Subject: Re: [PATCH RFC] ptr_ring: add ptr_ring_unconsume

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 24 Apr 2017 02:28:42 +0300

On Tue, Apr 18, 2017 at 11:07:42AM +0800, Jason Wang wrote:
> 
> 
> On 2017年04月17日 07:19, Michael S. Tsirkin wrote:
> > Applications that consume a batch of entries in one go
> > can benefit from ability to return some of them back
> > into the ring.
> > 
> > Add an API for that - assuming there's space. If there's no space
> > naturally we can't do this and have to drop entries, but this implies
> > ring is full so we'd likely drop some anyway.
> > 
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > ---
> > 
> > Jason, in my mind the biggest issue with your batching patchset is the
> > backet drops on disconnect.  This API will help avoid that in the common
> > case.
> 
> Ok, I will rebase the series on top of this. (Though I don't think we care
> the packet loss).

E.g. I care - I often start sending packets to VM before it's
fully booted. Several vhost resets might follow.

> > 
> > I would still prefer that we understand what's going on,
> 
> I try to reply in another thread, does it make sense?
> 
> >   and I would
> > like to know what's the smallest batch size that's still helpful,
> 
> Yes, I've replied in another thread, the result is:
> 
> 
> no batching   1.88Mpps
> RX_BATCH=1    1.93Mpps
> RX_BATCH=4    2.11Mpps
> RX_BATCH=16   2.14Mpps
> RX_BATCH=64   2.25Mpps
> RX_BATCH=256  2.18Mpps

Essentially 4 is enough, other stuf looks more like noise
to me. What about 2?

> >   but
> > I'm not going to block the patch on these grounds assuming packet drops
> > are fixed.
> 
> Thanks a lot.
> 
> > 
> > Lightly tested - this is on top of consumer batching patches.
> > 
> > Thanks!
> > 
> >   include/linux/ptr_ring.h | 57 ++++++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 57 insertions(+)
> > 
> > diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> > index 783e7f5..5fbeab4 100644
> > --- a/include/linux/ptr_ring.h
> > +++ b/include/linux/ptr_ring.h
> > @@ -457,6 +457,63 @@ static inline int ptr_ring_init(struct ptr_ring *r, int size, gfp_t gfp)
> >   	return 0;
> >   }
> > +/*
> > + * Return entries into ring. Destroy entries that don't fit.
> > + *
> > + * Note: this is expected to be a rare slow path operation.
> > + *
> > + * Note: producer lock is nested within consumer lock, so if you
> > + * resize you must make sure all uses nest correctly.
> > + * In particular if you consume ring in interrupt or BH context, you must
> > + * disable interrupts/BH when doing so.
> > + */
> > +static inline void ptr_ring_unconsume(struct ptr_ring *r, void **batch, int n,
> > +				      void (*destroy)(void *))
> > +{
> > +	unsigned long flags;
> > +	int head;
> > +
> > +	spin_lock_irqsave(&(r)->consumer_lock, flags);
> > +	spin_lock(&(r)->producer_lock);
> > +
> > +	if (!r->size)
> > +		goto done;
> > +
> > +	/*
> > +	 * Clean out buffered entries (for simplicity). This way following code
> > +	 * can test entries for NULL and if not assume they are valid.
> > +	 */
> > +	head = r->consumer_head - 1;
> > +	while (likely(head >= r->consumer_tail))
> > +		r->queue[head--] = NULL;
> > +	r->consumer_tail = r->consumer_head;
> > +
> > +	/*
> > +	 * Go over entries in batch, start moving head back and copy entries.
> > +	 * Stop when we run into previously unconsumed entries.
> > +	 */
> > +	while (n--) {
> > +		head = r->consumer_head - 1;
> > +		if (head < 0)
> > +			head = r->size - 1;
> > +		if (r->queue[head]) {
> > +			/* This batch entry will have to be destroyed. */
> > +			++n;
> > +			goto done;
> > +		}
> > +		r->queue[head] = batch[n];
> > +		r->consumer_tail = r->consumer_head = head;
> > +	}
> > +
> > +done:
> > +	/* Destroy all entries left in the batch. */
> > +	while (n--) {
> > +		destroy(batch[n]);
> > +	}
> > +	spin_unlock(&(r)->producer_lock);
> > +	spin_unlock_irqrestore(&(r)->consumer_lock, flags);
> > +}
> > +
> >   static inline void **__ptr_ring_swap_queue(struct ptr_ring *r, void **queue,
> >   					   int size, gfp_t gfp,
> >   					   void (*destroy)(void *))