Date:   Sat, 8 Apr 2017 14:14:08 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        brouer@...hat.com
Subject: Re: [PATCH 1/3] ptr_ring: batch ring zeroing

On Fri, 7 Apr 2017 08:49:57 +0300
"Michael S. Tsirkin" <mst@...hat.com> wrote:

> A known weakness in ptr_ring design is that it does not handle well the
> situation when ring is almost full: as entries are consumed they are
> immediately used again by the producer, so consumer and producer are
> writing to a shared cache line.
> 
> To fix this, add batching to consume calls: as entries are
> consumed do not write NULL into the ring until we get
> a multiple (in current implementation 2x) of cache lines
> away from the producer. At that point, write them all out.
> 
> We do the write out in the reverse order to keep
> producer from sharing cache with consumer for as long
> as possible.
> 
> Writeout also triggers when ring wraps around - there's
> no special reason to do this but it helps keep the code
> a bit simpler.
> 
> What should we do if getting away from producer by 2 cache lines
> would mean we are keeping the ring moe than half empty?
> Maybe we should reduce the batching in this case,
> current patch simply reduces the batching.
> 
> Notes:
> - it is no longer true that a call to consume guarantees
>   that the following call to produce will succeed.
>   No users seem to assume that.
> - batching can also in theory reduce the signalling rate:
>   users that would previously send interrups to the producer
>   to wake it up after consuming each entry would now only
>   need to do this once in a batch.
>   Doing this would be easy by returning a flag to the caller.
>   No users seem to do signalling on consume yet so this was not
>   implemented yet.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
> 
> Jason, I am curious whether the following gives you some of
> the performance boost that you see with vhost batching
> patches. Is vhost batching on top still helpful?
> 
>  include/linux/ptr_ring.h | 63 +++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 54 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index 6c70444..6b2e0dd 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -34,11 +34,13 @@
>  struct ptr_ring {
>  	int producer ____cacheline_aligned_in_smp;
>  	spinlock_t producer_lock;
> -	int consumer ____cacheline_aligned_in_smp;
> +	int consumer_head ____cacheline_aligned_in_smp; /* next valid entry */
> +	int consumer_tail; /* next entry to invalidate */
>  	spinlock_t consumer_lock;
>  	/* Shared consumer/producer data */
>  	/* Read-only by both the producer and the consumer */
>  	int size ____cacheline_aligned_in_smp; /* max entries in queue */
> +	int batch; /* number of entries to consume in a batch */
>  	void **queue;
>  };
>  
> @@ -170,7 +172,7 @@ static inline int ptr_ring_produce_bh(struct ptr_ring *r, void *ptr)
>  static inline void *__ptr_ring_peek(struct ptr_ring *r)
>  {
>  	if (likely(r->size))
> -		return r->queue[r->consumer];
> +		return r->queue[r->consumer_head];
>  	return NULL;
>  }
>  
> @@ -231,9 +233,38 @@ static inline bool ptr_ring_empty_bh(struct ptr_ring *r)
>  /* Must only be called after __ptr_ring_peek returned !NULL */
>  static inline void __ptr_ring_discard_one(struct ptr_ring *r)
>  {
> -	r->queue[r->consumer++] = NULL;
> -	if (unlikely(r->consumer >= r->size))
> -		r->consumer = 0;
> +	/* Fundamentally, what we want to do is update consumer
> +	 * index and zero out the entry so producer can reuse it.
> +	 * Doing it naively at each consume would be as simple as:
> +	 *       r->queue[r->consumer++] = NULL;
> +	 *       if (unlikely(r->consumer >= r->size))
> +	 *               r->consumer = 0;
> +	 * but that is suboptimal when the ring is full as producer is writing
> +	 * out new entries in the same cache line.  Defer these updates until a
> +	 * batch of entries has been consumed.
> +	 */
> +	int head = r->consumer_head++;
> +
> +	/* Once we have processed enough entries invalidate them in
> +	 * the ring all at once so producer can reuse their space in the ring.
> +	 * We also do this when we reach end of the ring - not mandatory
> +	 * but helps keep the implementation simple.
> +	 */
> +	if (unlikely(r->consumer_head - r->consumer_tail >= r->batch ||
> +		     r->consumer_head >= r->size)) {
> +		/* Zero out entries in the reverse order: this way we touch the
> +		 * cache line that producer might currently be reading the last;
> +		 * producer won't make progress and touch other cache lines
> +		 * besides the first one until we write out all entries.
> +		 */
> +		while (likely(head >= r->consumer_tail))
> +			r->queue[head--] = NULL;
> +		r->consumer_tail = r->consumer_head;
> +	}
> +	if (unlikely(r->consumer_head >= r->size)) {
> +		r->consumer_head = 0;
> +		r->consumer_tail = 0;
> +	}
>  }

I love this idea.  Reviewed and discussed the idea in-person with MST
during netdevconf[1] at this laptop.  I promised I will also run it
through my micro-benchmarking[2] once I return home (hint ptr_ring gets
used in network stack as skb_array).

Reviewed-by: Jesper Dangaard Brouer <brouer@...hat.com>

[1] http://netdevconf.org/2.1/
[2] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/skb_array_bench01.c
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

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