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Subject[PATCH 4.4 062/190] sched/numa: Use down_read_trylock() for the mmap_sem
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:[<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 <>
Signed-off-by: Peter Zijlstra (Intel) <>
Acked-by: Rik van Riel <>
Acked-by: Mel Gorman <>
Cc: Linus Torvalds <>
Cc: Peter Zijlstra <>
Cc: Thomas Gleixner <>
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

- down_read(&mm->mmap_sem);
+ if (!down_read_trylock(&mm->mmap_sem))
+ return;
vma = find_vma(mm, start);
if (!vma) {

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