Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751656Ab3FQVhI (ORCPT ); Mon, 17 Jun 2013 17:37:08 -0400 Received: from mail-pb0-f51.google.com ([209.85.160.51]:41416 "EHLO mail-pb0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751169Ab3FQVhF (ORCPT ); Mon, 17 Jun 2013 17:37:05 -0400 Date: Mon, 17 Jun 2013 14:37:02 -0700 (PDT) From: David Rientjes X-X-Sender: rientjes@chino.kir.corp.google.com To: Zhang Yanfei cc: Zhang Yanfei , Andrew Morton , Linux MM , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] mm: Add unlikely for current_order test In-Reply-To: <51BE6BFC.3030009@cn.fujitsu.com> Message-ID: References: <51BC4A83.50302@gmail.com> <51BE6BFC.3030009@cn.fujitsu.com> User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1501 Lines: 33 On Mon, 17 Jun 2013, Zhang Yanfei wrote: > > I don't understand the justification at all, current_order being unlikely > > greater than or equal to pageblock_order / 2 doesn't imply at all that > > it's unlikely that current_order is greater than or equal to > > pageblock_order. > > > > hmmm... I am confused. Since current_order is >= pageblock_order / 2 is unlikely, > why current_order is >= pageblock_order isn't unlikely. Or there are other > tips? > > Actually, I am also a little confused about why current_order should be > unlikely greater than or equal to pageblock_order / 2. When borrowing pages > with other migrate_type, we always search from MAX_ORDER-1, which is greater > or equal to pageblock_order. > Look at what is being done in the function: current_order loops down from MAX_ORDER-1 to the order passed. It is not at all "unlikely" that current_order is greater than pageblock_order, or pageblock_order / 2. MAX_ORDER is typically 11 and pageblock_order is typically 9 on x86. Integer division truncates, so pageblock_order / 2 is 4. For the first eight iterations, it's guaranteed that current_order >= pageblock_order / 2 if it even gets that far! So just remove the unlikely() entirely, it's completely bogus. -- 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/