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    Subject[PATCH AUTOSEL for 4.4 063/167] sched/numa: Use down_read_trylock() for the mmap_sem
    Date
    From: Vlastimil Babka <vbabka@suse.cz>

    [ 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 <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: Sasha Levin <alexander.levin@microsoft.com>
    ---
    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 812069b66f47..3b136fb4422c 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -2223,7 +2223,8 @@ 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);
    --
    2.14.1
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