Messages in this thread | | | Date | Thu, 18 Nov 2021 13:12:03 -0500 | Subject | Re: [PATCH] locking/rwsem: Optimize down_read_trylock() under highly contended case | From | Waiman Long <> |
| |
On 11/18/21 07:57, Peter Zijlstra wrote: > On Thu, Nov 18, 2021 at 05:44:55PM +0800, Muchun Song wrote: > >> By using the above benchmark, the real executing time on a x86-64 system >> before and after the patch were: > What kind of x86_64 ? > >> Before Patch After Patch >> # of Threads real real reduced by >> ------------ ------ ------ ---------- >> 1 65,373 65,206 ~0.0% >> 4 15,467 15,378 ~0.5% >> 40 6,214 5,528 ~11.0% >> >> For the uncontended case, the new down_read_trylock() is the same as >> before. For the contended cases, the new down_read_trylock() is faster >> than before. The more contended, the more fast. >> >> Signed-off-by: Muchun Song <songmuchun@bytedance.com> >> --- >> kernel/locking/rwsem.c | 11 ++++------- >> 1 file changed, 4 insertions(+), 7 deletions(-) >> >> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c >> index c51387a43265..ef2b2a3f508c 100644 >> --- a/kernel/locking/rwsem.c >> +++ b/kernel/locking/rwsem.c >> @@ -1249,17 +1249,14 @@ static inline int __down_read_trylock(struct rw_semaphore *sem) >> >> DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem); >> >> - /* >> - * Optimize for the case when the rwsem is not locked at all. >> - */ >> - tmp = RWSEM_UNLOCKED_VALUE; >> - do { >> + tmp = atomic_long_read(&sem->count); >> + while (!(tmp & RWSEM_READ_FAILED_MASK)) { >> if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, >> - tmp + RWSEM_READER_BIAS)) { >> + tmp + RWSEM_READER_BIAS)) { >> rwsem_set_reader_owned(sem); >> return 1; >> } >> - } while (!(tmp & RWSEM_READ_FAILED_MASK)); >> + } >> return 0; >> } > This is weird... so the only difference is that leading load, but given > contention you'd expect that load to stall, also, given it's a > non-exclusive load, to get stolen by a competing CPU. Whereas the old > code would start with a cmpxchg, which obviously will also stall, but > does an exclusive load. > > And the thinking is that the exclusive load and the presence of the > cmpxchg loop would keep the line on that CPU for a little while and > progress is made. > > Clearly this isn't working as expected. Also I suppose it would need > wider testing...
For a contended case, doing a shared read first doing an exclusive cmpxchg can certainly help to reduce cacheline trashing. I have no objection to making this change.
I believe most of the other trylock functions do a read first before doing an atomic operation. In essence, we assume the use of trylock means the callers are expecting an contended lock whereas callers of regular *lock() function are expecting an uncontended lock.
Acked-by: Waiman Long <longman@redhat.com>
-Longman
| |