linux-kernel - Re: [tip:sched/core] sched: Lower chances of cputime scaling overflow

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Date:	Sat, 13 Apr 2013 16:49:35 +0200
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched: Lower chances of cputime scaling overflow

On Fri, Apr 12, 2013 at 09:55:56AM +0200, Peter Zijlstra wrote:
> > The above is totally untested, but each step is pretty damn simple and
> > fairly cheap. Sure, it's a loop, but it's bounded to 32 (cheap)
> > iterations, and the normal case is that it's not done at all, or done
> > only a few times.
> 
> Right it gets gradually heavier the bigger the numbers get; which is
> more and more unlikely.
> 
> > And the advantage is that the end result is always that simple
> > 32x32/32 case that we started out with as the common case.
> > 
> > I dunno. Maybe I'm overlooking something, and the above is horrible,
> > but the above seems reasonably efficient if not optimal, and
> > *understandable*.
> 
> I suppose that entirely matters on what one is used to ;-) I had to
> stare rather hard at it for a little while.
> 
> But yes, you take it one step further and are willing to ditch rtime
> bits too and I suppose that's fine.
> 
> Should work,.. Stanislaw could you stick this into your userspace
> thingy and verify the numbers are sane enough? 

It works fine - gives relative error less than 0.1% for very big
numbers.

For the record I'm attaching test program and script.

Thanks
Stanislaw


View attachment "scale_stime5.c" of type "text/plain" (1587 bytes)

View attachment "scale_stime_test5.py" of type "text/plain" (1000 bytes)