From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx162.postini.com [74.125.245.162]) by kanga.kvack.org (Postfix) with SMTP id 81E036B002C for ; Fri, 2 Mar 2012 14:53:48 -0500 (EST) Date: Fri, 2 Mar 2012 13:53:45 -0600 (CST) From: Christoph Lameter Subject: Re: [PATCH] cpuset: mm: Remove memory barrier damage from the page allocator In-Reply-To: <20120302174349.GB3481@suse.de> Message-ID: References: <20120302112358.GA3481@suse.de> <20120302174349.GB3481@suse.de> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman Cc: Andrew Morton , Miao Xie , Peter Zijlstra , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Fri, 2 Mar 2012, Mel Gorman wrote: > I considered using a seqlock but it isn't cheap. The read side is heavy > with the possibility that it starts spinning and incurs a read barrier > (looking at read_seqbegin()) here. The retry block incurs another read > barrier so basically it would not be no better than what is there currently > (which at a 4% performance hit, sucks) Oh. You dont have a read barrier? So your approach is buggy? We could have read a state before someone else incremented the seq counter, then cached it, then we read the counter, did the processing and found that the sequid was not changed? > In the case of seqlocks, a reader will backoff if a writer is in progress > but the page allocator doesn't need that which is why I felt it was ok You can just not use the writer section if you think that is ok. Doubt it but lets at least start using a known serialization construct that would allow us to fix it up if we find that we need to update multiple variables protected by the seqlock. > Allocation failure is an unusual situation that can trigger application > exit or an OOM so it's ok to treat it as a slow path. A normal seqlock > would retry unconditionally and potentially have to handle the case > where it needs to free the page before retrying which is pointless. It will only retry as long as the writer hold the "lock". Like a spinlock the holdoff times depends on the size of the critical section and initially you could just avoid having write sections. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org