Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932371AbbHXGoS (ORCPT ); Mon, 24 Aug 2015 02:44:18 -0400 Received: from mail-wi0-f176.google.com ([209.85.212.176]:34689 "EHLO mail-wi0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932307AbbHXGoQ (ORCPT ); Mon, 24 Aug 2015 02:44:16 -0400 Date: Mon, 24 Aug 2015 08:44:12 +0200 From: Ingo Molnar To: "Paul E. McKenney" Cc: Frederic Weisbecker , Peter Zijlstra , LKML , Vatika Harlalka , Chris Metcalf , Thomas Gleixner , Preeti U Murthy , Christoph Lameter Subject: Re: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers Message-ID: <20150824064412.GA11581@gmail.com> References: <1439516774-4614-1-git-send-email-fweisbec@gmail.com> <20150823054032.GA28133@gmail.com> <20150823160101.GA11078@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150823160101.GA11078@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1833 Lines: 55 * Paul E. McKenney wrote: > > here it's fully set - triggering the bug I'm worried about. So what am I > > missing, what prevents CONFIG_NO_HZ_FULL_ALL from crashing? > > The boot CPU is excluded from tick_nohz_full_mask in tick_nohz_init(), which is > called from tick_init() which is called from start_kernel() shortly after > rcu_init(): > > cpu = smp_processor_id(); > > if (cpumask_test_cpu(cpu, tick_nohz_full_mask)) { > pr_warning("NO_HZ: Clearing %d from nohz_full range for timekeeping\n", cpu); > cpumask_clear_cpu(cpu, tick_nohz_full_mask); > } > > This happens after the call to tick_nohz_init_all() that does the > cpumask_setall() that you called out above. Ah, indeed - I somehow missed that. This brings up two other questions: 1) the 'housekeeping CPU' is essentially the boot CPU. Yet we dedicate a full mask to it (housekeeping_mask - a variable mask to begin with) and recover the housekeeping CPU via: + return cpumask_any_and(housekeeping_mask, cpu_online_mask); which can be pretty expensive, and which gets executed in two hotpaths: kernel/time/hrtimer.c: return &per_cpu(hrtimer_bases, get_nohz_timer_target()); kernel/time/timer.c: return per_cpu_ptr(&tvec_bases, get_nohz_timer_target()); ... why not just use a single housekeeping_cpu which would be way faster to pass down to the timer code? 2) What happens if the boot CPU is offlined? (under CONFIG_BOOTPARAM_HOTPLUG_CPU0=y) I don't see CPU hotplug callbacks fixing up the housekeeping_mask if the boot CPU is offlined. Thanks, Ingo -- 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/