03 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	umgwanakikbuti@...il.com, mingo@...e.hu
Cc:	ktkhai@...allels.com, rostedt@...dmis.org, tglx@...utronix.de,
	juri.lelli@...il.com, pang.xunlei@...aro.org, oleg@...hat.com,
	wanpeng.li@...ux.intel.com, linux-kernel@...r.kernel.org,
	peterz@...radead.org
Subject: [PATCH 0/9] sched: balance callbacks

Mike stumbled over a cute bug where the RT/DL balancing ops caused a bug.

The exact scenario is __sched_setscheduler() changing a (runnable) task from
FIFO to OTHER. In swiched_from_rt(), where we do pull_rt_task() we temporarity
drop rq->lock. This gap allows regular cfs load-balancing to step in and
migrate our task.

However, check_class_changed() will happily continue with switched_to_fair()
which assumes our task is still on the old rq and makes the kernel go boom.

Instead of trying to patch this up and make things complicated; simply disallow
these methods to drop rq->lock and extend the current post_schedule stuff into
a balancing callback list, and use that.

This survives Mike's testcase.

Changes since -v1:
 - make SMP=n build,
 - cured switched_from_dl()'s cancel_dl_timer().

no real tests on the new parts other than booting.

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