Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754756Ab0DBUuP (ORCPT ); Fri, 2 Apr 2010 16:50:15 -0400 Received: from mail-yw0-f172.google.com ([209.85.211.172]:60072 "EHLO mail-yw0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753798Ab0DBUuM convert rfc822-to-8bit (ORCPT ); Fri, 2 Apr 2010 16:50:12 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=Unnc6kg6RtFFuFYOD7wV5fW7B/S0hIWJPSB6eR8Y05WxRzMNx81zljvpbB0hdFLppM GdbLxvL6BZ7I3+etgna7Tg4FgasBGeqOMOeN+mOttw7Eq/Rc4/RSa8R1gevYy2CvNPVO S7N2BWhTkVOLCuDYz9b5K2CLQ2EEftzdB/4No= MIME-Version: 1.0 In-Reply-To: <20100402201648.GA15498@elte.hu> References: <1269858302.12097.272.camel@laptop> <20100402201648.GA15498@elte.hu> Date: Fri, 2 Apr 2010 16:50:10 -0400 Message-ID: Subject: Re: Discrepancy between comments for sched_find_first_bit From: Matt Turner To: Ingo Molnar Cc: Peter Zijlstra , LKML , linux-alpha@vger.kernel.org Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2343 Lines: 63 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? Thanks guys, Matt -- 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/