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SubjectRe: [PATCH RFC v2] ptr_ring: add ptr_ring_unconsume
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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@redhat.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 *))

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