From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932945AbcIENLx (ORCPT ); Mon, 5 Sep 2016 09:11:53 -0400 Received: from mx1.redhat.com ([209.132.183.28]:57980 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932127AbcIENLw (ORCPT ); Mon, 5 Sep 2016 09:11:52 -0400 Date: Mon, 5 Sep 2016 15:11:48 +0200 From: Oleg Nesterov To: cheng chao Cc: mingo@kernel.org, peterz@infradead.org, tj@kernel.org, akpm@linux-foundation.org, chris@chris-wilson.co.uk, linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched/core: simpler function for sched_exec migration Message-ID: <20160905131147.GA8552@redhat.com> References: <1473056403-7877-1-git-send-email-chengchao@kedacom.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1473056403-7877-1-git-send-email-chengchao@kedacom.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Mon, 05 Sep 2016 13:11:52 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/05, cheng chao wrote: > > @@ -2958,7 +2958,7 @@ void sched_exec(void) > struct migration_arg arg = { p, dest_cpu }; > > raw_spin_unlock_irqrestore(&p->pi_lock, flags); > - stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg); > + stop_one_cpu_sync(task_cpu(p), migration_cpu_stop, &arg); > return; > } > unlock: > diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c > index 4a1ca5f..24f8637 100644 > --- a/kernel/stop_machine.c > +++ b/kernel/stop_machine.c > @@ -130,6 +130,27 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg) > return done.ret; > } > > +/** > + * the caller keeps task_on_rq_queued, so it's more suitable for > + * sched_exec on the case when needs migration > + */ > +void stop_one_cpu_sync(unsigned int cpu, cpu_stop_fn_t fn, void *arg) > +{ > + struct cpu_stop_work work = { .fn = fn, .arg = arg, .done = NULL }; > + > + if (!cpu_stop_queue_work(cpu, &work)) > + return; > + > +#if defined(CONFIG_PREEMPT_NONE) || defined(CONFIG_PREEMPT_VOLUNTARY) > + /* > + * CONFIG_PREEMPT doesn't need call schedule here, because > + * preempt_enable already does the similar thing when call > + * cpu_stop_queue_work > + */ > + schedule(); > +#endif > +} Honestly, I don't really understand the changelog, but this looks wrong. stop_one_cpu_sync() assumes that cpu == smp_processor_id/task_cpu(current), and thus the stopper thread should preempt us at least after schedule() (if CONFIG_PREEMPT_NONE), so we do not need to synchronize. But this is not necessarily true? This task can migrate to another CPU before cpu_stop_queue_work() ? Oleg.