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 [thread overview]
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 other threads:[~2017-05-15 13:13 UTC|newest]
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 ` [PATCH] sched/numa: use down_read_trylock for mmap_sem 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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