linux-kernel - Bug 71331 - mlock yields processor to lower priority process

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Message-ID: <20140321200248.GB6264@owamsq.epam.com>
Date:	Fri, 21 Mar 2014 23:02:49 +0300
From:	Artem Fetishev <artem_fetishev@...m.com>
To:	<peterz@...radead.org>, <jmdavis@...k.com>,
	<kosaki.motohiro@...fujitsu.com>
CC:	<linux-kernel@...r.kernel.org>
Subject: Bug 71331 - mlock yields processor to lower priority process

Hi all,

I am looking at a use-case when a real-time task (B) of higher priority is sometimes preempted by another real-time task (A) of lower priority. Well, B is not really preempted. It calls mlockall() which forces task B to yield the CPU. Under certain conditions, mlockall() calls lru_add_drain_all() which schedules a deferred work and wants the calling task to wait until that work is complete by putting the task into TASK_UNINTERRUPTIBLE state and calling schedule_timeout().

Tasks utilize SCHED_FIFO policy.

See details here: https://bugzilla.kernel.org/show_bug.cgi?id=71331

Besides mlockall, there are other kernel paths which make use of lru_add_drain_all() and schedule_timeout(), so I guess there are bunch of other syscalls which may lead to the above use-case.

So the question is: is above use-case an expected behavior of real-time tasks or is it a bug in mlockall (i.e. it should not interrupt a real-time process)?

Thanks,
-Artem
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