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    SubjectRe: REGRESSION: Performance regressions from switching anon_vma->lock to mutex
    > On 2.6.39, the contention of anon_vma->lock occupies 3.25% of cpu.
    > However, after the switch of the lock to mutex on 3.0-rc2, the mutex
    > acquisition jumps to 18.6% of cpu. This seems to be the main cause of
    > the 52% throughput regression.
    >
    This patch makes the mutex in Tim's workload take a bit less CPU time
    (4% down) but it doesn't really fix the regression. When spinning for a
    value it's always better to read it first before attempting to write it.
    This saves expensive operations on the interconnect.

    So it's not really a fix for this, but may be a slight improvement for
    other workloads.

    -Andi

    From 34d4c1e579b3dfbc9a01967185835f5829bd52f0 Mon Sep 17 00:00:00 2001
    From: Andi Kleen <ak@linux.intel.com>
    Date: Tue, 14 Jun 2011 16:27:54 -0700
    Subject: [PATCH] mutex: while spinning read count before attempting cmpxchg

    Under heavy contention it's better to read first before trying
    to do an atomic operation on the interconnect.

    This gives a few percent improvement for the mutex CPU time
    under heavy contention and likely saves some power too.

    Signed-off-by: Andi Kleen <ak@linux.intel.com>
    ---
    kernel/mutex.c | 3 ++-
    1 files changed, 2 insertions(+), 1 deletions(-)

    diff --git a/kernel/mutex.c b/kernel/mutex.c
    index d607ed5..1abffa9 100644
    --- a/kernel/mutex.c
    +++ b/kernel/mutex.c
    @@ -170,7 +170,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
    if (owner && !mutex_spin_on_owner(lock, owner))
    break;

    - if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
    + if (atomic_read(&lock->count) == 1 &&
    + atomic_cmpxchg(&lock->count, 1, 0) == 1) {
    lock_acquired(&lock->dep_map, ip);
    mutex_set_owner(lock);
    preempt_enable();
    --
    1.7.4.4


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