From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752022AbbJJS4S (ORCPT ); Sat, 10 Oct 2015 14:56:18 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55296 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750737AbbJJS4R (ORCPT ); Sat, 10 Oct 2015 14:56:17 -0400 Date: Sat, 10 Oct 2015 20:52:55 +0200 From: Oleg Nesterov To: Peter Zijlstra Cc: heiko.carstens@de.ibm.com, Tejun Heo , Ingo Molnar , Rik van Riel , Thomas Gleixner , Vitaly Kuznetsov , linux-kernel@vger.kernel.org Subject: [PATCH 0/3] (Was: sched: start stopper early) Message-ID: <20151010185255.GA24075@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org To avoid the confusion, this has nothing to do with "stop_machine" changes we discuss in another thread, but On 10/09, Oleg Nesterov wrote: > > > case CPU_ONLINE: > > + stop_machine_unpark(cpu); > > /* > > * At this point a starting CPU has marked itself as online via > > * set_cpu_online(). But it might not yet have marked itself > > @@ -5337,7 +5340,7 @@ static int sched_cpu_active(struct notifier_block *nfb, > > * Thus, fall-through and help the starting CPU along. > > */ > > case CPU_DOWN_FAILED: > > - set_cpu_active((long)hcpu, true); > > + set_cpu_active(cpu, true); > > On a second thought, we can't do this (and your initial change has > the same problem). > > We can not wakeup it before set_cpu_active(). This can lead to the > same problem fixed by dd9d3843755da95f6 "sched: Fix cpu_active_mask/ > cpu_online_mask race". OTOH, I don't understand why do we actually need this fix... Or, iow I don't really understand the cpu_active() checks in select_fallback_rq(). Looks like we have some strange arch/ which has the "unsafe" online && !active window, but then it is not clear why it is safe to mark it active in sched_cpu_active(CPU_ONLINE). Confused. And I am even more confused by the fact that select_fallback_rq() checks cpu_active(), but select_task_rq() doesn't. This can't be right in any case. Oleg. kernel/sched/core.c | 41 +++++++++++++++++++++-------------------- 1 files changed, 21 insertions(+), 20 deletions(-)