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Subject: Re: [RFC patch 1/2] sched: dynamically adapt granularity with
 nr_running
From: Steven Rostedt <rostedt@goodmis.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
        Thomas Gleixner <tglx@linutronix.de>, Tony Lindgren <tony@atomide.com>,
        Mike Galbraith <efault@gmx.de>
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Date: Mon, 13 Sep 2010 10:43:10 -0400
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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