linux-kernel - Re: [PATCH V2 net-next 1/7] ptr_ring: introduce batch dequeuing

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Date:   Thu, 30 Mar 2017 16:53:07 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 net-next 1/7] ptr_ring: introduce batch dequeuing

On Thu, Mar 30, 2017 at 03:22:24PM +0800, Jason Wang wrote:
> This patch introduce a batched version of consuming, consumer can
> dequeue more than one pointers from the ring at a time. We don't care
> about the reorder of reading here so no need for compiler barrier.
> 
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>  include/linux/ptr_ring.h | 65 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 65 insertions(+)
> 
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index 6c70444..2be0f350 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -247,6 +247,22 @@ static inline void *__ptr_ring_consume(struct ptr_ring *r)
>  	return ptr;
>  }
>  
> +static inline int __ptr_ring_consume_batched(struct ptr_ring *r,
> +					     void **array, int n)

Can we use a shorter name? ptr_ring_consume_batch?

> +{
> +	void *ptr;
> +	int i;
> +
> +	for (i = 0; i < n; i++) {
> +		ptr = __ptr_ring_consume(r);
> +		if (!ptr)
> +			break;
> +		array[i] = ptr;
> +	}
> +
> +	return i;
> +}
> +
>  /*
>   * Note: resize (below) nests producer lock within consumer lock, so if you
>   * call this in interrupt or BH context, you must disable interrupts/BH when

I'd like to add a code comment here explaining why we don't
care about cpu or compiler reordering. And I think the reason is
in the way you use this API: in vhost it does not matter
if you get less entries than present in the ring.
That's ok but needs to be noted
in a code comment so people use this function correctly.

Also, I think you need to repeat the comment about cpu_relax
near this function: if someone uses it in a loop,
a compiler barrier is needed to prevent compiler from
optimizing it out.

I note that ptr_ring_consume currently lacks any of these
comments so I'm ok with merging as is, and I'll add
documentation on top.
Like this perhaps?

/* Consume up to n entries and return the number of entries consumed
 * or 0 on ring empty.
 * Note: this might return early with less entries than present in the
 * ring.
 * Note: callers invoking this in a loop must use a compiler barrier,
 * for example cpu_relax(). Callers must take consumer_lock
 * if the ring is ever resized - see e.g. ptr_ring_consume_batch.
 */

> @@ -297,6 +313,55 @@ static inline void *ptr_ring_consume_bh(struct ptr_ring *r)
>  	return ptr;
>  }
>  
> +static inline int ptr_ring_consume_batched(struct ptr_ring *r,
> +					   void **array, int n)
> +{
> +	int ret;
> +
> +	spin_lock(&r->consumer_lock);
> +	ret = __ptr_ring_consume_batched(r, array, n);
> +	spin_unlock(&r->consumer_lock);
> +
> +	return ret;
> +}
> +
> +static inline int ptr_ring_consume_batched_irq(struct ptr_ring *r,
> +					       void **array, int n)
> +{
> +	int ret;
> +
> +	spin_lock_irq(&r->consumer_lock);
> +	ret = __ptr_ring_consume_batched(r, array, n);
> +	spin_unlock_irq(&r->consumer_lock);
> +
> +	return ret;
> +}
> +
> +static inline int ptr_ring_consume_batched_any(struct ptr_ring *r,
> +					       void **array, int n)
> +{
> +	unsigned long flags;
> +	int ret;
> +
> +	spin_lock_irqsave(&r->consumer_lock, flags);
> +	ret = __ptr_ring_consume_batched(r, array, n);
> +	spin_unlock_irqrestore(&r->consumer_lock, flags);
> +
> +	return ret;
> +}
> +
> +static inline int ptr_ring_consume_batched_bh(struct ptr_ring *r,
> +					      void **array, int n)
> +{
> +	int ret;
> +
> +	spin_lock_bh(&r->consumer_lock);
> +	ret = __ptr_ring_consume_batched(r, array, n);
> +	spin_unlock_bh(&r->consumer_lock);
> +
> +	return ret;
> +}
> +
>  /* Cast to structure type and call a function without discarding from FIFO.
>   * Function must return a value.
>   * Callers must take consumer_lock.
> -- 
> 2.7.4