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    SubjectRe: Performance regression from switching lock to rw-sem for anon-vma tree
    On 06/14/2013 07:43 AM, Davidlohr Bueso wrote:
    > I was hoping that the lack of spin on owner was the main difference with
    > rwsems and am/was in the middle of implementing it. Could you send your
    > patch so I can give it a try on my workloads?
    >
    > Note that there have been a few recent (3.10) changes to mutexes that
    > give a nice performance boost, specially on large systems, most
    > noticeably:
    >
    > commit 2bd2c92c (mutex: Make more scalable by doing less atomic
    > operations)
    >
    > commit 0dc8c730 (mutex: Queue mutex spinners with MCS lock to reduce
    > cacheline contention)
    >
    > It might be worth looking into doing something similar to commit
    > 0dc8c730, in addition to the optimistic spinning.

    It is a good tunning for large machine. I just following what the commit
    0dc8c730 done, give a RFC patch here. I tried it on my NHM EP machine. seems no
    clear help on aim7. but maybe it is helpful on large machine. :)


    diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
    index bb1e2cd..240729a 100644
    --- a/include/asm-generic/rwsem.h
    +++ b/include/asm-generic/rwsem.h
    @@ -70,11 +70,11 @@ static inline void __down_write(struct rw_semaphore *sem)

    static inline int __down_write_trylock(struct rw_semaphore *sem)
    {
    - long tmp;
    + if (unlikely(&sem->count != RWSEM_UNLOCKED_VALUE))
    + return 0;

    - tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
    - RWSEM_ACTIVE_WRITE_BIAS);
    - return tmp == RWSEM_UNLOCKED_VALUE;
    + return cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
    + RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_UNLOCKED_VALUE;
    }

    /*
    diff --git a/lib/rwsem.c b/lib/rwsem.c
    index 19c5fa9..9e54e20 100644
    --- a/lib/rwsem.c
    +++ b/lib/rwsem.c
    @@ -64,7 +64,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
    struct rwsem_waiter *waiter;
    struct task_struct *tsk;
    struct list_head *next;
    - long oldcount, woken, loop, adjustment;
    + long woken, loop, adjustment;

    waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
    if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
    @@ -75,7 +75,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
    * will block as they will notice the queued writer.
    */
    wake_up_process(waiter->task);
    - goto out;
    + return sem;
    }

    /* Writers might steal the lock before we grant it to the next reader.
    @@ -85,15 +85,28 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
    adjustment = 0;
    if (wake_type != RWSEM_WAKE_READ_OWNED) {
    adjustment = RWSEM_ACTIVE_READ_BIAS;
    - try_reader_grant:
    - oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
    - if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
    - /* A writer stole the lock. Undo our reader grant. */
    + while (1) {
    + long oldcount;
    +
    + /* A writer stole the lock. */
    + if (unlikely(sem->count & RWSEM_ACTIVE_MASK))
    + return sem;
    +
    + if (unlikely(sem->count < RWSEM_WAITING_BIAS)) {
    + cpu_relax();
    + continue;
    + }
    +
    + oldcount = rwsem_atomic_update(adjustment, sem)
    + - adjustment;
    + if (likely(oldcount >= RWSEM_WAITING_BIAS))
    + break;
    +
    + /* A writer stole the lock. Undo our reader grant. */
    if (rwsem_atomic_update(-adjustment, sem) &
    RWSEM_ACTIVE_MASK)
    - goto out;
    + return sem;
    /* Last active locker left. Retry waking readers. */
    - goto try_reader_grant;
    }
    }

    @@ -136,7 +149,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
    sem->wait_list.next = next;
    next->prev = &sem->wait_list;

    - out:
    return sem;
    }

    --
    Thanks
    Alex

    --
    Thanks
    Alex


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