From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757965AbcILLlO (ORCPT ); Mon, 12 Sep 2016 07:41:14 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:59394 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756697AbcILLlL (ORCPT ); Mon, 12 Sep 2016 07:41:11 -0400 Date: Mon, 12 Sep 2016 13:41:08 +0200 From: Peter Zijlstra To: Cheng Chao Cc: mingo@kernel.org, oleg@redhat.com, tj@kernel.org, akpm@linux-foundation.org, chris@chris-wilson.co.uk, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT_NONE Message-ID: <20160912114108.GP10168@twins.programming.kicks-ass.net> References: <1473408834-6122-1-git-send-email-chengchao@kedacom.com> <1473497532-5440-1-git-send-email-cs.os.kernel@gmail.com> <20160912113727.GQ10153@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160912113727.GQ10153@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 12, 2016 at 01:37:27PM +0200, Peter Zijlstra wrote: > So what you're saying is that migration_stop_cpu() doesn't work because > wait_for_completion() dequeues the task. > > True I suppose. Not sure I like your solution, nor your implementation > of the solution much though. > > I would much prefer an unconditional cond_resched() there, but also, I > think we should do what __migrate_swap_task() does, and set wake_cpu. > > So something like so.. > > --- > kernel/sched/core.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index ddd5f48551f1..ade772aa9610 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -1063,8 +1063,12 @@ static int migration_cpu_stop(void *data) > * holding rq->lock, if p->on_rq == 0 it cannot get enqueued because > * we're holding p->pi_lock. > */ > - if (task_rq(p) == rq && task_on_rq_queued(p)) > - rq = __migrate_task(rq, p, arg->dest_cpu); > + if (task_rq(p) == rq) { > + if (task_on_rq_queued(p)) > + rq = __migrate_task(rq, p, arg->dest_cpu); > + else > + p->wake_cpu = arg->dest_cpu; > + } > raw_spin_unlock(&rq->lock); > raw_spin_unlock(&p->pi_lock); > And this, too narrow a constraint do git diff made it go away. --- kernel/stop_machine.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c index ae6f41fb9cba..637798d6b554 100644 --- a/kernel/stop_machine.c +++ b/kernel/stop_machine.c @@ -121,6 +121,11 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg) cpu_stop_init_done(&done, 1); if (!cpu_stop_queue_work(cpu, &work)) return -ENOENT; + /* + * In case @cpu == smp_proccessor_id() we can avoid a sleep+wakeup + * by doing a preemption. + */ + cond_resched(); wait_for_completion(&done.completion); return done.ret; }