Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754903Ab0DBVZ2 (ORCPT ); Fri, 2 Apr 2010 17:25:28 -0400 Received: from mx3.mail.elte.hu ([157.181.1.138]:50598 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754551Ab0DBVZV (ORCPT ); Fri, 2 Apr 2010 17:25:21 -0400 Date: Fri, 2 Apr 2010 23:25:15 +0200 From: Ingo Molnar To: Matt Turner Cc: Peter Zijlstra , LKML , linux-alpha@vger.kernel.org Subject: Re: Discrepancy between comments for sched_find_first_bit Message-ID: <20100402212515.GC17041@elte.hu> References: <1269858302.12097.272.camel@laptop> <20100402201648.GA15498@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.20 (2009-08-17) X-ELTE-SpamScore: -2.0 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-2.0 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.5 -2.0 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2648 Lines: 70 * Matt Turner wrote: > On Fri, Apr 2, 2010 at 4:16 PM, Ingo Molnar wrote: > > > > * Peter Zijlstra wrote: > > > >> On Sun, 2010-03-28 at 23:37 -0400, Matt Turner wrote: > >> > include/asm-generic/bitops/sched.h says > >> > /* > >> > ?* Every architecture must define this function. It's the fastest > >> > ?* way of searching a 100-bit bitmap. ?It's guaranteed that at least > >> > ?* one of the 100 bits is cleared. > >> > ?*/ > >> > > >> > arch/alpha/include/asm/bitops.h says > >> > /* > >> > ?* Every architecture must define this function. It's the fastest > >> > ?* way of searching a 140-bit bitmap where the first 100 bits are > >> > ?* unlikely to be set. It's guaranteed that at least one of the 140 > >> > ?* bits is set. > >> > ?*/ > >> > > >> > Is the guarantee that one of the first 100-bits set (and that the last > >> > 40 are useless?), or 140-bits? If it's just the first 100 bits we care > >> > about, then the alpha version needs to be fixed. > >> > > >> > I'm just checking this out, because gcc produces horrendous code for > >> > sched_find_first_bit on alpha. I rewrote it in assembly and it's > >> > better than 4 times faster. > >> > > >> > Also, is it even worth optimizing that function? It looks like it's > >> > only used in kernel/sched_rt.c. > >> > >> (might help if you CC the scheduler people on scheduler functions :-) > >> > >> Right, so it used to be 140 bits with the old O(1) scheduler, currently > >> (as you noted) sched_rt is the only user left and will therefore only > >> care about the first 100 bits. > >> > >> As it stands I think it might still make sense to optimize this as for > >> rt loads it still on a hot path. > >> > >> As to the 100 vs 140 length, would it really make much of difference to > >> shorten the implementation to 100? If not I'd leave it at 140. > >> > >> Ingo, any comments? > > > > I guess getting below the 128 bits boundary would shave an instruction and a > > branch off or so? > > > > ? ? ? ?Ingo > > > > That's right. I should be able to get rid of a cmov, which kind of > counts as two instructions in EV6 scheduling. > > So I should send a patch to reduce this to the first 100 (128) bits? Sure, why not, every instruction counts :-) Note, if you do it then please also include a disassembly of the area that changed, so that we document the effect. Ingo -- 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/