From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752565AbbKKKPn (ORCPT ); Wed, 11 Nov 2015 05:15:43 -0500 Received: from LGEAMRELO13.lge.com ([156.147.23.53]:51431 "EHLO lgeamrelo13.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752057AbbKKKPm (ORCPT ); Wed, 11 Nov 2015 05:15:42 -0500 X-Original-SENDERIP: 156.147.1.121 X-Original-MAILFROM: byungchul.park@lge.com X-Original-SENDERIP: 10.177.222.33 X-Original-MAILFROM: byungchul.park@lge.com Date: Wed, 11 Nov 2015 19:15:32 +0900 From: Byungchul Park To: Peter Zijlstra Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, yuyang.du@intel.com, pjt@google.com, efault@gmx.de, tglx@linutronix.de Subject: Re: [PATCH v4 3/3] sched: optimize migration by forcing rmb() and updating to be called once Message-ID: <20151111101532.GE7136@byungchulpark-X58A-UD3R> References: <1445616981-29904-1-git-send-email-byungchul.park@lge.com> <1445616981-29904-4-git-send-email-byungchul.park@lge.com> <20151109132914.GK17308@twins.programming.kicks-ass.net> <20151110010905.GD4164@byungchulpark-X58A-UD3R> <20151110121647.GZ17308@twins.programming.kicks-ass.net> <20151110235147.GE4164@byungchulpark-X58A-UD3R> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151110235147.GE4164@byungchulpark-X58A-UD3R> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 11, 2015 at 08:51:47AM +0900, Byungchul Park wrote: > On Tue, Nov 10, 2015 at 01:16:47PM +0100, Peter Zijlstra wrote: > > On Tue, Nov 10, 2015 at 10:09:05AM +0900, Byungchul Park wrote: > > > On Mon, Nov 09, 2015 at 02:29:14PM +0100, Peter Zijlstra wrote: > > > > On Sat, Oct 24, 2015 at 01:16:21AM +0900, byungchul.park@lge.com wrote: > > > > > +++ b/kernel/sched/core.c > > > > > @@ -1264,6 +1264,8 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr); > > > > > > > > > > void set_task_cpu(struct task_struct *p, unsigned int new_cpu) > > > > > { > > > > > + unsigned int prev_cpu = task_cpu(p); > > > > > + > > > > > #ifdef CONFIG_SCHED_DEBUG > > > > > /* > > > > > * We should never call set_task_cpu() on a blocked task, > > > > > @@ -1289,15 +1291,14 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu) > > > > > #endif > > > > > > > > > > trace_sched_migrate_task(p, new_cpu); > > > > > + __set_task_cpu(p, new_cpu); > > > > > > > > > > - if (task_cpu(p) != new_cpu) { > > > > > + if (prev_cpu != new_cpu) { > > > > > if (p->sched_class->migrate_task_rq) > > > > > - p->sched_class->migrate_task_rq(p, new_cpu); > > > > > + p->sched_class->migrate_task_rq(p, prev_cpu); > > > > > p->se.nr_migrations++; > > > > > perf_event_task_migrate(p); > > > > > } > > > > > - > > > > > - __set_task_cpu(p, new_cpu); > > > > > } > > > > > > > > I don't think this is safe, see the comment in __set_task_cpu(). We want > > > > that to be last. > > > > > > I am sorry but I don't understand what you said. I checked the comment in > > > __set_task_cpu(). > > > > > > /* > > > * After ->cpu is set up to a new value, task_rq_lock(p, ...) can be > > > * successfuly executed on another CPU. We must ensure that updates of > > > * per-task data have been completed by this moment. > > > */ > > > > > > Of course, ->cpu should be set up to a new value for task_rq_lock() to be > > > executed successfully on another CPU. Is this the case? Is there something > > > i missed? I think it would be ok if task->pi_lock can work correctly within > > > "if" statement in set_task_cpu(). Is there problem to do that? > > > > So the problem is that as soon as that ->cpu store comes through, the > > other rq->lock can happen, even though we might still hold a rq->lock > > thinking we're serialized. > > > > Take for instance move_queued_tasks(), it does: > > > > dequeue_task(rq, p, 0); > > p->on_rq = TASK_ON_RQ_MIGRATING; > > set_task_cpu(p, new_cpu) { > > __set_task_cpu(); > > > > ^^^ here holding rq->lock is insufficient and the below: > > > > p->sched_class->migrate_task_rq() > > Thank you for explaning in detail, but this's why i asked you. > Yes, rq->lock is insufficient in this place as you said, but > should migrate_task_rq() be serialized by rq->lock? I might have > agreed with you if the migrate_task_rq() should be serialized by > rq->lock, but I think it's not the case. I think it would be of rq->lock, but I think it's not the case. I think it would be *ok* (sorry for typo) > if task->pi_lock can work correcly within *if statement* in > set_task_cpu(). Wrong? > > > > > would no longer be serialized by rq->lock. > > > > } > > raw_spin_unlock(&rq->lock); > > > > > > -- > > 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/ > -- > 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/