Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755389AbbBBX3M (ORCPT ); Mon, 2 Feb 2015 18:29:12 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:35427 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752461AbbBBX3K (ORCPT ); Mon, 2 Feb 2015 18:29:10 -0500 Date: Mon, 2 Feb 2015 15:29:09 -0800 From: Andrew Morton To: "Wang, Yalin" Cc: "'arnd@arndb.de'" , "'linux-arch@vger.kernel.org'" , "'linux-kernel@vger.kernel.org'" , "'linux@arm.linux.org.uk'" , "'linux-arm-kernel@lists.infradead.org'" Subject: Re: [RFC] change non-atomic bitops method Message-Id: <20150202152909.13bfd11f192fb0268b2ab4bf@linux-foundation.org> In-Reply-To: <35FD53F367049845BC99AC72306C23D1044A02027E0A@CNBJMBX05.corpusers.net> References: <35FD53F367049845BC99AC72306C23D1044A02027E0A@CNBJMBX05.corpusers.net> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1226 Lines: 33 On Mon, 2 Feb 2015 11:55:03 +0800 "Wang, Yalin" wrote: > This patch change non-atomic bitops, > add a if() condition to test it, before set/clear the bit. > so that we don't need dirty the cache line, if this bit > have been set or clear. On SMP system, dirty cache line will > need invalidate other processors cache line, this will have > some impact on SMP systems. > > --- a/include/asm-generic/bitops/non-atomic.h > +++ b/include/asm-generic/bitops/non-atomic.h > @@ -17,7 +17,9 @@ static inline void __set_bit(int nr, volatile unsigned long *addr) > unsigned long mask = BIT_MASK(nr); > unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr); > > - *p |= mask; > + if ((*p & mask) == 0) > + *p |= mask; > + > } hm, maybe. It will speed up set_bit on an already-set bit. But it will slow down set_bit on a not-set bit. And the latter case is presumably much, much more common. How do we know the patch is a net performance gain? -- 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/