linux-kernel - Re: [PATCH RFC v2] ptr_ring: add ptr_ring

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Date:   Wed, 26 Apr 2017 17:09:42 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH RFC v2] ptr_ring: add ptr_ring_unconsume



On 2017年04月25日 00:01, 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 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 <mst@...hat.com>
> ---
>
> Jason, if you add this and unconsume the outstanding packets
> on backend disconnect, vhost close and reset, I think
> we should apply your patch even if we don't yet know 100%
> why it helps.
>
> changes from v1:
> - fix up coding style issues reported by Sergei Shtylyov
>
>
>   include/linux/ptr_ring.h | 56 ++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 56 insertions(+)
>
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index 783e7f5..902afc2 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -457,6 +457,62 @@ 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;

Looks like something wrong here (bad page state reported), uncomment the 
above while() solving the issue. But after staring it for a while I 
didn't find anything interesting, maybe you have some idea on this?

Thanks


> +	}
> +
> +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 *))