From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756648AbZEOB3B (ORCPT ); Thu, 14 May 2009 21:29:01 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754270AbZEOB2v (ORCPT ); Thu, 14 May 2009 21:28:51 -0400 Received: from yw-out-2324.google.com ([74.125.46.31]:43652 "EHLO yw-out-2324.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753796AbZEOB2u convert rfc822-to-8bit (ORCPT ); Thu, 14 May 2009 21:28:50 -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=lrs8Arx6vl03paJSws/e2m8FkHi6ufdhDA/SkOJg35tLd9Qczndgiay92OXiYzAL0H 819BJT+1p5m+f3/gsvO1gCks1reAaDAaX06nFzEWHohu6AlHo/N7vqrN34InrucWvCLa PUdFLKDVyCTuWgQVzECfJEOs34NuYG3wj/RDs= MIME-Version: 1.0 In-Reply-To: <20090514162201.GA2361@cmpxchg.org> References: <20090514231555.f52c81eb.minchan.kim@gmail.com> <2f11576a0905140727j5ba02b07t94826f57dd99839c@mail.gmail.com> <44c63dc40905140739n271d3d2w2e0cc364c0012d71@mail.gmail.com> <20090514162201.GA2361@cmpxchg.org> Date: Fri, 15 May 2009 10:28:50 +0900 Message-ID: <28c262360905141828v6c9503e9q12cd0e6157a8b5e9@mail.gmail.com> Subject: Re: [PATCH] mmtom: Prevent shrinking of active anon lru list in case of no swap space V3 From: Minchan Kim To: Johannes Weiner Cc: Minchan Kim , KOSAKI Motohiro , Andrew Morton , LKML , linux-mm , Rik van Riel Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 15, 2009 at 1:22 AM, Johannes Weiner wrote: > On Thu, May 14, 2009 at 11:39:49PM +0900, Minchan Kim wrote: >> On Thu, May 14, 2009 at 11:27 PM, KOSAKI Motohiro >> wrote: >> >>  mm/vmscan.c |    2 +- >> >>  1 files changed, 1 insertions(+), 1 deletions(-) >> >> >> >> diff --git a/mm/vmscan.c b/mm/vmscan.c >> >> index 2f9d555..621708f 100644 >> >> --- a/mm/vmscan.c >> >> +++ b/mm/vmscan.c >> >> @@ -1577,7 +1577,7 @@ static void shrink_zone(int priority, struct zone *zone, >> >>         * Even if we did not try to evict anon pages at all, we want to >> >>         * rebalance the anon lru active/inactive ratio. >> >>         */ >> >> -       if (inactive_anon_is_low(zone, sc)) >> >> +       if (inactive_anon_is_low(zone, sc) && nr_swap_pages > 0) >> >>                shrink_active_list(SWAP_CLUSTER_MAX, zone, sc, priority, 0); >> > >> > >> >       if (nr_swap_pages > 0 && inactive_anon_is_low(zone, sc)) >> > >> > is better? >> > compiler can't swap evaluate order around &&. >> >> If GCC optimizes away that branch with CONFIG_SWAP=n as Rik mentioned, >> we don't have a concern. > > It can only optimize it away when the condition is a compile time > constant. > > But inactive_anon_is_low() contains atomic operations which the > compiler is not allowed to drop and so the && semantics lead to > >        atomic_read() && 0 > > emitting the read while still knowing the whole expression is 0 at > compile-time, optimizing away only the branch itself but leaving the > read in place! > > Compared to > >        0 && atomic_read() > > where the && short-circuitry leads to atomic_read() not being > executed.  And since the 0 is a compile time constant, no code has to > be emitted for the read. > > So KOSAKI-san's is right.  Your version results in bigger object code. You're right. I realized it from you. I will repost this. Thanks for great review, Hannes :) >        Hannes > -- Kinds regards, Minchan Kim