Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

Darren Hart
2013-11-23T07:20:21Z

On Fri, 2013-11-22 at 16:56 -0800, Davidlohr Bueso wrote:
> In futex_wake() there is clearly no point in taking the hb->lock if
> we know beforehand that there are no tasks to be woken. This comes
> at the smaller cost of doing some atomic operations to keep track of
> the list's size. Specifically, increment the counter when an element is
> added to the list, and decrement when it is removed. Of course, if the
> counter is 0, then there are no tasks blocked on a futex. Some special
> considerations:
>
> - increment the counter at queue_lock() as we always end up calling
> queue_me() which adds the element to the list. Upon any error,
> queue_unlock() is called for housekeeping, for which we decrement
> to mach the increment done in queue_lock().

^match

>
> - decrement the counter at __unqueue_me() to reflect when an element is
> removed from the queue for wakeup related purposes.

__unqueue_futex (not __unqueue_me)

> @@ -999,6 +1001,10 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
> goto out;
>
> hb = hash_futex(&key);
> + /* make sure we really have tasks to wakeup */

Nit, but please use proper sentence formatting for consistency with the
rest of the comments in futex.c (most of them anyway).

/* Make sure we really have tasks to wake up. */

Now... I'm not thrilled with adding atomics if we don't need to,
especially for an optimization since the atomics themselves cause enough
problems, especially across a large number of CPUs... I'll respond to
Linus's thread though.

--
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel