linux-kernel - Re: [PATCH 1/2] sched: allow SCHED_BATCH to preempt SCHED

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Date:	Wed, 23 Feb 2011 06:31:58 +0100
From:	Mike Galbraith <efault@....de>
To:	Darren Hart <dvhart@...ux.intel.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, richard.purdie@...uxfoundation.org
Subject: Re: [PATCH 1/2] sched: allow SCHED_BATCH to preempt SCHED_IDLE
 tasks

On Wed, 2011-02-23 at 05:20 +0100, Mike Galbraith wrote:
> On Tue, 2011-02-22 at 13:04 -0800, Darren Hart wrote:
> > Perform the test for SCHED_IDLE before testing for SCHED_BATCH (and ensure idle
> > tasks don't preempt idle tasks) so the non-interactive, but still important,
> > SCHED_BATCH tasks will run in favor of the very low priority SCHED_IDLE tasks.
> 
> Yeah, that could be construed as a fairness fix for light SCHED_BATCH vs
> a heavy SCHED_IDLE.  It should lower latencies for both when mixed.

Hm.  Seems SCHED_IDLE _always_ being preempted is a potential terminal
starvation bug though, unless preempt_tick() checks spread to guarantee
that the preempted task will eventually get the CPU back, even in the
face of massive non-idle wakeup driven load.. which it does not.  (idle
task holds resource?)

Maybe my imagination has had too much java though.

	-Mike

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