Messages in this thread | | | Date | Wed, 11 May 2016 11:33:28 -0700 | From | Davidlohr Bueso <> | Subject | Re: [PATCH] locking/rwsem: Optimize write lock slowpath |
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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
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