[PATCH 20.11 v2] mempool: fix get objects from mempool with cache [ upstream commit a2833ecc5ea4adcbc3b77e7aeac2a6fd945da6a0 ]

[PATCH 20.11 v2] mempool: fix get objects from mempool with cache [ upstream commit a2833ecc5ea4adcbc3b77e7aeac2a6fd945da6a0 ]

[Morten Brørup]

A flush threshold for the mempool cache was introduced in DPDK version
1.3, but rte_mempool_do_generic_get() was not completely updated back
then, and some inefficiencies were introduced.

Fix the following in rte_mempool_do_generic_get():

1. The code that initially screens the cache request was not updated
with the change in DPDK version 1.3.
The initial screening compared the request length to the cache size,
which was correct before, but became irrelevant with the introduction of
the flush threshold. E.g. the cache can hold up to flushthresh objects,
which is more than its size, so some requests were not served from the
cache, even though they could be.
The initial screening has now been corrected to match the initial
screening in rte_mempool_do_generic_put(), which verifies that a cache
is present, and that the length of the request does not overflow the
memory allocated for the cache.

This bug caused a major performance degradation in scenarios where the
application burst length is the same as the cache size. In such cases,
the objects were not ever fetched from the mempool cache, regardless if
they could have been.
This scenario occurs e.g. if an application has configured a mempool
with a size matching the application's burst size.

2. The function is a helper for rte_mempool_generic_get(), so it must
behave according to the description of that function.
Specifically, objects must first be returned from the cache,
subsequently from the backend.
After the change in DPDK version 1.3, this was not the behavior when
the request was partially satisfied from the cache; instead, the objects
from the backend were returned ahead of the objects from the cache.
This bug degraded application performance on CPUs with a small L1 cache,
which benefit from having the hot objects first in the returned array.
(This is probably also the reason why the function returns the objects
in reverse order, which it still does.)
Now, all code paths first return objects from the cache, subsequently
from the backend.

The function was not behaving as described (by the function using it)
and expected by applications using it. This in itself is also a bug.

3. If the cache could not be backfilled, the function would attempt
to get all the requested objects from the backend (instead of only the
number of requested objects minus the objects available in the backend),
and the function would fail if that failed.
Now, the first part of the request is always satisfied from the cache,
and if the subsequent backfilling of the cache from the backend fails,
only the remaining requested objects are retrieved from the backend.

The function would fail despite there are enough objects in the cache
plus the common pool.

4. The code flow for satisfying the request from the cache was slightly
inefficient:
The likely code path where the objects are simply served from the cache
was treated as unlikely. Now it is treated as likely.

v2 changes:
- Fix one incorrect use of __MEMPOOL_STAT_ADD().

Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Morten Brørup <mb@smartsharesystems.com>
---
 lib/librte_mempool/rte_mempool.h | 83 ++++++++++++++++++++------------
 1 file changed, 52 insertions(+), 31 deletions(-)

diff --git a/lib/librte_mempool/rte_mempool.h b/lib/librte_mempool/rte_mempool.h
index 7516d90824..19cfcd4db7 100644
--- a/lib/librte_mempool/rte_mempool.h
+++ b/lib/librte_mempool/rte_mempool.h
@@ -1403,61 +1403,82 @@ rte_mempool_put(struct rte_mempool *mp, void *obj)
  *   A pointer to a mempool cache structure. May be NULL if not needed.
  * @return
  *   - >=0: Success; number of objects supplied.
- *   - <0: Error; code of ring dequeue function.
+ *   - <0: Error; code of driver dequeue function.
  */
 static __rte_always_inline int
 __mempool_generic_get(struct rte_mempool *mp, void **obj_table,
 		      unsigned int n, struct rte_mempool_cache *cache)
 {
 	int ret;
+	unsigned int remaining = n;
 	uint32_t index, len;
 	void **cache_objs;
 
-	/* No cache provided or cannot be satisfied from cache */
-	if (unlikely(cache == NULL || n >= cache->size))
-		goto ring_dequeue;
+	/* No cache provided */
+	if (unlikely(cache == NULL))
+		goto driver_dequeue;
 
-	cache_objs = cache->objs;
+	/* Use the cache as much as we have to return hot objects first */
+	len = RTE_MIN(remaining, cache->len);
+	cache_objs = &cache->objs[cache->len];
+	cache->len -= len;
+	remaining -= len;
+	for (index = 0; index < len; index++)
+		*obj_table++ = *--cache_objs;
 
-	/* Can this be satisfied from the cache? */
-	if (cache->len < n) {
-		/* No. Backfill the cache first, and then fill from it */
-		uint32_t req = n + (cache->size - cache->len);
+	if (remaining == 0) {
+		/* The entire request is satisfied from the cache. */
 
-		/* How many do we require i.e. number to fill the cache + the request */
-		ret = rte_mempool_ops_dequeue_bulk(mp,
-			&cache->objs[cache->len], req);
-		if (unlikely(ret < 0)) {
-			/*
-			 * In the off chance that we are buffer constrained,
-			 * where we are not able to allocate cache + n, go to
-			 * the ring directly. If that fails, we are truly out of
-			 * buffers.
-			 */
-			goto ring_dequeue;
-		}
+		__MEMPOOL_STAT_ADD(mp, get_success, n);
 
-		cache->len += req;
+		return 0;
 	}
 
-	/* Now fill in the response ... */
-	for (index = 0, len = cache->len - 1; index < n; ++index, len--, obj_table++)
-		*obj_table = cache_objs[len];
+	/* if dequeue below would overflow mem allocated for cache */
+	if (unlikely(remaining > RTE_MEMPOOL_CACHE_MAX_SIZE))
+		goto driver_dequeue;
+
+	/* Fill the cache from the backend; fetch size + remaining objects. */
+	ret = rte_mempool_ops_dequeue_bulk(mp, cache->objs,
+			cache->size + remaining);
+	if (unlikely(ret < 0)) {
+		/*
+		 * We are buffer constrained, and not able to allocate
+		 * cache + remaining.
+		 * Do not fill the cache, just satisfy the remaining part of
+		 * the request directly from the backend.
+		 */
+		goto driver_dequeue;
+	}
 
-	cache->len -= n;
+	/* Satisfy the remaining part of the request from the filled cache. */
+	cache_objs = &cache->objs[cache->size + remaining];
+	for (index = 0; index < remaining; index++)
+		*obj_table++ = *--cache_objs;
+
+	cache->len = cache->size;
 
 	__MEMPOOL_STAT_ADD(mp, get_success, n);
 
 	return 0;
 
-ring_dequeue:
+driver_dequeue:
+
+	/* Get remaining objects directly from the backend. */
+	ret = rte_mempool_ops_dequeue_bulk(mp, obj_table, remaining);
 
-	/* get remaining objects from ring */
-	ret = rte_mempool_ops_dequeue_bulk(mp, obj_table, n);
+	if (ret < 0) {
+		if (likely(cache != NULL)) {
+			cache->len = n - remaining;
+			/*
+			 * No further action is required to roll the first part
+			 * of the request back into the cache, as objects in
+			 * the cache are intact.
+			 */
+		}
 
-	if (ret < 0)
 		__MEMPOOL_STAT_ADD(mp, get_fail, n);
-	else
+	} else
 		__MEMPOOL_STAT_ADD(mp, get_success, n);
 
 	return ret;
-- 
2.17.1