<snip>
>
> Micro-optimization:
> Reduced the most likely code path in the generic put function by moving an
> unlikely check out of the most likely code path and further down.
>
> Also updated the comments in the function.
>
> v2 (feedback from Andrew Rybchenko):
> * Modified comparison to prevent overflow if n is really huge and len is
> non-zero.
> * Added assertion about the invariant preventing overflow in the
> comparison.
> * Crossing the threshold is not extremely unlikely, so removed likely()
> from that comparison.
> The compiler will generate code with optimal static branch prediction
> here anyway.
>
> Signed-off-by: Morten Brørup <m...@smartsharesystems.com>
> ---
> lib/mempool/rte_mempool.h | 36 ++++++++++++++++++++----------------
> 1 file changed, 20 insertions(+), 16 deletions(-)
>
> diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h
> index 9f530db24b..dd1a3177d6 100644
> --- a/lib/mempool/rte_mempool.h
> +++ b/lib/mempool/rte_mempool.h
> @@ -1364,32 +1364,36 @@ rte_mempool_do_generic_put(struct
> rte_mempool *mp, void * const *obj_table, {
> void **cache_objs;
>
> - /* No cache provided */
> + /* No cache provided? */
> if (unlikely(cache == NULL))
> goto driver_enqueue;
>
> - /* increment stat now, adding in mempool always success */
> + /* Increment stats now, adding in mempool always succeeds. */
> RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_bulk, 1);
> RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_objs, n);
>
> - /* The request itself is too big for the cache */
> - if (unlikely(n > cache->flushthresh))
> - goto driver_enqueue_stats_incremented;
> -
> - /*
> - * The cache follows the following algorithm:
> - * 1. If the objects cannot be added to the cache without crossing
> - * the flush threshold, flush the cache to the backend.
> - * 2. Add the objects to the cache.
> - */
> + /* Assert the invariant preventing overflow in the comparison below.
> */
> + RTE_ASSERT(cache->len <= cache->flushthresh);
>
> - if (cache->len + n <= cache->flushthresh) {
> + if (n <= cache->flushthresh - cache->len) {
> + /*
> + * The objects can be added to the cache without crossing the
> + * flush threshold.
> + */
> cache_objs = &cache->objs[cache->len];
> cache->len += n;
> - } else {
> + } else if (likely(n <= cache->flushthresh)) {
IMO, this is a misconfiguration on the application part. In the PMDs I have
looked at, max value of 'n' is controlled by compile time constants.
Application could do a compile time check on the cache threshold or we could
have another RTE_ASSERT on this.
> + /*
> + * The request itself fits into the cache.
> + * But first, the cache must be flushed to the backend, so
> + * adding the objects does not cross the flush threshold.
> + */
> cache_objs = &cache->objs[0];
> rte_mempool_ops_enqueue_bulk(mp, cache_objs, cache-
> >len);
> cache->len = n;
> + } else {
> + /* The request itself is too big for the cache. */
> + goto driver_enqueue_stats_incremented;
> }
>
> /* Add the objects to the cache. */
> @@ -1399,13 +1403,13 @@ rte_mempool_do_generic_put(struct
> rte_mempool *mp, void * const *obj_table,
>
> driver_enqueue:
>
> - /* increment stat now, adding in mempool always success */
> + /* Increment stats now, adding in mempool always succeeds. */
> RTE_MEMPOOL_STAT_ADD(mp, put_bulk, 1);
> RTE_MEMPOOL_STAT_ADD(mp, put_objs, n);
>
> driver_enqueue_stats_incremented:
>
> - /* push objects to the backend */
> + /* Push the objects to the backend. */
> rte_mempool_ops_enqueue_bulk(mp, obj_table, n); }
>
> --
> 2.17.1
