[PATCH] ring: optimize corner case for enqueue/dequeue

[Morten Brørup]

> From: Andrzej Ostruszka [mailto:amo at semihalf.com]
> Sent: Tuesday, 11 January 2022 12.38
> 
> When enqueueing/dequeueing to/from the ring we try to optimize by
> manual
> loop unrolling.  The check for this optimization looks like:
> 
> 	if (likely(idx + n < size)) {
> 
> where 'idx' points to the first usable element (empty slot for enqueue,
> data for dequeue).  The correct comparison here should be '<=' instead
> of '<'.
> 
> This is not a functional error since we fall back to the loop with
> correct checks on indexes.  Just a minor suboptimal behaviour for the
> case when we want to enqueue/dequeue exactly the number of elements
> that
> we have in the ring before wrapping to its beginning.
> 
> Fixes: cc4b218790f6 ("ring: support configurable element size")
> Fixes: 286bd05bf70d ("ring: optimisations")
> 
> Signed-off-by: Andrzej Ostruszka <amo at semihalf.com>
> Reviewed-by: Olivier Matz <olivier.matz at 6wind.com>
> Acked-by: Konstantin Ananyev <konstantin.ananyev at intel.com>
> ---
>  lib/ring/rte_ring_elem_pvt.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/lib/ring/rte_ring_elem_pvt.h
> b/lib/ring/rte_ring_elem_pvt.h
> index 275ec55393..83788c56e6 100644
> --- a/lib/ring/rte_ring_elem_pvt.h
> +++ b/lib/ring/rte_ring_elem_pvt.h
> @@ -17,7 +17,7 @@ __rte_ring_enqueue_elems_32(struct rte_ring *r, const
> uint32_t size,
>  	unsigned int i;
>  	uint32_t *ring = (uint32_t *)&r[1];
>  	const uint32_t *obj = (const uint32_t *)obj_table;
> -	if (likely(idx + n < size)) {
> +	if (likely(idx + n <= size)) {
>  		for (i = 0; i < (n & ~0x7); i += 8, idx += 8) {
>  			ring[idx] = obj[i];
>  			ring[idx + 1] = obj[i + 1];
> @@ -62,7 +62,7 @@ __rte_ring_enqueue_elems_64(struct rte_ring *r,
> uint32_t prod_head,
>  	uint32_t idx = prod_head & r->mask;
>  	uint64_t *ring = (uint64_t *)&r[1];
>  	const unaligned_uint64_t *obj = (const unaligned_uint64_t
> *)obj_table;
> -	if (likely(idx + n < size)) {
> +	if (likely(idx + n <= size)) {
>  		for (i = 0; i < (n & ~0x3); i += 4, idx += 4) {
>  			ring[idx] = obj[i];
>  			ring[idx + 1] = obj[i + 1];
> @@ -95,7 +95,7 @@ __rte_ring_enqueue_elems_128(struct rte_ring *r,
> uint32_t prod_head,
>  	uint32_t idx = prod_head & r->mask;
>  	rte_int128_t *ring = (rte_int128_t *)&r[1];
>  	const rte_int128_t *obj = (const rte_int128_t *)obj_table;
> -	if (likely(idx + n < size)) {
> +	if (likely(idx + n <= size)) {
>  		for (i = 0; i < (n & ~0x1); i += 2, idx += 2)
>  			memcpy((void *)(ring + idx),
>  				(const void *)(obj + i), 32);
> @@ -151,7 +151,7 @@ __rte_ring_dequeue_elems_32(struct rte_ring *r,
> const uint32_t size,
>  	unsigned int i;
>  	uint32_t *ring = (uint32_t *)&r[1];
>  	uint32_t *obj = (uint32_t *)obj_table;
> -	if (likely(idx + n < size)) {
> +	if (likely(idx + n <= size)) {
>  		for (i = 0; i < (n & ~0x7); i += 8, idx += 8) {
>  			obj[i] = ring[idx];
>  			obj[i + 1] = ring[idx + 1];
> @@ -196,7 +196,7 @@ __rte_ring_dequeue_elems_64(struct rte_ring *r,
> uint32_t prod_head,
>  	uint32_t idx = prod_head & r->mask;
>  	uint64_t *ring = (uint64_t *)&r[1];
>  	unaligned_uint64_t *obj = (unaligned_uint64_t *)obj_table;
> -	if (likely(idx + n < size)) {
> +	if (likely(idx + n <= size)) {
>  		for (i = 0; i < (n & ~0x3); i += 4, idx += 4) {
>  			obj[i] = ring[idx];
>  			obj[i + 1] = ring[idx + 1];
> @@ -229,7 +229,7 @@ __rte_ring_dequeue_elems_128(struct rte_ring *r,
> uint32_t prod_head,
>  	uint32_t idx = prod_head & r->mask;
>  	rte_int128_t *ring = (rte_int128_t *)&r[1];
>  	rte_int128_t *obj = (rte_int128_t *)obj_table;
> -	if (likely(idx + n < size)) {
> +	if (likely(idx + n <= size)) {
>  		for (i = 0; i < (n & ~0x1); i += 2, idx += 2)
>  			memcpy((void *)(obj + i), (void *)(ring + idx), 32);
>  		switch (n & 0x1) {
> --
> 2.34.1.575.g55b058a8bb-goog
> 

Also this version of the patch:

Reviewed-by: Morten Brørup <mb at smartsharesystems.com>