From: Vlastimil Babka <vbabka@suse.cz> To: Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org> Cc: Mel Gorman <mgorman@techsingularity.net>, Rik van Riel <riel@redhat.com>, linux-kernel@vger.kernel.org, Vlastimil Babka <vbabka@suse.cz> Subject: [PATCH] sched/numa: use down_read_trylock for mmap_sem Date: Mon, 15 May 2017 15:13:16 +0200 Message-ID: <20170515131316.21909-1-vbabka@suse.cz> (raw) A customer has reported a soft-lockup when running a proprietary intensive memory stress test, where the trace on multiple CPU's looks like this: RIP: 0010:[<ffffffff810c53fe>] [<ffffffff810c53fe>] native_queued_spin_lock_slowpath+0x10e/0x190 ... Call Trace: [<ffffffff81182d07>] queued_spin_lock_slowpath+0x7/0xa [<ffffffff811bc331>] change_protection_range+0x3b1/0x930 [<ffffffff811d4be8>] change_prot_numa+0x18/0x30 [<ffffffff810adefe>] task_numa_work+0x1fe/0x310 [<ffffffff81098322>] 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 <vbabka@suse.cz> --- kernel/sched/fair.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index dea138964b91..d70f9026defc 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2475,7 +2475,8 @@ void task_numa_work(struct callback_head *work) 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); -- 2.12.2
next reply index Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-05-15 13:13 Vlastimil Babka [this message] 2017-05-15 14:27 ` Rik van Riel 2017-05-15 14:35 ` Mel Gorman 2017-05-16 8:15 ` Peter Zijlstra 2017-05-23 8:47 ` [tip:sched/core] sched/numa: Use down_read_trylock() for the mmap_sem tip-bot for Vlastimil Babka
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