Re: [git pull] scheduler fixes

Matthew Wilcox
2008-05-11T14:22:33Z

On Sun, May 11, 2008 at 03:54:14PM +0200, Ingo Molnar wrote:
> the gain is rather obvious: two parallel up()s (or just up()s which come
> close enough after each other) will wake up two tasks in parallel. With
> your patch, the first guy wakes up and then it wakes up the second guy.
> I.e. your patch serializes the wakeup chain, mine keeps it parallel.

Yup. I explained why that's actually beneficial in an earlier email.

> the cache line dirtying is rather secondary to any solution - the first
> goal for any locking primitive is to get scheduling precise: to not wake
> up more tasks than optimal and to not wake up less tasks than optimal.

That's a laudable goal, but ultimately it's secondary to performance (or
this thread wouldn't exist).

> i.e. can you see any conceptual hole in the patch below?

No conceptual holes, just a performance one.

Either we want your patch below or mine; definitely not both.

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;
}

> Ingo
>
> ---
> kernel/semaphore.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> Index: linux/kernel/semaphore.c
> ===================================================================
> --- linux.orig/kernel/semaphore.c
> +++ linux/kernel/semaphore.c
> @@ -258,5 +258,11 @@ static noinline void __sched __up(struct
> {
> struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
> struct semaphore_waiter, list);
> + /*
> + * Rotate sleepers - to make sure all of them get woken in case
> + * of parallel up()s:
> + */
> + list_move_tail(&waiter->list, &sem->wait_list);
> +
> wake_up_process(waiter->task);
> }

--
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"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
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