From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933214Ab0CKQU6 (ORCPT ); Thu, 11 Mar 2010 11:20:58 -0500 Received: from mx1.redhat.com ([209.132.183.28]:53763 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933204Ab0CKQU4 (ORCPT ); Thu, 11 Mar 2010 11:20:56 -0500 Date: Thu, 11 Mar 2010 17:19:09 +0100 From: Oleg Nesterov To: Peter Zijlstra Cc: Ingo Molnar , Lai Jiangshan , Tejun Heo , linux-kernel@vger.kernel.org Subject: Re: Q: select_fallback_rq() && cpuset_lock() Message-ID: <20100311161909.GA16008@redhat.com> References: <20100309180615.GA11681@redhat.com> <1268239242.5279.46.camel@twins> <20100310173018.GA1294@redhat.com> <1268244075.5279.53.camel@twins> <20100310183259.GA23648@redhat.com> <20100311145248.GA12907@redhat.com> <1268321759.5037.113.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1268321759.5037.113.camel@laptop> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/11, Peter Zijlstra wrote: > > On Thu, 2010-03-11 at 15:52 +0100, Oleg Nesterov wrote: > > > Btw, select_fallback_rq() takes > > rcu_read_lock around cpuset_cpus_allowed_locked(). Why? I must have > > missed something, but afaics this buys nothing. > > for task_cs() iirc. it should be stable under task_lock()... Never mind. > > How can we fix this later? Perhaps we can change > > cpuset_track_online_cpus(CPU_DEAD) to scan all affected cpusets and > > fixup the tasks with the wrong ->cpus_allowed == cpu_possible_mask. > > Problem is, we can't really fix up tasks, wakeup must be able to find a > suitable cpu. Yes sure. I meant, wakeup()->select_fallback_rq() sets cpus_allowed = cpu_possible_map as we discussed. Then cpuset_track_online_cpus(CPU_DEAD) fixes the affected tasks. > > At first glance this should work in try_to_wake_up(p) case, we can't > > race with cpuset_change_cpumask()/etc because of TASK_WAKING logic. > > Well, cs->cpus_possible can still go funny on us. What do you mean? Afaics, cpusets always uses set_cpus_allowed() to change task->cpus_allowed. > > But I am not sure how can we fix move_task_off_dead_cpu(). I think > > __migrate_task_irq() itself is fine, but if select_fallback_rq() is > > called from move_task_off_dead_cpu() nothing protects ->cpus_allowed. > > It has that retry loop in case the migration fails, right? > > > We can race with cpusets, or even with the plain set_cpus_allowed(). > > Probably nothing really bad can happen, if the resulting cpumask > > doesn't have online cpus due to the racing memcpys, we should retry > > after __migrate_task_irq() fails. Or we can take cpu_rq(cpu)-lock > > around cpumask_copy(p->cpus_allowed, cpu_possible_mask). > > It does the retry thing. Yes, I mentioned retry logic too. But it can't always help, even without cpusets. Suppose a task T is bound to the dead CPU, and move_task_off_dead_cpu() races with set_cpus_allowed(new_mask). I think it is fine if T gets either new_mask or cpu_possible_map in ->cpus_allowed. But, it can get a "random" mix if 2 memcpy() run in parallel. And it is possible that __migrate_task_irq() will not fail if dest_cpu falls into resulting mask. > > @@ -2289,10 +2289,9 @@ static int select_fallback_rq(int cpu, s > > > > /* No more Mr. Nice Guy. */ > > if (dest_cpu >= nr_cpu_ids) { > > - rcu_read_lock(); > > - cpuset_cpus_allowed_locked(p, &p->cpus_allowed); > > - rcu_read_unlock(); > > - dest_cpu = cpumask_any_and(cpu_active_mask, &p->cpus_allowed); > > + // XXX: take cpu_rq(cpu)->lock ??? > > + cpumask_copy(&p->cpus_allowed, cpu_possible_mask); > > + dest_cpu = cpumask_any(cpu_active_mask); > > > Right, this seems safe. OK, I'll try to read this code a bit more and then send this patch. Oleg.