From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755105Ab2APP3B (ORCPT ); Mon, 16 Jan 2012 10:29:01 -0500 Received: from mx1.redhat.com ([209.132.183.28]:30881 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754637Ab2APP3A (ORCPT ); Mon, 16 Jan 2012 10:29:00 -0500 Date: Mon, 16 Jan 2012 10:28:53 -0500 From: Vivek Goyal To: Tejun Heo Cc: KAMEZAWA Hiroyuki , Andrew Morton , avi@redhat.com, nate@cpanel.net, cl@linux-foundation.org, oleg@redhat.com, axboe@kernel.dk, linux-kernel@vger.kernel.org Subject: Re: [PATCHSET] block, mempool, percpu: implement percpu mempool and fix blkcg percpu alloc deadlock Message-ID: <20120116152853.GB9129@redhat.com> References: <20111222220911.GK17084@google.com> <20111222142058.41316ee0.akpm@linux-foundation.org> <20111223014043.GC12738@redhat.com> <20111222175834.66559c8b.akpm@linux-foundation.org> <20111223025629.GE12738@redhat.com> <20111226150531.3c22f2f0.kamezawa.hiroyu@jp.fujitsu.com> <20111227175249.GE17712@google.com> <20111228091402.c45a08f6.kamezawa.hiroyu@jp.fujitsu.com> <20111228004102.GK17712@google.com> <20120105012842.GQ31746@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120105012842.GQ31746@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 04, 2012 at 05:28:42PM -0800, Tejun Heo wrote: > On Tue, Dec 27, 2011 at 04:41:02PM -0800, Tejun Heo wrote: > > Hello, > > > > On Wed, Dec 28, 2011 at 09:14:02AM +0900, KAMEZAWA Hiroyuki wrote: > > > > This is essentially more specialized form of the mempool approach. It > > > > doesn't seem any simpler to me while being less generic. I don't see > > > > what the upside would be. > > > > > > Hm, but this never causes -ENOMEM error, at all. > > > > Ooh, I missed the part it falls back to the global counter if percpu > > counters aren't allocated yet. Yeah, this is an interesting approach. > > I'll think more about it. > > I've been staring at the blkcg stats code and commit logs and am > wondering whether we can just scrap percpu counters there. It seems > the reason why it was introduced in the first place is to avoid > stats->lock, which BTW is extremely heavy handed for gathering stats, > overhead in fast paths and I think there can be easier ways to avoid > stats->lock. Tejun, If you get rid of stats->lock, and also don't use per cpu data strucutres how would we avoid races in case of concurrent update (IO submission?). Thanks Vivek