From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756067Ab1LBWmo (ORCPT ); Fri, 2 Dec 2011 17:42:44 -0500 Received: from www.linutronix.de ([62.245.132.108]:49523 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753260Ab1LBWmn (ORCPT ); Fri, 2 Dec 2011 17:42:43 -0500 Date: Fri, 2 Dec 2011 23:42:39 +0100 (CET) From: Thomas Gleixner To: Dimitri Sivanich cc: Andrew Morton , linux-kernel@vger.kernel.org Subject: Re: [PATCH] specific do_timer_cpu value for nohz off mode In-Reply-To: <20111202201452.GF2164@sgi.com> Message-ID: References: <20111108191149.GA7236@sgi.com> <20111122160802.e99d6218.akpm@linux-foundation.org> <20111130152959.GA19205@sgi.com> <20111130161131.31cdccff.akpm@linux-foundation.org> <20111130161610.69c516f7.akpm@linux-foundation.org> <20111201020727.GB30097@sgi.com> <20111130181318.38f4659d.akpm@linux-foundation.org> <20111201163740.GA11693@sgi.com> <20111201145623.d2bf252e.akpm@linux-foundation.org> <20111202201452.GF2164@sgi.com> User-Agent: Alpine 2.02 (LFD 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2 Dec 2011, Dimitri Sivanich wrote: > On Thu, Dec 01, 2011 at 02:56:23PM -0800, Andrew Morton wrote: > Well, we do have to take the write_seqlock() in tick_periodic, so there's > no danger of do_timer running exactly concurrently. > > But yes, we may end up with 2 jiffies ticks occurring close together > (when 5 runs do_timer while 4 waits for the seqlock), or we might end up > missing a jiffies update for almost a full tick (when it changes from 5 > to 4 immediately after 4 has done the 'tick_do_timer_cpu == cpu' check). > > So at that time, we could be off +- almost a tick. The question is, how > critical is that? When you down a cpu, the same sort of thing could > happen via tick_handover_do_timer(), which itself does nothing more than > change tick_do_timer_cpu. It's uncritical as long as you are not using clocksource=jiffies. With all other clocksources you just miss a jiffies update, which does not affect timekeeping at all. It just might expire your network timeout a jiffie earlier or later. So there is no damage to expect. Thanks, tglx