On 06/22/2013 03:21 PM, Peter Hurley wrote:
> On 06/21/2013 07:51 PM, Tim Chen wrote:
>> Doing cmpxchg will cause cache bouncing when checking
>> sem->count. This could cause scalability issue
>> in a large machine (e.g. a 80 cores box).
>>
>> A pre-read of sem->count can mitigate this.
>>
>> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxx>
>> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
>> ---
>> include/asm-generic/rwsem.h | 8 ++++----
>> lib/rwsem.c | 21 +++++++++++++--------
>> 2 files changed, 17 insertions(+), 12 deletions(-)
>>
>> diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
>> index bb1e2cd..052d973 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))
> ^^^^^^^^^^^
>
> This is probably not what you want.
>
this function logical is quite simple. check the sem->count before
cmpxchg is no harm this logical.
So could you like to tell us what should we want?
>
>> + 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..2072af5 100644
>> --- a/lib/rwsem.c
>> +++ b/lib/rwsem.c
>> @@ -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;
>
> Please put these flow control changes in a separate patch.
I had sent the split patches to Tim&Davidlohr. They will send them out
as a single patchset.
>
>
>> }
>>
>> /* Writers might steal the lock before we grant it to the next
>> reader.
>> @@ -85,15 +85,21 @@ __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) {
>> + /* A writer stole the lock. */
>> + if (sem->count < RWSEM_WAITING_BIAS)
>> + return sem;
>
> I'm all for structured looping instead of goto labels but this optimization
> is only useful on the 1st iteration. IOW, on the second iteration you
> already
> know that you need to try for reclaiming the lock.
>
sorry. could you like to say more clear, what's the 1st or 2nd iteration
or others?
>
>> +
>> + 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 +142,6 @@ __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;
>> }
>
>
> Alex and Tim,
>
> Was there a v1 of this series; ie., is this v2 (or higher)?
>
> How are you validating lock correctness/behavior with this series?
some benchmark tested against this patch, mainly aim7. plus by eyes, we
didn't change the logical except check the lock value before do locking
>
> Regards,
> Peter Hurley
>
--
Thanks
Alex
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