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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 24 Apr 2017 15:00:10 +0300

On Mon, Apr 24, 2017 at 07:54:18PM +0800, Jason Wang wrote:
> 
> 
> On 2017年04月24日 07:28, Michael S. Tsirkin wrote:
> > 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.
> 
> Ok.
> 
> > 
> > > > 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?
> 
> The numbers are pretty stable, so probably not noise. Retested on top of
> batch zeroing:
> 
> no  1.97Mpps
> 1   2.09Mpps
> 2   2.11Mpps
> 4   2.16Mpps
> 8   2.19Mpps
> 16  2.21Mpps
> 32  2.25Mpps
> 64  2.30Mpps
> 128 2.21Mpps
> 256 2.21Mpps
> 
> 64 performs best.
> 
> Thanks

OK but it might be e.g. a function of the ring size, host cache size or
whatever. As we don't really understand the why, if we just optimize for
your setup we risk regressions in others.  64 entries is a lot, it
increases the queue size noticeably.  Could this be part of the effect?
Could you try changing the queue size to see what happens?

> > 
> > > >    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 *))