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

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Date:	Fri, 22 Nov 2013 23:20:05 -0800
From:	Darren Hart <dvhart@...ux.intel.com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	peterz@...radead.org, tglx@...utronix.de, efault@....de,
	jeffm@...e.com, torvalds@...ux-foundation.org, scott.norton@...com,
	tom.vaden@...com, aswin@...com, Waiman.Long@...com,
	jason.low2@...com
Subject: Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

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


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