sched/numa: Use down_read_trylock() for the mmap_sem

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.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170515131316.21909-1-vbabka@suse.cz
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -2470,7 +2470,8 @@ 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);