linux-kernel - Re: [RFC patch 1/2] sched: dynamically adapt granularity with nr

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Date:	Mon, 13 Sep 2010 10:43:10 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	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>,
	Tony Lindgren <tony@...mide.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [RFC patch 1/2] sched: dynamically adapt granularity with
 nr_running

On Mon, 2010-09-13 at 16:16 +0200, Peter Zijlstra wrote:

> > > -     if (delta_exec < sysctl_sched_min_granularity)
> > > -             return;
> > 
> > Well, the reason why this test is here seems to be that we don't want to trigger
> > "resched_task" more often than needed, and here it's defined by the granularity.
> 
> Right, but its wrong for the weighted case. Letting a light task run
> that long will make its latency suck.
> 
> > I don't quite see with what you are replacing this, other than "let's set the
> > resched flag all the time to save a 32-bit division". I figure out it's more
> > expensive the call the scheduler than to do a 32-bit div.
> 
> The more divs we put it, the more expensive it all becomes.
> 

What about:

	if (delta_exec < sysctl_sched_min_granularity ||
	    delta_exec < __sched_gran(cfs_rq->nr_running))

This way we avoid the div when delta_exec is less than the
min_granularity, and then we can do the div to perhaps avoid a needless
resched?

-- Steve


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