Re: [RFC PATCH v2] ptr_ring: linked list fallback

[Jason Wang <jasowang@redhat.com>]

On 2018年02月26日 09:17, Michael S. Tsirkin wrote:
> So pointer rings work fine, but they have a problem: make them too small
> and not enough entries fit.  Make them too large and you start flushing
> your cache and running out of memory.
>
> This is a new idea of mine: a ring backed by a linked list. Once you run
> out of ring entries, instead of a drop you fall back on a list with a
> common lock.
>
> Should work well for the case where the ring is typically sized
> correctly, but will help address the fact that some user try to set e.g.
> tx queue length to 1000000.
>
> In other words, the idea is that if a user sets a really huge TX queue
> length, we allocate a ptr_ring which is smaller, and use the backup
> linked list when necessary to provide the requested TX queue length
> legitimately.
>
> My hope this will move us closer to direction where e.g. fw codel can
> use ptr rings without locking at all.  The API is still very rough, and
> I really need to take a hard look at lock nesting.
>
> Compiled only, sending for early feedback/flames.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>
> changes from v1:
> - added clarifications by DaveM in the commit log
> - build fixes
>
>   include/linux/ptr_ring.h | 64 +++++++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 61 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index d72b2e7..8aa8882 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -31,11 +31,18 @@
>   #include <asm/errno.h>
>   #endif
>   
> +/* entries must start with the following structure */
> +struct plist {
> +	struct plist *next;
> +	struct plist *last; /* only valid in the 1st entry */
> +};

So I wonder whether or not it's better to do this in e.g skb_array 
implementation. Then it can use its own prev/next field.

> +
>   struct ptr_ring {
>   	int producer ____cacheline_aligned_in_smp;
>   	spinlock_t producer_lock;
>   	int consumer_head ____cacheline_aligned_in_smp; /* next valid entry */
>   	int consumer_tail; /* next entry to invalidate */
> +	struct plist *consumer_list;
>   	spinlock_t consumer_lock;
>   	/* Shared consumer/producer data */
>   	/* Read-only by both the producer and the consumer */
> @@ -120,10 +127,40 @@ static inline int __ptr_ring_produce(struct ptr_ring *r, void *ptr)
>   }
>   
>   /*
> - * Note: resize (below) nests producer lock within consumer lock, so if you
> - * consume in interrupt or BH context, you must disable interrupts/BH when
> - * calling this.
> + * Note: resize API with the _fallback should be used when calling this.
>    */
> +static inline int ptr_ring_produce_fallback(struct ptr_ring *r, void *ptr)
> +{
> +	int ret;
> +	unsigned long flags;
> +	struct plist *p = ptr;
> +
> +	p->next = NULL;
> +	p->last = p;
> +
> +	spin_lock_irqsave(&r->producer_lock, flags);
> +	ret = __ptr_ring_produce(r, ptr);
> +	if (ret) {
> +		spin_lock(&r->consumer_lock);
> +		ret = __ptr_ring_produce(r, ptr);
> +		if (ret) {
> +			int producer = r->producer ? r->producer - 1 :
> +				r->size - 1;
> +			struct plist *first = r->queue[producer];
> +
> +			BUG_ON(!first);
> +
> +			first->last->next = p;
> +			first->last = p;

I believe we still need a limitation on the total size of the queue.

Thanks

> +		}
> +		spin_unlock(&r->consumer_lock);
> +	}
> +
> +	spin_unlock_irqrestore(&r->producer_lock, flags);
> +
> +	return ret;
> +}
> +
>   static inline int ptr_ring_produce(struct ptr_ring *r, void *ptr)
>   {
>   	int ret;
> @@ -135,6 +172,7 @@ static inline int ptr_ring_produce(struct ptr_ring *r, void *ptr)
>   	return ret;
>   }
>   
> +
>   static inline int ptr_ring_produce_irq(struct ptr_ring *r, void *ptr)
>   {
>   	int ret;
> @@ -359,6 +397,26 @@ static inline void *ptr_ring_consume_bh(struct ptr_ring *r)
>   	return ptr;
>   }
>   
> +static inline void *ptr_ring_consume_fallback(struct ptr_ring *r)
> +{
> +	unsigned long flags;
> +	struct plist *ptr;
> +
> +	spin_lock_irqsave(&r->consumer_lock, flags);
> +	if (r->consumer_list) {
> +		ptr = r->consumer_list;
> +		r->consumer_list = ptr->next;
> +	} else {
> +		ptr = __ptr_ring_consume(r);
> +		if (ptr) {
> +			r->consumer_list = ptr->next;
> +		}
> +	}
> +	spin_unlock_irqrestore(&r->consumer_lock, flags);
> +
> +	return ptr;
> +}
> +
>   static inline int ptr_ring_consume_batched(struct ptr_ring *r,
>   					   void **array, int n)
>   {