linux-kernel - Re: [PATCH RFC] sched/rt: preserve global runtime/period ratio in do_balance

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Message-ID: <20130522081601.GP26912@twins.programming.kicks-ass.net>
Date:	Wed, 22 May 2013 10:16:01 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Peter Boonstoppel <pboonstoppel@...dia.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Walmsley <pwalmsley@...dia.com>
Subject: Re: [PATCH RFC] sched/rt: preserve global runtime/period ratio in
 do_balance_runtime()

On Tue, May 21, 2013 at 02:30:09PM -0700, Peter Boonstoppel wrote:
> RT throttling aims to prevent starvation of non-SCHED_FIFO threads
> when a rogue RT thread is hogging the CPU. It does so by piggybacking
> on the rt_bandwidth system and allocating at most rt_runtime per
> rt_period to SCHED_FIFO tasks (e.g. 950ms out of every second,
> allowing 'regular' tasks to run for at least 50ms every second).
> 
> However, when multiple cores are available, rt_bandwidth allows cores
> to borrow rt_runtime from one another. This means that a core with a
> rogue RT thread, consuming 100% CPU cycles, can borrow enough runtime
> from other cores to allow the RT thread to run continuously, with no
> runtime for regular tasks on this core.
> 
> Although regular tasks can get scheduled on other available cores
> (which are guaranteed to have some non-RT runtime avaible, since they
> just lent some RT time to us), tasks that are specifically affined to
> a particular core may not be able to make progress (e.g. workqueues,
> timer functions). This can break e.g. watchdog-like functionality that
> is supposed to kill the rogue RT thread.
> 
> This patch changes do_balance_runtime() in such a way that no core can
> aquire (borrow) more runtime than the globally set rt_runtime /
> rt_period ratio. This guarantees there will always be some non-RT
> runtime available on every individual core.
> 
> Signed-off-by: Peter Boonstoppel <pboonstoppel@...dia.com>

I thought we already had a knob for that; RT_RUNTIME_SHARE. And as Mike
said, we might consider flipping the default on that.
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