@@ -180,16 +180,16 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
}
/*
- * Grant an infinite number of read locks to the readers at the front
- * of the queue. We know that woken will be at least 1 as we accounted
- * for above. Note we increment the 'active part' of the count by the
+ * Grant an infinite number of read locks to all the readers in the
+ * queue. We know that woken will be at least 1 as we accounted for
+ * above. Note we increment the 'active part' of the count by the
* number of readers before waking any processes up.
*/
list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
struct task_struct *tsk;
if (waiter->type == RWSEM_WAITING_FOR_WRITE)
- break;
+ continue;
woken++;
tsk = waiter->task;
When the front of the wait queue is a reader, other readers immediately following the first reader will also be woken up at the same time. However, if there is a writer in between. Those readers behind the writer will not be woken up. Because of optimistic spinning, the lock acquisition order is not FIFO anyway. The lock handoff mechanism will ensure that lock starvation will not happen. Assuming that the lock hold times of the other readers still in the queue will be about the same as the readers that are being woken up, there is really not much additional cost other than the additional latency due to the wakeup of additional tasks by the waker. Therefore all the readers in the queue are woken up when the first waiter is a reader to improve reader throughput. With a locking microbenchmark running on 5.0 based kernel, the total locking rates (in kops/s) of the benchmark on a 4-socket 56-core x86-64 system with equal numbers of readers and writers before all the reader spining patches, before this patch and after this patch were as follows: # of Threads Pre-rspin Pre-Patch Post-patch ------------ --------- --------- ---------- 2 1,926 8,057 7,397 4 1,391 7,680 6,161 8 716 7,284 6,405 16 618 6,542 6,768 32 501 1,449 6,550 64 61 480 5,548 112 75 769 5,216 At low contention level, there is a slight drop in performance. At high contention level, however, this patch gives a big performance boost. Signed-off-by: Waiman Long <longman@redhat.com> --- kernel/locking/rwsem-xadd.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)