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


Nick Piggin wrote:

>
> The following patch moves the rwsem's up_read wake ups out of the
> wait_lock. Wakeups need to aquire a runqueue lock which is probably
> getting contended. The following graph is a best of 3 runs average.
> http://www.kerneltrap.org/~npiggin/rwsem.png
>
> The part to look at is the tail. I need to do some more testing to
> see if its significant.


Err, here's one with a comment changed to match, sorry.


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


linux-2.6-npiggin/lib/rwsem.c | 41 +++++++++++++++++++++++++++--------------
1 files changed, 27 insertions(+), 14 deletions(-)

diff -puN lib/rwsem.c~rwsem-scale lib/rwsem.c
--- linux-2.6/lib/rwsem.c~rwsem-scale 2003-12-12 13:01:28.000000000 +1100
+++ linux-2.6-npiggin/lib/rwsem.c 2003-12-12 13:01:45.000000000 +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 +105,21 @@ 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_init(&waiter->wake_list);
+ mb();
+ wake_up_process(waiter->task);
+ }
rwsemtrace(sem,"Leaving __rwsem_do_wake");
return sem;

@@ -130,9 +143,9 @@ 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 +156,8 @@ 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 +217,8 @@ 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 +239,8 @@ 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;
_
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