linux-kernel - Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new forked task

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Message-ID: <20130506152850.GA17623@dyad.programming.kicks-ass.net>
Date:	Mon, 6 May 2013 17:28:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alex Shi <alex.shi@...el.com>
Cc:	Paul Turner <pjt@...gle.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>
Subject: Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new
 forked task

On Mon, May 06, 2013 at 11:26:06PM +0800, Alex Shi wrote:
> On 05/06/2013 06:22 PM, Paul Turner wrote:
> >>> >> This is missing a scale_load() right?  Further: Why not put this in
> >>> >> __sched_fork?
> >> >
> >> > scale_load is not working now. Anyway I can add this.
> > I believe someone tracked down a plausible cause for this:
> > A governor was examining the values and making a mess with the scaled
> > ones.  I'm sorry, I don't have the post off hand.
> > 
> > You actually likely ideally want this _on_ for these patches; the
> > available resolution with SCHED_LOAD_SHIFT=10 disappears really
> > quickly and scaling by runnable_avg only further accelerates that.
> 
> Sorry for can not follow you. Do you mean the scaling by runnable_avg is
> better than scale_load?
> 
> In fact, after think twice of scale_load, guess better to figure out a
> good usage for it before enabling blindly.

He's saying its way too easy to run out of fractional bits with the current
setup; adding the 10 extra fractional bits provided by SCHED_LOAD_SHIFT is
advantagous and we should re-enable that.
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