From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754083AbdBGLnc (ORCPT ); Tue, 7 Feb 2017 06:43:32 -0500 Received: from mx2.suse.de ([195.135.220.15]:60796 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753497AbdBGLnb (ORCPT ); Tue, 7 Feb 2017 06:43:31 -0500 Date: Tue, 7 Feb 2017 12:43:27 +0100 From: Michal Hocko To: Mel Gorman Cc: Vlastimil Babka , Dmitry Vyukov , Tejun Heo , Christoph Lameter , "linux-mm@kvack.org" , LKML , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , syzkaller , Andrew Morton Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc Message-ID: <20170207114327.GI5065@dhcp22.suse.cz> References: <20170206220530.apvuknbagaf2rdlw@techsingularity.net> <20170207084855.GC5065@dhcp22.suse.cz> <20170207094300.cuxfqi35wflk5nr5@techsingularity.net> <2cdef192-1939-d692-1224-8ff7d7ff7203@suse.cz> <20170207102809.awh22urqmfrav5r6@techsingularity.net> <20170207103552.GH5065@dhcp22.suse.cz> <20170207113435.6xthczxt2cx23r4t@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170207113435.6xthczxt2cx23r4t@techsingularity.net> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 07-02-17 11:34:35, Mel Gorman wrote: > On Tue, Feb 07, 2017 at 11:35:52AM +0100, Michal Hocko wrote: > > On Tue 07-02-17 10:28:09, Mel Gorman wrote: > > > On Tue, Feb 07, 2017 at 10:49:28AM +0100, Vlastimil Babka wrote: > > > > On 02/07/2017 10:43 AM, Mel Gorman wrote: > > > > > If I'm reading this right, a hot-remove will set the pool POOL_DISASSOCIATED > > > > > and unbound. A workqueue queued for draining get migrated during hot-remove > > > > > and a drain operation will execute twice on a CPU -- one for what was > > > > > queued and a second time for the CPU it was migrated from. It should still > > > > > work with flush_work which doesn't appear to block forever if an item > > > > > got migrated to another workqueue. The actual drain workqueue function is > > > > > using the CPU ID it's currently running on so it shouldn't get confused. > > > > > > > > Is the worker that will process this migrated workqueue also guaranteed > > > > to be pinned to a cpu for the whole work, though? drain_local_pages() > > > > needs that guarantee. > > > > > > > > > > It should be by running on a workqueue handler bound to that CPU (queued > > > on wq->cpu_pwqs in __queue_work) > > > > Are you sure? The comment in kernel/workqueue.c says > > * While DISASSOCIATED, the cpu may be offline and all workers have > > * %WORKER_UNBOUND set and concurrency management disabled, and may > > * be executing on any CPU. The pool behaves as an unbound one. > > > > I might be misreadig but an unbound pool can be handled by workers which > > are not pinned on any cpu AFAIU. > > Right. The unbind operation can set a mask that is any allowable CPU and > the final process_work is not done in a context that prevents > preemption. > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 3b93879990fd..7af165d308c4 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -2342,7 +2342,14 @@ void drain_local_pages(struct zone *zone) > > static void drain_local_pages_wq(struct work_struct *work) > { > + /* > + * Ordinarily a drain operation is bound to a CPU but may be unbound > + * after a CPU hotplug operation so it's necessary to disable > + * preemption for the drain to stabilise the CPU ID. > + */ > + preempt_disable(); > drain_local_pages(NULL); > + preempt_enable_no_resched(); > } > > /* [...] > @@ -6711,7 +6714,16 @@ static int page_alloc_cpu_dead(unsigned int cpu) > { > > lru_add_drain_cpu(cpu); > + > + /* > + * A per-cpu drain via a workqueue from drain_all_pages can be > + * rescheduled onto an unrelated CPU. That allows the hotplug > + * operation and the drain to potentially race on the same > + * CPU. Serialise hotplug versus drain using pcpu_drain_mutex > + */ > + mutex_lock(&pcpu_drain_mutex); > drain_pages(cpu); > + mutex_unlock(&pcpu_drain_mutex); You cannot put sleepable lock inside the preempt disbaled section... We can make it a spinlock right? -- Michal Hocko SUSE Labs