Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161644Ab3DKSIH (ORCPT ); Thu, 11 Apr 2013 14:08:07 -0400 Received: from merlin.infradead.org ([205.233.59.134]:36426 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1161570Ab3DKSIE (ORCPT ); Thu, 11 Apr 2013 14:08:04 -0400 Message-ID: <1365703670.10217.10.camel@laptop> Subject: Re: [tip:sched/core] sched: Lower chances of cputime scaling overflow From: Peter Zijlstra To: Linus Torvalds Cc: Stanislaw Gruszka , Linux Kernel Mailing List , Ingo Molnar , "H. Peter Anvin" , =?ISO-8859-1?Q?Fr=E9d=E9ric?= Weisbecker , Steven Rostedt , Andrew Morton , Thomas Gleixner , linux-tip-commits@vger.kernel.org Date: Thu, 11 Apr 2013 20:07:50 +0200 In-Reply-To: References: <20130326140147.GB2029@redhat.com> <1365687946.8824.3.camel@laptop> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.6.2-0ubuntu0.1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1426 Lines: 38 On Thu, 2013-04-11 at 08:38 -0700, Linus Torvalds wrote: > On Thu, Apr 11, 2013 at 6:45 AM, Peter Zijlstra 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. But that would mean fls() and shifting again for nsec based cputime. I'll have a better read and think about the rest of your email but that'll have to be tomorrow :/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/