linux-kernel - Re: [PATCH] sched/rt: fix SCHED

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Date:	Fri, 18 May 2012 10:56:35 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Colin Cross <ccross@...roid.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Hillf Danton <dhillf@...il.com>
Subject: Re: [PATCH] sched/rt: fix SCHED_RR across cgroups

On Wed, 2012-05-16 at 21:34 -0700, Colin Cross wrote:
> task_tick_rt has an optimization to only reschedule SCHED_RR tasks
> if they were the only element on their rq.  However, with cgroups
> a SCHED_RR task could be the only element on its per-cgroup rq but
> still be competing with other SCHED_RR tasks in its parent's
> cgroup.  In this case, the SCHED_RR task in the child cgroup would
> never yield at the end of its timeslice.  If the child cgroup
> rt_runtime_us was the same as the parent cgroup rt_runtime_us,
> the task in the parent cgroup would starve completely.
> 
> Modify task_tick_rt to check that the task is the only task on its
> rq, and that the each of the scheduling entities of its ancestors
> is also the only entity on its rq.
> 
> Signed-off-by: Colin Cross <ccross@...roid.com>

OK, fair enough.. one does wonder though, WTH is android doing with
SCHED_RR?
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