Re: [PATCH] locking/rwsem: Optimize write lock slowpath

[Davidlohr Bueso <dave@stgolabs.net>]

On Wed, 11 May 2016, Peter Zijlstra wrote:

>On Mon, May 09, 2016 at 12:16:37PM -0700, Jason Low wrote:
>> When acquiring the rwsem write lock in the slowpath, we first try
>> to set count to RWSEM_WAITING_BIAS. When that is successful,
>> we then atomically add the RWSEM_WAITING_BIAS in cases where
>> there are other tasks on the wait list. This causes write lock
>> operations to often issue multiple atomic operations.
>>
>> We can instead make the list_is_singular() check first, and then
>> set the count accordingly, so that we issue at most 1 atomic
>> operation when acquiring the write lock and reduce unnecessary
>> cacheline contention.
>>
>> Signed-off-by: Jason Low <jason.low2@hp.com>

Acked-by: Davidlohr Bueso <dave@stgolabs.net>

(one nit: the patch title could be more informative to what
optimization we are talking about here... ie: reduce atomic ops
in writer slowpath' or something.)


>> ---
>>  kernel/locking/rwsem-xadd.c | 20 +++++++++++++-------
>>  1 file changed, 13 insertions(+), 7 deletions(-)
>>
>> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
>> index df4dcb8..23c33e6 100644
>> --- a/kernel/locking/rwsem-xadd.c
>> +++ b/kernel/locking/rwsem-xadd.c
>> @@ -258,14 +258,20 @@ EXPORT_SYMBOL(rwsem_down_read_failed);
>>  static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
>>  {
>>	/*
>> +	 * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
>>	 */
>> +	if (count != RWSEM_WAITING_BIAS)
>> +		return false;
>> +
>> +	/*
>> +	 * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
>> +	 * are other tasks on the wait list, we need to add on WAITING_BIAS.
>> +	 */
>> +	count = list_is_singular(&sem->wait_list) ?
>> +			RWSEM_ACTIVE_WRITE_BIAS :
>> +			RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;
>> +
>> +	if (cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count) == RWSEM_WAITING_BIAS) {
>>		rwsem_set_owner(sem);
>>		return true;
>>	}
>
>Right; so that whole thing works because we're holding sem->wait_lock.
>Should we clarify that someplace?

Yes exactly, rwsem_try_write_lock() is always called with the wait_lock held,
unlike the unqueued cousin.

Thanks,
Davidlohr