Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752237Ab3HUOYP (ORCPT ); Wed, 21 Aug 2013 10:24:15 -0400 Received: from merlin.infradead.org ([205.233.59.134]:58243 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751725Ab3HUOYL (ORCPT ); Wed, 21 Aug 2013 10:24:11 -0400 Date: Wed, 21 Aug 2013 16:23:56 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: Arjan van de Ven , Fernando Luis =?iso-8859-1?Q?V=E1zquez?= Cao , Frederic Weisbecker , Ingo Molnar , Thomas Gleixner , LKML , Tetsuo Handa , Andrew Morton Subject: Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock Message-ID: <20130821142356.GC31370@twins.programming.kicks-ass.net> References: <521313D8.9080500@lab.ntt.co.jp> <20130820084405.GC3258@twins.programming.kicks-ass.net> <52138BE9.5090005@linux.intel.com> <20130820160146.GG3258@twins.programming.kicks-ass.net> <20130820163312.GA17957@redhat.com> <20130820175429.GI3258@twins.programming.kicks-ass.net> <20130820182553.GB22287@redhat.com> <20130821083130.GM3258@twins.programming.kicks-ass.net> <20130821113551.GA1472@redhat.com> <20130821123311.GA31370@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130821123311.GA31370@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1278 Lines: 31 On Wed, Aug 21, 2013 at 02:33:11PM +0200, Peter Zijlstra wrote: > On Wed, Aug 21, 2013 at 01:35:51PM +0200, Oleg Nesterov wrote: > > On 08/21, Peter Zijlstra wrote: > > > > Yes its the right rq, but the wrong time. > > > > Hmm. Just in case, it is not that I think this patch really makes sense, > > but I'd like to understand why do you think it is wrong. > > > But it is not "after it switches out", it is after it switched back. > > D'uh I was being particularly dense it seems :/ > > Yes I think it is correct. You're now trading two atomic ops on a > different cacheline than rq->lock for one atomic op on the rq->lock. > > Might be a win, esp. since hopefully there's a fair chance its the same > runqueue. The other consideration is that this adds two branches to the normal schedule path. I really don't know what the regular ratio between schedule() and io_schedule() is -- and I suspect it can very much depend on workload -- but it might be a net loss due to that, even if it makes io_schedule() 'lots' cheaper. -- 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/