From 70800c3c0cc525baa38fd0fe4660f2c27f1bfeeb Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Fri, 5 Aug 2016 01:04:45 -0700 Subject: [PATCH] locking/rwsem: Scan the wait_list for readers only once When wanting to wakeup readers, __rwsem_mark_wakeup() currently iterates the wait_list twice while looking to wakeup the first N queued reader-tasks. While this can be quite inefficient, it was there such that a awoken reader would be first and foremost acknowledged by the lock counter. Keeping the same logic, we can further benefit from the use of wake_qs and avoid entirely the first wait_list iteration that sets the counter as wake_up_process() isn't going to occur right away, and therefore we maintain the counter->list order of going about things. Other than saving cycles with O(n) "scanning", this change also nicely cleans up a good chunk of __rwsem_mark_wakeup(); both visually and less tedious to read. For example, the following improvements where seen on some will it scale microbenchmarks, on a 48-core Haswell: v4.7 v4.7-rwsem-v1 Hmean signal1-processes-8 5792691.42 ( 0.00%) 5771971.04 ( -0.36%) Hmean signal1-processes-12 6081199.96 ( 0.00%) 6072174.38 ( -0.15%) Hmean signal1-processes-21 3071137.71 ( 0.00%) 3041336.72 ( -0.97%) Hmean signal1-processes-48 3712039.98 ( 0.00%) 3708113.59 ( -0.11%) Hmean signal1-processes-79 4464573.45 ( 0.00%) 4682798.66 ( 4.89%) Hmean signal1-processes-110 4486842.01 ( 0.00%) 4633781.71 ( 3.27%) Hmean signal1-processes-141 4611816.83 ( 0.00%) 4692725.38 ( 1.75%) Hmean signal1-processes-172 4638157.05 ( 0.00%) 4714387.86 ( 1.64%) Hmean signal1-processes-203 4465077.80 ( 0.00%) 4690348.07 ( 5.05%) Hmean signal1-processes-224 4410433.74 ( 0.00%) 4687534.43 ( 6.28%) Stddev signal1-processes-8 6360.47 ( 0.00%) 8455.31 ( 32.94%) Stddev signal1-processes-12 4004.98 ( 0.00%) 9156.13 (128.62%) Stddev signal1-processes-21 3273.14 ( 0.00%) 5016.80 ( 53.27%) Stddev signal1-processes-48 28420.25 ( 0.00%) 26576.22 ( -6.49%) Stddev signal1-processes-79 22038.34 ( 0.00%) 18992.70 (-13.82%) Stddev signal1-processes-110 23226.93 ( 0.00%) 17245.79 (-25.75%) Stddev signal1-processes-141 6358.98 ( 0.00%) 7636.14 ( 20.08%) Stddev signal1-processes-172 9523.70 ( 0.00%) 4824.75 (-49.34%) Stddev signal1-processes-203 13915.33 ( 0.00%) 9326.33 (-32.98%) Stddev signal1-processes-224 15573.94 ( 0.00%) 10613.82 (-31.85%) Other runs that saw improvements include context_switch and pipe; and as expected, this is particularly highlighted on larger thread counts as it becomes more expensive to walk the list twice. No change in wakeup ordering or semantics. Signed-off-by: Davidlohr Bueso Signed-off-by: Peter Zijlstra (Intel) Cc: Andrew Morton Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Waiman.Long@hp.com Cc: dave@stgolabs.net Cc: jason.low2@hpe.com Cc: wanpeng.li@hotmail.com Link: http://lkml.kernel.org/r/1470384285-32163-4-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar --- kernel/locking/rwsem-xadd.c | 58 +++++++++++++++++-------------------- 1 file changed, 26 insertions(+), 32 deletions(-) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index e02fe3289b5a..2337b4bb2366 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -125,12 +125,14 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type, struct wake_q_head *wake_q) { - struct rwsem_waiter *waiter; - struct task_struct *tsk; - struct list_head *next; - long loop, oldcount, woken = 0, adjustment = 0; + struct rwsem_waiter *waiter, *tmp; + long oldcount, woken = 0, adjustment = 0; - waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); + /* + * Take a peek at the queue head waiter such that we can determine + * the wakeup(s) to perform. + */ + waiter = list_first_entry(&sem->wait_list, struct rwsem_waiter, list); if (waiter->type == RWSEM_WAITING_FOR_WRITE) { if (wake_type == RWSEM_WAKE_ANY) { @@ -180,36 +182,21 @@ 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. Note we increment the 'active part' of the count by - * the number of readers before waking any processes up. + * 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 + * number of readers before waking any processes up. */ - do { - woken++; + list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) { + struct task_struct *tsk; - if (waiter->list.next == &sem->wait_list) + if (waiter->type == RWSEM_WAITING_FOR_WRITE) break; - waiter = list_entry(waiter->list.next, - struct rwsem_waiter, list); - - } while (waiter->type != RWSEM_WAITING_FOR_WRITE); - - adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment; - if (waiter->type != RWSEM_WAITING_FOR_WRITE) - /* hit end of list above */ - adjustment -= RWSEM_WAITING_BIAS; - - if (adjustment) - atomic_long_add(adjustment, &sem->count); - - next = sem->wait_list.next; - loop = woken; - do { - waiter = list_entry(next, struct rwsem_waiter, list); - next = waiter->list.next; + woken++; tsk = waiter->task; wake_q_add(wake_q, tsk); + list_del(&waiter->list); /* * Ensure that the last operation is setting the reader * waiter to nil such that rwsem_down_read_failed() cannot @@ -217,10 +204,16 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, * to the task to wakeup. */ smp_store_release(&waiter->task, NULL); - } while (--loop); + } - sem->wait_list.next = next; - next->prev = &sem->wait_list; + adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment; + if (list_empty(&sem->wait_list)) { + /* hit end of list above */ + adjustment -= RWSEM_WAITING_BIAS; + } + + if (adjustment) + atomic_long_add(adjustment, &sem->count); } /* @@ -245,7 +238,8 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) /* we're now waiting on the lock, but no longer actively locking */ count = atomic_long_add_return(adjustment, &sem->count); - /* If there are no active locks, wake the front queued process(es). + /* + * If there are no active locks, wake the front queued process(es). * * If there are no writers and we are first in the queue, * wake our own waiter to join the existing active readers ! -- 2.20.1