Re: [PATCH v3] wfcqueue: functions for local append and enqueue

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Eric Wong (normalperson@yhbt.net) wrote:
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> > * Eric Wong (normalperson@yhbt.net) wrote:
> > >  /*
> > > + * ___wfcq_append: append one local queue to another local queue
> > > + *
> 
> > __wfcq_append() and ___wfcq_append() are meant to be private to
> > wfcqueue.h. Therefore, the comment above should be removed, since it is
> > not part of the API.
> > 
> > I notice that I should have used ___wfcq_append() for the original
> > append function for consistency (other private helpers in this header
> > are prefixed with ___).
> > 
> > So maybe we should rename __wfcq_append to ___wfcq_append (making it
> > clear that it is a private helper), and introduce your helper as
> > ___wfcq_append_local() (I don't care about having "local" in there since
> > it is not part of the exposed API).
> 
> Thanks for the explanation, I've squashed that renames into my patch
> below and removed the comment.
> 
> > > +/*
> > > + * __wfcq_enqueue: enqueue a node into a local queue
> > 
> > The concept of "local queue" is not clearly defined.
> > 
> > Perhaps it would be clearer to state:
> > 
> >  * __wfcq_enqueue: enqueue a node into a queue, requiring mutual exclusion.
> 
> Sounds good to me.  Updated patch below:
> 
> -------------------------------8<-----------------------------
> From: Eric Wong <normalperson@yhbt.net>
> Date: Sat, 23 Mar 2013 19:07:26 +0000
> Subject: [PATCH] wfcqueue: functions for local append and enqueue
> 
> With level-triggered epoll, append/enqueue operations to the
> local/locked queues increase performance by avoiding unnecessary atomic
> operations and barriers.  These are necessary to avoid performance
> regressions when looping through ep_send_events and appending many items
> to a local queue where the caller already manages mutual exclusion.

Looks good to me. Thanks!

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

> 
> Changes since v1 and v2:
> * Function renaming and documentation updates
> * rename the existing private __wfcq_append to ___wfcq_append
> 
> Signed-off-by: Eric Wong <normalperson@yhbt.net>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Stephen Hemminger <shemminger@vyatta.com>
> Cc: Davide Libenzi <davidel@xmailserver.org>
> ---
>  include/linux/wfcqueue.h | 56 +++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 46 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/wfcqueue.h b/include/linux/wfcqueue.h
> index 9464a0c..a452ab9 100644
> --- a/include/linux/wfcqueue.h
> +++ b/include/linux/wfcqueue.h
> @@ -55,14 +55,16 @@
>   * [4] __wfcq_splice (source queue)
>   * [5] __wfcq_first
>   * [6] __wfcq_next
> + * [7] __wfcq_enqueue
>   *
> - *     [1] [2] [3] [4] [5] [6]
> - * [1]  -   -   -   -   -   -
> - * [2]  -   -   -   -   -   -
> - * [3]  -   -   X   X   X   X
> - * [4]  -   -   X   -   X   X
> - * [5]  -   -   X   X   -   -
> - * [6]  -   -   X   X   -   -
> + *     [1] [2] [3] [4] [5] [6] [7]
> + * [1]  -   -   -   -   -   -   X
> + * [2]  -   -   -   -   -   -   X
> + * [3]  -   -   X   X   X   X   X
> + * [4]  -   -   X   -   X   X   X
> + * [5]  -   -   X   X   -   -   X
> + * [6]  -   -   X   X   -   -   X
> + * [7]  X   X   X   X   X   X   X
>   *
>   * Besides locking, mutual exclusion of dequeue, splice and iteration
>   * can be ensured by performing all of those operations from a single
> @@ -147,7 +149,7 @@ static inline bool wfcq_empty(struct wfcq_head *head,
>  		&& CMM_LOAD_SHARED(tail->p) == &head->node;
>  }
>  
> -static inline bool __wfcq_append(struct wfcq_head *head,
> +static inline bool ___wfcq_append(struct wfcq_head *head,
>  		struct wfcq_tail *tail,
>  		struct wfcq_node *new_head,
>  		struct wfcq_node *new_tail)
> @@ -201,7 +203,41 @@ static inline bool wfcq_enqueue(struct wfcq_head *head,
>  		struct wfcq_tail *tail,
>  		struct wfcq_node *new_tail)
>  {
> -	return __wfcq_append(head, tail, new_tail, new_tail);
> +	return ___wfcq_append(head, tail, new_tail, new_tail);
> +}
> +
> +static inline bool ___wfcq_append_local(struct wfcq_head *head,
> +		struct wfcq_tail *tail,
> +		struct wfcq_node *new_head,
> +		struct wfcq_node *new_tail)
> +{
> +	struct wfcq_node *old_tail;
> +
> +	old_tail = tail->p;
> +	tail->p = new_tail;
> +	old_tail->next = new_head;
> +
> +	/*
> +	 * Return false if queue was empty prior to adding the node,
> +	 * else return true.
> +	 */
> +	return old_tail != &head->node;
> +}
> +
> +/*
> + * __wfcq_enqueue: enqueue a node into a queue, requiring mutual exclusion.
> + *
> + * No memory barriers are issued.  Mutual exclusion is the responsibility
> + * of the caller.
> + *
> + * Returns false if the queue was empty prior to adding the node.
> + * Returns true otherwise.
> + */
> +static inline bool __wfcq_enqueue(struct wfcq_head *head,
> +		struct wfcq_tail *tail,
> +		struct wfcq_node *new_tail)
> +{
> +	return ___wfcq_append_local(head, tail, new_tail, new_tail);
>  }
>  
>  /*
> @@ -398,7 +434,7 @@ static inline enum wfcq_ret __wfcq_splice(
>  	 * Append the spliced content of src_q into dest_q. Does not
>  	 * require mutual exclusion on dest_q (wait-free).
>  	 */
> -	if (__wfcq_append(dest_q_head, dest_q_tail, head, tail))
> +	if (___wfcq_append(dest_q_head, dest_q_tail, head, tail))
>  		return WFCQ_RET_DEST_NON_EMPTY;
>  	else
>  		return WFCQ_RET_DEST_EMPTY;
> -- 
> 1.8.2.rc3.2.geae6cf5
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com