From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753882Ab0CYKWS (ORCPT ); Thu, 25 Mar 2010 06:22:18 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:59444 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753781Ab0CYKWQ (ORCPT ); Thu, 25 Mar 2010 06:22:16 -0400 Subject: Re: [PATCH 0/6] sched/cpusets fixes, more changes are needed From: Peter Zijlstra To: Oleg Nesterov Cc: Ingo Molnar , Ben Blum , Jiri Slaby , Lai Jiangshan , Li Zefan , Miao Xie , Paul Menage , "Rafael J. Wysocki" , Tejun Heo , linux-kernel@vger.kernel.org In-Reply-To: <20100324180912.GA21774@redhat.com> References: <20100315090958.GA9116@redhat.com> <1269452296.5109.508.camel@twins> <20100324180912.GA21774@redhat.com> Content-Type: text/plain; charset="UTF-8" Date: Thu, 25 Mar 2010 11:22:11 +0100 Message-ID: <1269512531.12097.67.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2010-03-24 at 19:09 +0100, Oleg Nesterov wrote: > On 03/24, Peter Zijlstra wrote: > > > > On Mon, 2010-03-15 at 10:09 +0100, Oleg Nesterov wrote: > > > > > > - do_fork() clears PF_STARTING and then calls wake_up_new_task() > > > which finally does s/WAKING/RUNNING. > > > > > > But. Nobody can take rq->lock in between. This means a signal > > > from irq (quite possible with CLONE_THREAD) or another rt > > > thread which preempts us can lockup. > > > > Hmm, the signal case might indeed be a problem, however I cannot see how > > the RT thread can be a problem because until we do wake_up_new_task() > > the child will not be runnable and can thus not be preempted. > > Indeed, but I meant the _parent_ can be preempted ;) I still can't see how that would be a problem.. > In short. TASK_WAKING acts as a spinlock in fact. And since ttwu() can > be called from any context, it should be irq-safe: any owner must disable > inerrupts and preemption. Agreed, and I think that's corrected with my patch. > > The reason we have that TASK_WAKING stuff for fork is because > > wake_up_new_task() needs p->cpus_allowed to be stable > > Sure! But it is very easy to change wake_up_new_task() to set TASK_WAKING > like ttwu() does. Of course, this needs raw_spin_lock_irq(rq->lock) for > a moment, but afaics that is all? My patch does that. > > So the below patch makes select_task_rq_fair unlock the rq when needed, > > and then puts all ->select_task_rq() calls under rq->lock. This should > > allow us to remove the TASK_WAKING thing from fork which in turn allows > > us to remove the PF_STARTING check in task_is_waking. > > > > How does that look? > > I'll try to read this patch tomorrow. But could you please consider > the suggestion above? I think I do all those :-) I was still looking at removing the TASK_WAKING check from task_rq_lock() since now we do set_task_cpu() under rq->lock again so it should be good again. Hmm, except for sched_fork() that still does set_task_cpu() without holding rq->lock, but that is before the child gets exposed so there should not be any concurrency.