linux-kernel - [RFC][PATCH 0/7] sched: balance callbacks

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Message-Id: <20150601135818.506080835@infradead.org>
Date:	Mon, 01 Jun 2015 15:58:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	umgwanakikbuti@...il.com, mingo@...e.hu
Cc:	ktkhai@...allels.com, rostedt@...dmis.org, juri.lelli@...il.com,
	pang.xunlei@...aro.org, oleg@...hat.com,
	linux-kernel@...r.kernel.org,
	"Peter Zijlstra" <peterz@...radead.org>
Subject: [RFC][PATCH 0/7] sched: balance callbacks

Hi,

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.

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 for well over an hour on my ivb-ep. I've not yet
tested it on anything bigger.

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