From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id 4C9E86B0047 for ; Tue, 31 Aug 2010 17:28:32 -0400 (EDT) Received: by wyb36 with SMTP id 36so10104350wyb.14 for ; Tue, 31 Aug 2010 14:28:31 -0700 (PDT) Subject: Re: [PATCH 03/10] Use percpu stats From: Eric Dumazet In-Reply-To: References: <1281374816-904-1-git-send-email-ngupta@vflare.org> <1281374816-904-4-git-send-email-ngupta@vflare.org> Content-Type: text/plain; charset="UTF-8" Date: Tue, 31 Aug 2010 23:28:26 +0200 Message-ID: <1283290106.2198.26.camel@edumazet-laptop> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org To: Nitin Gupta Cc: Christoph Lameter , Pekka Enberg , Minchan Kim , Andrew Morton , Greg KH , Linux Driver Project , linux-mm , linux-kernel List-ID: Le mardi 31 aoA>>t 2010 A 16:31 -0400, Nitin Gupta a A(C)crit : > On Mon, Aug 30, 2010 at 12:20 PM, Christoph Lameter wrote: > > On Mon, 9 Aug 2010, Nitin Gupta wrote: > > > >> -static void zram_stat_inc(u32 *v) > >> +static void zram_add_stat(struct zram *zram, > >> + enum zram_stats_index idx, s64 val) > >> { > >> - *v = *v + 1; > >> + struct zram_stats_cpu *stats; > >> + > >> + preempt_disable(); > >> + stats = __this_cpu_ptr(zram->stats); > >> + u64_stats_update_begin(&stats->syncp); > >> + stats->count[idx] += val; > >> + u64_stats_update_end(&stats->syncp); > >> + preempt_enable(); > > > > Maybe do > > > > #define zram_add_stat(zram, index, val) > > this_cpu_add(zram->stats->count[index], val) > > > > instead? It creates an add in a single "atomic" per cpu instruction and > > deals with the fallback scenarios for processors that cannot handle 64 > > bit adds. > > > > > > 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. -- 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