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[PATCH 4.12 20/27] sched/numa: Use down_read_trylock() for the mmap_sem

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4.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Vlastimil Babka <vbabka@suse.cz>

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.

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>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -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);

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[PATCH 4.12 00/27] 4.12.1-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:20 +0200
  [PATCH 4.12 14/27] tpm: Issue a TPM2_Shutdown for TPM2 devices. Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:20 +0200
  [PATCH 4.12 26/27] xen: avoid deadlock in xenbus driver Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:20 +0200
  [PATCH 4.12 05/27] usb: usbip: set buffer pointers to NULL after free Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:20 +0200
  [PATCH 4.12 02/27] RDMA/uverbs: Check port number supplied by user verbs cmds Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:20 +0200
  [PATCH 4.12 19/27] sched/core: Implement new approach to scale select_idle_cpu() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:20 +0200
  [PATCH 4.12 01/27] driver core: platform: fix race condition with driver_override Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:20 +0200
  [PATCH 4.12 06/27] Add USB quirk for HVR-950q to avoid intermittent device resets Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:20 +0200
  [PATCH 4.12 25/27] sched/numa: Hide numa_wake_affine() from UP build Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:30 +0200
  [PATCH 4.12 17/27] x86/uaccess: Optimize copy_user_enhanced_fast_string() for short strings Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:30 +0200
  [PATCH 4.12 04/27] USB: serial: cp210x: add ID for CEL EM3588 USB ZigBee stick Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  [PATCH 4.12 13/27] Add "shutdown" to "struct class". Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  [PATCH 4.12 11/27] xhci: Limit USB2 port wake support for AMD Promontory hosts Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  [PATCH 4.12 18/27] sched/fair, cpumask: Export for_each_cpu_wrap() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  [PATCH 4.12 23/27] sched/numa: Implement NUMA node level wake_affine() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  [PATCH 4.12 24/27] sched/fair: Remove effective_load() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  [PATCH 4.12 15/27] tpm: fix a kernel memory leak in tpm-sysfs.c Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  [PATCH 4.12 20/27] sched/numa: Use down_read_trylock() for the mmap_sem Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  [PATCH 4.12 08/27] USB: core: fix device node leak Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  [PATCH 4.12 12/27] gfs2: Fix glock rhashtable rcu bug Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  [PATCH 4.12 03/27] usb: dwc3: replace %p with %pK Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  [PATCH 4.12 27/27] crypto: drbg - Fixes panic in wait_for_completion call Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  [PATCH 4.12 22/27] sched/fair: Simplify wake_affine() for the single socket case Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  [PATCH 4.12 07/27] usb: Fix typo in the definition of Endpoint[out]Request Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:40 +0200
  Re: [PATCH 4.12 00/27] 4.12.1-stable review Guenter Roeck <linux@roeck-us.net> - 2017-07-11 03:30 +0200
    Re: [PATCH 4.12 00/27] 4.12.1-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-11 11:50 +0200

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