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

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Message-ID: <1365753356.17140.18.camel@laptop>
Date:	Fri, 12 Apr 2013 09:55:56 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Stanislaw Gruszka <sgruszka@...hat.com>,
	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 Thu, 2013-04-11 at 08:38 -0700, Linus Torvalds wrote:
> So *now*, once we are in the uncommon case, let's start counting bits.
> Like this:
> 
>     /* We know one of the values has a bit set in the high 32 bits */
>     for (;;) {
>         /* Make sure "stime" is the bigger of stime/rtime */
>         if (rtime > stime) {
>             u64 tmp = stime; stime = rtime; rtime = tmp;
>         }
> 
>         /* Do we need to balance stime/rtime bits? */
>         if (stime >> 32) {
>             if (rtime >> 31)
>                 goto drop_precision;
> 
>             /* We can grow rtime and shrink stime and try to make them
> both fit */
>             rtime <<= 1;
>             stime >>= 1;
>             continue;
>         }
> 
>         /* stime/rtime fits in 32 bits, how about total? */
>         if (!(total >> 32))
>             break;
> 
> drop_precision:
>         /* We drop from stime, it has more bits than rtime */
>         stime >>= 1;
>         total >>= 1;
>     }
> 
> 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? 


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