From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935590AbcIFPXj (ORCPT ); Tue, 6 Sep 2016 11:23:39 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41186 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933176AbcIFPXg (ORCPT ); Tue, 6 Sep 2016 11:23:36 -0400 Date: Tue, 6 Sep 2016 17:22:54 +0200 From: Oleg Nesterov To: chengchao 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: <20160906152253.GB17586@redhat.com> References: <1473056403-7877-1-git-send-email-chengchao@kedacom.com> <20160905131147.GA8552@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Tue, 06 Sep 2016 15:23:35 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/06, chengchao wrote: > > the key point is for CONFIG_PREEMPT_NONE=y, > ... > it is too much overhead for one task(fork()+exec()), isn't it? Yes, yes, I see, this is suboptimal. Not sure we actually do care, but yes, perhaps another helper which migrates the current task makes sense, I dunno. But, > > 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. > > > yes. the stop_one_cpu_sync is not a good name, stop_one_cpu_schedule is better? > there is nothing about synchronization. We need to synchronize with the stopper to ensure it can't touch cpu_stop_work on stack after stop_one_cpu_sync() returns, and > > But this is not necessarily true? This task can migrate to another CPU > > before cpu_stop_queue_work() ? > > > before sched_exec() calls stop_one_cpu()/cpu_stop_queue_work(), this > task(current) cannot migrate to another cpu,because this task is running > on the cpu. Why? The running task can migrate to another CPU at any moment. Unless it runs with preemption disabled or CONFIG_PREEMPT_NONE=y. And this means that cpu_stop_queue_work() can queue the work on another CPU != smp_processor_id(), and in this case the kernel can crash because the pending cpu_stop_work can be overwritten right after return. So you need something like void stop_one_cpu_sync(cpu_stop_fn_t fn, void *arg) { struct cpu_stop_work work = { .fn = fn, .arg = arg, .done = NULL }; preempt_disable(); cpu_stop_queue_work(raw_smp_processor_id(), &work); preempt_enable_no_resched(); schedule(); } or I am totally confused. Note that it doesn't (and shouldn't) have the "int cpu" argument. Oleg.