Patch "sched/numa: Use down_read_trylock() for the mmap_sem" has been added to the 4.12-stable tree

This is a note to let you know that I've just added the patch titled

sched/numa: Use down_read_trylock() for the mmap_sem

to the 4.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sched-numa-use-down_read_trylock-for-the-mmap_sem.patch
and it can be found in the queue-4.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Vlastimil Babka <[email protected]>
Date: Mon, 15 May 2017 15:13:16 +0200
Subject: sched/numa: Use down_read_trylock() for the mmap_sem

From: Vlastimil Babka <[email protected]>

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:[<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.

---
 kernel/sched/fair.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

@@ -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);

Patches currently in stable-queue which might be from [email protected] are

queue-4.12/sched-numa-use-down_read_trylock-for-the-mmap_sem.patch