Messages in this thread | | | Subject | Re: [PATCH v2] locking/rwsem: Take read lock immediate if queue empty with no writer | From | Waiman Long <> | Date | Wed, 18 Jul 2018 11:15:40 -0400 |
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On 07/13/2018 02:30 PM, Waiman Long wrote: > It was discovered that a constant stream of readers might cause the > count to go negative most of the time after an initial trigger by a > writer even if no writer was present afterward. As a result, most of the > readers would have to go through the slowpath reducing their performance. > > To avoid that from happening, an additional check is added to detect > the special case that the reader in the critical section is the only > one in the wait queue and no writer is present. When that happens, it > can just have the lock and return immediately without further action. > Other incoming readers won't see a waiter is present and be forced > into the slowpath. > > The additional code is in the slowpath and so should not have an impact > on rwsem performance. However, in the special case listed above, it may > greatly improve performance. > > The issue was found in a customer site where they had an application > that pounded on the pread64 syscalls heavily on an XFS filesystem. The > application was run in a recent 4-socket boxes with a lot of CPUs. They > saw significant spinlock contention in the rwsem_down_read_failed() call. > With this patch applied, the system CPU usage went from 85% to 57%, > and the spinlock contention in the pread64 syscalls was gone. > > v2: Add customer testing results and remove wording that may cause > confusion. > > Signed-off-by: Waiman Long <longman@redhat.com> > --- > kernel/locking/rwsem-xadd.c | 13 ++++++++++++- > 1 file changed, 12 insertions(+), 1 deletion(-) > > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c > index 3064c50..bf0570e 100644 > --- a/kernel/locking/rwsem-xadd.c > +++ b/kernel/locking/rwsem-xadd.c > @@ -233,8 +233,19 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, > waiter.type = RWSEM_WAITING_FOR_READ; > > raw_spin_lock_irq(&sem->wait_lock); > - if (list_empty(&sem->wait_list)) > + if (list_empty(&sem->wait_list)) { > + /* > + * In the unlikely event that the task is the only one in > + * the wait queue and a writer isn't present, it can have > + * the lock and return immediately without going through > + * the remaining slowpath code. > + */ > + if (unlikely(atomic_long_read(&sem->count) >= 0)) { > + raw_spin_unlock_irq(&sem->wait_lock); > + return sem; > + } > adjustment += RWSEM_WAITING_BIAS; > + } > list_add_tail(&waiter.list, &sem->wait_list); > > /* we're now waiting on the lock, but no longer actively locking */
Ping!
Any comment on this one?
Cheers, Longman
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