> From: Honnappa Nagarahalli [mailto:honnappa.nagaraha...@arm.com]
> Sent: Wednesday, 16 November 2022 16.51
>
> <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.
There could be applications using a mempool for something else than mbufs.
In that case, the application should be allowed to get/put many objects in one
transaction.
>
> > + /*
> > + * 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
