From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754612Ab1H2SAg (ORCPT ); Mon, 29 Aug 2011 14:00:36 -0400 Received: from merlin.infradead.org ([205.233.59.134]:45427 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754174Ab1H2SAc convert rfc822-to-8bit (ORCPT ); Mon, 29 Aug 2011 14:00:32 -0400 Subject: Re: [PATCH 09/32] nohz: Move ts->idle_calls into strict idle logic From: Peter Zijlstra To: Frederic Weisbecker Cc: LKML , Andrew Morton , Anton Blanchard , Avi Kivity , Ingo Molnar , Lai Jiangshan , "Paul E . McKenney" , Paul Menage , Stephen Hemminger , Thomas Gleixner , Tim Pepper Date: Mon, 29 Aug 2011 19:59:49 +0200 In-Reply-To: <20110829173426.GE9748@somewhere.redhat.com> References: <1313423549-27093-1-git-send-email-fweisbec@gmail.com> <1313423549-27093-10-git-send-email-fweisbec@gmail.com> <1314629267.2816.82.camel@twins> <20110829173426.GE9748@somewhere.redhat.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT X-Mailer: Evolution 3.0.2- Message-ID: <1314640789.2816.122.camel@twins> Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2011-08-29 at 19:34 +0200, Frederic Weisbecker wrote: > On Mon, Aug 29, 2011 at 04:47:47PM +0200, Peter Zijlstra wrote: > > On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote: > > > +static bool tick_nohz_can_stop_tick(int cpu, struct tick_sched *ts) > > > +{ > > > + /* > > > + * If this cpu is offline and it is the one which updates > > > + * jiffies, then give up the assignment and let it be taken by > > > + * the cpu which runs the tick timer next. If we don't drop > > > + * this here the jiffies might be stale and do_timer() never > > > + * invoked. > > > + */ > > > + if (unlikely(!cpu_online(cpu))) { > > > + if (cpu == tick_do_timer_cpu) > > > + tick_do_timer_cpu = TICK_DO_TIMER_NONE; > > > + } > > > + > > > + if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE)) > > > + return false; > > > + > > > + if (need_resched()) > > > + return false; > > > + > > > + if (unlikely(local_softirq_pending() && cpu_online(cpu))) { > > > + static int ratelimit; > > > + > > > + if (ratelimit < 10) { > > > + printk(KERN_ERR "NOHZ: local_softirq_pending %02x\n", > > > + (unsigned int) local_softirq_pending()); > > > + ratelimit++; > > > + } > > > + return false; > > > + } > > > + > > > + return true; > > > +} > > > > Why aren't rcu_needs_cpu(), printk_needs_cpu() and arch_needs_cpu() not > > in there? > > > > That are typical 'can we go sleep now?' functions. > > Because when one of these functions are positive, the ts->next_jiffies and > ts->last_jiffies stats are updated. Not with the above. > Also I start to think the above checks are only useful in the idle case. Then call it tick_nohz_can_stop_tick_idle() or so, and create tick_nohz_can_stop_tick() to deal with all stuff. > We still want tick_nohz_stop_sched_tick() to have the *needs_cpu() checks > so that they can restore a HZ periodic behaviour on interrupt return if > needed. Well, no, on interrupt return you shouldn't do anything. If you've stopped the tick it stays stopped until you do something that needs it, then that action will re-enable it. > That said I wonder if some of the above conditions should restore a periodic > behaviour on interrupt return... I would expect the tick not to be stopped when tick_nohz_can_stop_tick() returns false. If it returns true, then I expect anything that needs it to re-enable it.