Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755678AbbHYNqE (ORCPT ); Tue, 25 Aug 2015 09:46:04 -0400 Received: from mail-wi0-f171.google.com ([209.85.212.171]:35657 "EHLO mail-wi0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752414AbbHYNqC (ORCPT ); Tue, 25 Aug 2015 09:46:02 -0400 Date: Tue, 25 Aug 2015 15:45:41 +0200 From: Frederic Weisbecker To: Ingo Molnar Cc: "Paul E. McKenney" , 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: <20150825134538.GA3497@lerouge> References: <1439516774-4614-1-git-send-email-fweisbec@gmail.com> <20150823054032.GA28133@gmail.com> <20150823160101.GA11078@linux.vnet.ibm.com> <20150824064412.GA11581@gmail.com> <20150824133559.GA27147@lerouge> <20150825082904.GA20562@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150825082904.GA20562@gmail.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: 2209 Lines: 45 On Tue, Aug 25, 2015 at 10:29:04AM +0200, Ingo Molnar wrote: > > * Frederic Weisbecker wrote: > > > On Mon, Aug 24, 2015 at 08:44:12AM +0200, Ingo Molnar wrote: > > > 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. > > > > We have tick_nohz_cpu_down_callback() which makes sure that the timekeeper, which > > is the boot CPU in nohz full, never gets offlined. > > That solution really sucks - it essentially regresses a feature the user > explicitly asked for! I also see no way for the user to migrate the timekeeping > functionality over to another CPU without rebooting. > > If this is the last timekeeping CPU then it should migrate the timekeeping > functionality to another CPU, and perhaps printk a warning if all other CPUs are > nohz-full and we have to mark one of them as the timekeeper. > > Also, the nohz-full and timekeeper functionality should not be a boot parameter > only thing, but should be runtime configurable. When I tried to allow moving the timekeeping duty over all housekeeping CPUs, Thomas got angry because it broke the KISS current nohz full code. Indeed, there must be at least one running all the time on behalf of nohz full CPUs that can run anytime. Thus balancing the timekeeping duty over housekeepers is a bit more complicated than in normal configurations. Now surely we can do that using an IPI from CPU_DOWN_PREPARE to a housekeeper if any remains or to a nohz full one. Then we must make sure the new timekeeper never goes to idle. But nohz_full is a corner usecase and I'm not sure it's worth the complexity. If a nohz full user came and complained about CPU0 hotplog not working, I would definetly retry it but I haven't heard about that yet. Besides, hotplug is very isolation-unfriendly in general due to stop machine. -- 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/