From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933127AbdGJRbh (ORCPT ); Mon, 10 Jul 2017 13:31:37 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:46574 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932285AbdGJRLT (ORCPT ); Mon, 10 Jul 2017 13:11:19 -0400 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Vlastimil Babka , "Peter Zijlstra (Intel)" , Rik van Riel , Mel Gorman , Linus Torvalds , Thomas Gleixner , Ingo Molnar Subject: [PATCH 4.12 20/27] sched/numa: Use down_read_trylock() for the mmap_sem Date: Mon, 10 Jul 2017 19:10:17 +0200 Message-Id: <20170710170104.741374954@linuxfoundation.org> X-Mailer: git-send-email 2.13.2 In-Reply-To: <20170710170103.831324799@linuxfoundation.org> References: <20170710170103.831324799@linuxfoundation.org> User-Agent: quilt/0.65 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 4.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: Vlastimil Babka commit 8655d5497735b288f8a9b458bd22e7d1bf95bb61 upstream. A customer has reported a soft-lockup when running an intensive memory stress test, where the trace on multiple CPU's looks like this: RIP: 0010:[] [] native_queued_spin_lock_slowpath+0x10e/0x190 ... Call Trace: [] queued_spin_lock_slowpath+0x7/0xa [] change_protection_range+0x3b1/0x930 [] change_prot_numa+0x18/0x30 [] task_numa_work+0x1fe/0x310 [] task_work_run+0x72/0x90 Further investigation showed that the lock contention here is pmd_lock(). The task_numa_work() function makes sure that only one thread is let to perform the work in a single scan period (via cmpxchg), but if there's a thread with mmap_sem locked for writing for several periods, multiple threads in task_numa_work() can build up a convoy waiting for mmap_sem for read and then all get unblocked at once. This patch changes the down_read() to the trylock version, which prevents the build up. For a workload experiencing mmap_sem contention, it's probably better to postpone the NUMA balancing work anyway. This seems to have fixed the soft lockups involving pmd_lock(), which is in line with the convoy theory. Signed-off-by: Vlastimil Babka Signed-off-by: Peter Zijlstra (Intel) Acked-by: Rik van Riel Acked-by: Mel Gorman Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/20170515131316.21909-1-vbabka@suse.cz Signed-off-by: Ingo Molnar Signed-off-by: Mel Gorman Signed-off-by: Greg Kroah-Hartman --- kernel/sched/fair.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2469,7 +2469,8 @@ void task_numa_work(struct callback_head return; - down_read(&mm->mmap_sem); + if (!down_read_trylock(&mm->mmap_sem)) + return; vma = find_vma(mm, start); if (!vma) { reset_ptenuma_scan(p);