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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 31 Mar 2017 17:31:41 +0300

On Fri, Mar 31, 2017 at 11:52:24AM +0800, Jason Wang wrote:
> 
> 
> On 2017年03月30日 21:53, Michael S. Tsirkin wrote:
> > 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 <[email-masked]>
> > > ---
> > >   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?
> 
> Ok, but at least we need to keep the prefix since there's a locked version.
> 
> 
> 
> > 
> > > +{
> > > +	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.
> 
> Interesting, but I still think it's not necessary.
> 
> If consumer is doing a busy polling, it will eventually get the entries. If
> the consumer need notification from producer, it should drain the queue
> which means it need enable notification before last try of consuming call,
> otherwise it was a bug. The batch consuming function in this patch can
> guarantee return at least one pointer if there's many, this looks sufficient
> for the correctness?
> 
> Thanks

You ask for N entries but get N-1. This seems to imply the
ring is now empty. Do we guarantee this?

> > 
> > 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