linux-kernel - Re: [PATCH] sched: fair scheduler should not resched rt tasks

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Date:	Sat, 11 Oct 2008 09:55:37 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <clark.williams@...il.com>
Subject: Re: [PATCH] sched: fair scheduler should not resched rt tasks


On Sat, 11 Oct 2008, Peter Zijlstra wrote:

> On Sat, 2008-10-11 at 03:01 -0400, Steven Rostedt wrote:
> > Using ftrace, I noticed latencies in real-time tasks where they were
> > needlessly calling schedule due to sched_fair sending out time slices.
> > 
> > This patch prevents a call to resched_task by the sched fair class if
> > the task it wants to reschedule is an rt task.
> 
> Right, thats not a good thing, however this patch looks wrong, we should
> never call hrtick_start_fair() on a rt task to begin with.

Great! I don't care which way we fix this. This was making noticeable
latencies against rt tasks.

> 
> The way I can see that happening is through enqueue/dequeue_task_fair()
> where we want to re-programm the hrtick because nr_running changes (and
> thus the current tasks desired runtime).

My original patch was more like this one, but I wasn't sure if I was 
skipping any calculations that were assumed to be made. Thus, I wrote the 
bare minimum and sent it out CC'ing the experts. Looks like my plan worked 
;-)

-- Steve


> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>

Acked-by: Steven Rostedt <srostedt@...hat.com>

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