From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with SMTP id 5A6136B0047 for ; Tue, 31 Aug 2010 17:35:42 -0400 (EDT) Date: Tue, 31 Aug 2010 16:35:37 -0500 (CDT) From: Christoph Lameter Subject: Re: [PATCH 03/10] Use percpu stats In-Reply-To: <1283290106.2198.26.camel@edumazet-laptop> Message-ID: References: <1281374816-904-1-git-send-email-ngupta@vflare.org> <1281374816-904-4-git-send-email-ngupta@vflare.org> <1283290106.2198.26.camel@edumazet-laptop> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org To: Eric Dumazet Cc: Nitin Gupta , Pekka Enberg , Minchan Kim , Andrew Morton , Greg KH , Linux Driver Project , linux-mm , linux-kernel List-ID: On Tue, 31 Aug 2010, Eric Dumazet wrote: > > Yes, this_cpu_add() seems sufficient. I can't recall why I used u64_stats_* > > but if it's not required for atomic access to 64-bit then why was it added to > > the mainline in the first place? > > Because we wanted to have fast 64bit counters, even on 32bit arches, and > this has litle to do with 'atomic' on one entity, but a group of > counters. (check drivers/net/loopback.c, lines 91-94). No lock prefix > used in fast path. > > We also wanted readers to read correct values, not a value being changed > by a writer, with inconsistent 32bit halves. SNMP applications want > monotonically increasing counters. > > this_cpu_add()/this_cpu_read() doesnt fit. > > Even for single counter, this_cpu_read(64bit) is not using an RMW > (cmpxchg8) instruction, so you can get very strange results when low > order 32bit wraps. How about fixing it so that everyone benefits? -- 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/ . Don't email: email@kvack.org