Re: [PATCH] improve rwsem scalability (was Re: [CFT][RFC] HT scheduler)

[Nick Piggin]

Martin J. Bligh wrote:

> > > What do you think? Is there any other sorts of benchmarks I should try?
> > > The improvement I think is significant, although volanomark is quite
> > > erratic and doesn't show it well.
> > >
> > > I don't see any problem with moving the wakeups out of the rwsem's 
> > > spinlock.
> > Actually my implementation does have a race because list_del_init isn't
> > atomic. Shouldn't be difficult to fix if anyone cares about it... otherwise
> > I won't bother.
>
> If you can fix it up, I'll get people here to do some more testing on the
> patch on other benchmarks, etc.

OK, this one should work. There isn't much that uses rwsems, but mmap_sem
is the obvious one.

So if the patch helps anywhere, it will be something heavily threaded, that
is taking the mmap sem a lot (mostly for reading, sometimes writing), with
a lot of CPUs and a lot of runqueue activity (ie waking up, sleeping, tasks
running).


Move rwsem's up_read wakeups out of the semaphore's wait_lock


 linux-2.6-npiggin/lib/rwsem.c |   42 ++++++++++++++++++++++++++++--------------
 1 files changed, 28 insertions(+), 14 deletions(-)

diff -puN lib/rwsem.c~rwsem-scale lib/rwsem.c
--- linux-2.6/lib/rwsem.c~rwsem-scale	2003-12-20 02:17:36.000000000 +1100
+++ linux-2.6-npiggin/lib/rwsem.c	2003-12-20 02:23:59.000000000 +1100
@@ -8,7 +8,7 @@
 #include <linux/module.h>
 
 struct rwsem_waiter {
-	struct list_head	list;
+	struct list_head	list, wake_list;
 	struct task_struct	*task;
 	unsigned int		flags;
 #define RWSEM_WAITING_FOR_READ	0x00000001
@@ -35,9 +35,12 @@ void rwsemtrace(struct rw_semaphore *sem
  * - the spinlock must be held by the caller
  * - woken process blocks are discarded from the list after having flags zeroised
  * - writers are only woken if wakewrite is non-zero
+ *
+ * The spinlock will be dropped by this function
  */
 static inline struct rw_semaphore *__rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
 {
+	LIST_HEAD(wake_list);
 	struct rwsem_waiter *waiter;
 	struct list_head *next;
 	signed long oldcount;
@@ -65,7 +68,8 @@ static inline struct rw_semaphore *__rws
 
 	list_del(&waiter->list);
 	waiter->flags = 0;
-	wake_up_process(waiter->task);
+	if (list_empty(&waiter->wake_list))
+		list_add_tail(&waiter->wake_list, &wake_list);
 	goto out;
 
 	/* don't want to wake any writers */
@@ -74,9 +78,10 @@ static inline struct rw_semaphore *__rws
 	if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
 		goto out;
 
-	/* 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 (less one
-	 *   for the activity decrement we've already done) before waking any processes up
+	/* 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 (less one for the activity decrement we've
+	 * already done) before waking any processes up
 	 */
  readers_only:
 	woken = 0;
@@ -100,13 +105,22 @@ static inline struct rw_semaphore *__rws
 		waiter = list_entry(next,struct rwsem_waiter,list);
 		next = waiter->list.next;
 		waiter->flags = 0;
-		wake_up_process(waiter->task);
+		if (list_empty(&waiter->wake_list))
+			list_add_tail(&waiter->wake_list, &wake_list);
 	}
 
 	sem->wait_list.next = next;
 	next->prev = &sem->wait_list;
 
  out:
+	spin_unlock(&sem->wait_lock);
+	while (!list_empty(&wake_list)) {
+		waiter = list_entry(wake_list.next,struct rwsem_waiter,wake_list);
+		list_del(&waiter->wake_list);
+		waiter->wake_list.next = &waiter->wake_list;
+		wmb(); /* Mustn't lose wakeups */
+		wake_up_process(waiter->task);
+	}
 	rwsemtrace(sem,"Leaving __rwsem_do_wake");
 	return sem;
 
@@ -130,9 +144,9 @@ static inline struct rw_semaphore *rwsem
 	set_task_state(tsk,TASK_UNINTERRUPTIBLE);
 
 	/* set up my own style of waitqueue */
-	spin_lock(&sem->wait_lock);
 	waiter->task = tsk;
-
+	INIT_LIST_HEAD(&waiter->wake_list);
+	spin_lock(&sem->wait_lock);
 	list_add_tail(&waiter->list,&sem->wait_list);
 
 	/* note that we're now waiting on the lock, but no longer actively read-locking */
@@ -143,8 +157,8 @@ static inline struct rw_semaphore *rwsem
 	 */
 	if (!(count & RWSEM_ACTIVE_MASK))
 		sem = __rwsem_do_wake(sem,1);
-
-	spin_unlock(&sem->wait_lock);
+	else
+		spin_unlock(&sem->wait_lock);
 
 	/* wait to be given the lock */
 	for (;;) {
@@ -204,8 +218,8 @@ struct rw_semaphore *rwsem_wake(struct r
 	/* do nothing if list empty */
 	if (!list_empty(&sem->wait_list))
 		sem = __rwsem_do_wake(sem,1);
-
-	spin_unlock(&sem->wait_lock);
+	else
+		spin_unlock(&sem->wait_lock);
 
 	rwsemtrace(sem,"Leaving rwsem_wake");
 
@@ -226,8 +240,8 @@ struct rw_semaphore *rwsem_downgrade_wak
 	/* do nothing if list empty */
 	if (!list_empty(&sem->wait_list))
 		sem = __rwsem_do_wake(sem,0);
-
-	spin_unlock(&sem->wait_lock);
+	else
+		spin_unlock(&sem->wait_lock);
 
 	rwsemtrace(sem,"Leaving rwsem_downgrade_wake");
 	return sem;

_