From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.linuxfoundation.org ([140.211.169.12]:59278 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755860AbeDKSrX (ORCPT ); Wed, 11 Apr 2018 14:47:23 -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 , Sasha Levin Subject: [PATCH 4.4 062/190] sched/numa: Use down_read_trylock() for the mmap_sem Date: Wed, 11 Apr 2018 20:35:08 +0200 Message-Id: <20180411183553.939186174@linuxfoundation.org> In-Reply-To: <20180411183550.114495991@linuxfoundation.org> References: <20180411183550.114495991@linuxfoundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: stable-owner@vger.kernel.org List-ID: 4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Vlastimil Babka [ Upstream commit 8655d5497735b288f8a9b458bd22e7d1bf95bb61 ] 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: Sasha Levin 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 @@ -2223,7 +2223,8 @@ 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);