Re: [PATCH RFC] sched/rt: preserve global runtime/period ratio indo_balance_runtime()

From: Peter Zijlstra
Date: Wed May 22 2013 - 04:16:18 EST

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@xxxxxxxxxx>

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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