Commit 70800c3c authored Aug 05, 2016 by Davidlohr Bueso Committed by Ingo Molnar Aug 18, 2016

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 <dbueso@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
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 <mingo@kernel.org>

parent c2867bba

kernel/locking/rwsem-xadd.c

+26 −32

Original line number	Diff line number	Diff line
		@@ -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 !