From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wj0-f199.google.com (mail-wj0-f199.google.com [209.85.210.199]) by kanga.kvack.org (Postfix) with ESMTP id BC1536B0033 for ; Tue, 7 Feb 2017 06:34:37 -0500 (EST) Received: by mail-wj0-f199.google.com with SMTP id kq3so24914314wjc.1 for ; Tue, 07 Feb 2017 03:34:37 -0800 (PST) Received: from outbound-smtp04.blacknight.com (outbound-smtp04.blacknight.com. [81.17.249.35]) by mx.google.com with ESMTPS id y187si11852571wmd.87.2017.02.07.03.34.36 for (version=TLS1 cipher=AES128-SHA bits=128/128); Tue, 07 Feb 2017 03:34:36 -0800 (PST) Received: from mail.blacknight.com (pemlinmail06.blacknight.ie [81.17.255.152]) by outbound-smtp04.blacknight.com (Postfix) with ESMTPS id EC02F98CD2 for ; Tue, 7 Feb 2017 11:34:35 +0000 (UTC) Date: Tue, 7 Feb 2017 11:34:35 +0000 From: Mel Gorman Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc Message-ID: <20170207113435.6xthczxt2cx23r4t@techsingularity.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20170207103552.GH5065@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Vlastimil Babka , Dmitry Vyukov , Tejun Heo , Christoph Lameter , "linux-mm@kvack.org" , LKML , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , syzkaller , Andrew Morton 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(); } /* @@ -2377,13 +2384,10 @@ void drain_all_pages(struct zone *zone) mutex_lock(&pcpu_drain_mutex); } - get_online_cpus(); - /* - * We don't care about racing with CPU hotplug event - * as offline notification will cause the notified - * cpu to drain that CPU pcps and on_each_cpu_mask - * disables preemption as part of its processing + * We don't care about racing with CPU hotplug event as offline + * notification will cause the notified cpu to drain that CPU pcps + * and it is serialised against here via pcpu_drain_mutex. */ for_each_online_cpu(cpu) { struct per_cpu_pageset *pcp; @@ -2418,7 +2422,6 @@ void drain_all_pages(struct zone *zone) for_each_cpu(cpu, &cpus_with_pcps) flush_work(per_cpu_ptr(&pcpu_drain, cpu)); - put_online_cpus(); mutex_unlock(&pcpu_drain_mutex); } @@ -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); /* * Spill the event counters of the dead processor -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org