Re: [git pull] scheduler fixes

Matthew Wilcox
2008-05-11T11:03:18Z

On Thu, May 08, 2008 at 05:10:28PM +0200, Ingo Molnar wrote:
> @@ -258,7 +256,5 @@ static noinline void __sched __up(struct semaphore *sem)
> {
> struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
> struct semaphore_waiter, list);
> - list_del(&waiter->list);
> - waiter->up = 1;
> wake_up_process(waiter->task);
> }

This might be the problem that causes the missing wakeups. If you have a
semaphore with n=2, and four processes calling down(), tasks A and B
acquire the semaphore and tasks C and D go to sleep. Task A calls up()
and wakes up C. Then task B calls up() and doesn't wake up anyone
because C hasn't run yet. I think we need another wakeup when task C
finishes in __down_common, like this (on top of your patch):

diff --git a/kernel/semaphore.c b/kernel/semaphore.c
index 5e41217..e520ad4 100644
--- a/kernel/semaphore.c
+++ b/kernel/semaphore.c
@@ -229,6 +229,11 @@ static inline int __sched __down_common(struct semaphore *sem, long state,
}

list_del(&waiter.list);
+
+ /* It's possible we need to wake up the next task on the list too */
+ if (unlikely(sem->count > 1) && !list_empty(&sem->wait_list))
+ __up(sem);
+
return ret;
}

Sven, can you try this with your workload? I suspect this might be it
because XFS does use semaphores with n>1.

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