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

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Message-ID: <20130411182201.GE17129@somewhere.redhat.com>
Date:	Thu, 11 Apr 2013 20:22:03 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>,
	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, Apr 11, 2013 at 08:07:50PM +0200, Peter Zijlstra wrote:
> On Thu, 2013-04-11 at 08:38 -0700, Linus Torvalds wrote:
> > On Thu, Apr 11, 2013 at 6:45 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > > On Tue, 2013-03-26 at 15:01 +0100, Stanislaw Gruszka wrote:
> > >> Thoughts?
> > >
> > > Would something like the below work?
> > 
> > Ugh, this is hard to think about, it's also fairly inefficient.
> > 
> > >  static cputime_t scale_stime(u64 stime, u64 rtime, u64 total)
> > >  {
> > > -       u64 rem, res, scaled;
> > > +       int stime_fls = fls64(stime);
> > > +       int total_fls = fls64(total);
> > > +       int rtime_fls = fls64(rtime);
> > 
> > Doing "fls64()" unconditionally is quite expensive on some
> > architectures, 
> 
> Oh, I (wrongly it appears) assumed that fls was something cheap :/
> 
> > and if I am not mistaken, the *common* case (by far) is
> > that all these values fit in 32 bits, no?
> 
> It depends on if we use cputime_jiffies.h or cputime_nsec.h and I'm
> completely lost as to which we default to atm. But we sure can reduce
> to 32 bits in most cases without too much problems.

We default to the jiffies. The nsecs case is used only for full dynticks
accounting and ia64 precise accounting.
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