RE: [PATCH v8] mempool cache: add zero-copy get and put functions

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

Hi Morten,
	Apologies for the late comments.

> -----Original Message-----
> From: Morten Brørup <mb@smartsharesystems.com>
> Sent: Thursday, February 9, 2023 8:59 AM
> To: olivier.matz@6wind.com; andrew.rybchenko@oktetlabs.ru; Honnappa
> Nagarahalli <Honnappa.Nagarahalli@arm.com>; Kamalakshitha Aligeri
> <Kamalakshitha.Aligeri@arm.com>; bruce.richardson@intel.com;
> konstantin.ananyev@huawei.com; dev@dpdk.org
> Cc: nd <nd@arm.com>; david.marchand@redhat.com; Morten Brørup
> <mb@smartsharesystems.com>
> Subject: [PATCH v8] mempool cache: add zero-copy get and put functions
> 
> Zero-copy access to mempool caches is beneficial for PMD performance, and
> must be provided by the mempool library to fix [Bug 1052] without a
> performance regression.
> 
> [Bug 1052]: https://bugs.dpdk.org/show_bug.cgi?id=1052
> 
> Bugzilla ID: 1052
> 
> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> Acked-by: Konstantin Ananyev <konstantin.v.ananyev@yandex.ru>
> 
> v8:
> * Actually include the rte_errno header file.
>   Note to self: The changes only take effect on the disk after the file in
>   the text editor has been saved.
> v7:
> * Fix typo in function description. (checkpatch)
> * Zero-copy functions may set rte_errno; include rte_errno header file.
>   (ci/loongarch-compilation)
> v6:
> * Improve description of the 'n' parameter to the zero-copy get function.
>   (Konstantin, Bruce)
> * The caches used for zero-copy may not be user-owned, so remove this word
>   from the function descriptions. (Kamalakshitha)
> v5:
> * Bugfix: Compare zero-copy get request to the cache size instead of the
>   flush threshold; otherwise refill could overflow the memory allocated
>   for the cache. (Andrew)
> * Split the zero-copy put function into an internal function doing the
>   work, and a public function with trace.
> * Avoid code duplication by rewriting rte_mempool_do_generic_put() to use
>   the internal zero-copy put function. (Andrew)
> * Corrected the return type of rte_mempool_cache_zc_put_bulk() from void
> *
>   to void **; it returns a pointer to an array of objects.
> * Fix coding style: Add missing curly brackets. (Andrew)
> v4:
> * Fix checkpatch warnings.
> v3:
> * Bugfix: Respect the cache size; compare to the flush threshold instead
>   of RTE_MEMPOOL_CACHE_MAX_SIZE.
> * Added 'rewind' function for incomplete 'put' operations. (Konstantin)
> * Replace RTE_ASSERTs with runtime checks of the request size.
>   Instead of failing, return NULL if the request is too big. (Konstantin)
> * Modified comparison to prevent overflow if n is really huge and len is
>   non-zero.
> * Updated the comments in the code.
> v2:
> * Fix checkpatch warnings.
> * Fix missing registration of trace points.
> * The functions are inline, so they don't go into the map file.
> v1 changes from the RFC:
> * Removed run-time parameter checks. (Honnappa)
>   This is a hot fast path function; requiring correct application
>   behaviour, i.e. function parameters must be valid.
> * Added RTE_ASSERT for parameters instead.
>   Code for this is only generated if built with RTE_ENABLE_ASSERT.
> * Removed fallback when 'cache' parameter is not set. (Honnappa)
> * Chose the simple get function; i.e. do not move the existing objects in
>   the cache to the top of the new stack, just leave them at the bottom.
> * Renamed the functions. Other suggestions are welcome, of course. ;-)
> * Updated the function descriptions.
> * Added the functions to trace_fp and version.map.
> ---
>  lib/mempool/mempool_trace_points.c |   9 ++
>  lib/mempool/rte_mempool.h          | 238 +++++++++++++++++++++++++----
>  lib/mempool/rte_mempool_trace_fp.h |  23 +++
>  lib/mempool/version.map            |   5 +
>  4 files changed, 246 insertions(+), 29 deletions(-)
> 
> diff --git a/lib/mempool/mempool_trace_points.c
> b/lib/mempool/mempool_trace_points.c
> index 4ad76deb34..83d353a764 100644
> --- a/lib/mempool/mempool_trace_points.c
> +++ b/lib/mempool/mempool_trace_points.c
> @@ -77,3 +77,12 @@
> RTE_TRACE_POINT_REGISTER(rte_mempool_trace_ops_free,
> 
>  RTE_TRACE_POINT_REGISTER(rte_mempool_trace_set_ops_byname,
>  	lib.mempool.set.ops.byname)
> +
> +RTE_TRACE_POINT_REGISTER(rte_mempool_trace_cache_zc_put_bulk,
> +	lib.mempool.cache.zc.put.bulk)
> +
> +RTE_TRACE_POINT_REGISTER(rte_mempool_trace_cache_zc_put_rewind,
> +	lib.mempool.cache.zc.put.rewind)
> +
> +RTE_TRACE_POINT_REGISTER(rte_mempool_trace_cache_zc_get_bulk,
> +	lib.mempool.cache.zc.get.bulk)
> diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h
> index 9f530db24b..711a9d1c16 100644
> --- a/lib/mempool/rte_mempool.h
> +++ b/lib/mempool/rte_mempool.h
> @@ -42,6 +42,7 @@
>  #include <rte_config.h>
>  #include <rte_spinlock.h>
>  #include <rte_debug.h>
> +#include <rte_errno.h>
>  #include <rte_lcore.h>
>  #include <rte_branch_prediction.h>
>  #include <rte_ring.h>
> @@ -1346,6 +1347,198 @@ rte_mempool_cache_flush(struct
> rte_mempool_cache *cache,
>  	cache->len = 0;
>  }
> 
> +
> +/**
> + * @internal used by rte_mempool_cache_zc_put_bulk() and
> rte_mempool_do_generic_put().
> + *
> + * Zero-copy put objects in a mempool cache backed by the specified
> mempool.
> + *
> + * @param cache
> + *   A pointer to the mempool cache.
> + * @param mp
> + *   A pointer to the mempool.
> + * @param n
> + *   The number of objects to be put in the mempool cache.
> + * @return
> + *   The pointer to where to put the objects in the mempool cache.
> + *   NULL if the request itself is too big for the cache, i.e.
> + *   exceeds the cache flush threshold.
> + */
> +static __rte_always_inline void **
> +__rte_mempool_cache_zc_put_bulk(struct rte_mempool_cache *cache,
> +		struct rte_mempool *mp,
> +		unsigned int n)
> +{
> +	void **cache_objs;
> +
> +	RTE_ASSERT(cache != NULL);
> +	RTE_ASSERT(mp != NULL);
> +
> +	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 if (likely(n <= cache->flushthresh)) {
> +		/*
> +		 * 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);
This is a flush of the cache. It is probably worth having a counter for this.

> +		cache->len = n;
> +	} else {
> +		/* The request itself is too big for the cache. */
This is possibly an error condition. Do we need to set rte_errno? Do we need a counter here to capture that?

> +		return NULL;
> +	}
> +
> +	RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_bulk, 1);
> +	RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_objs, n);
> +
> +	return cache_objs;
> +}
> +
> +/**
> + * @warning
> + * @b EXPERIMENTAL: This API may change, or be removed, without prior
> notice.
> + *
> + * Zero-copy put objects in a mempool cache backed by the specified
> mempool.
> + *
> + * @param cache
> + *   A pointer to the mempool cache.
> + * @param mp
> + *   A pointer to the mempool.
> + * @param n
> + *   The number of objects to be put in the mempool cache.
> + * @return
> + *   The pointer to where to put the objects in the mempool cache.
> + *   NULL if the request itself is too big for the cache, i.e.
> + *   exceeds the cache flush threshold.
> + */
> +__rte_experimental
> +static __rte_always_inline void **
> +rte_mempool_cache_zc_put_bulk(struct rte_mempool_cache *cache,
> +		struct rte_mempool *mp,
> +		unsigned int n)
> +{
> +	RTE_ASSERT(cache != NULL);
> +	RTE_ASSERT(mp != NULL);
> +
> +	rte_mempool_trace_cache_zc_put_bulk(cache, mp, n);
> +	return __rte_mempool_cache_zc_put_bulk(cache, mp, n); }
> +
> +/**
> + * @warning
> + * @b EXPERIMENTAL: This API may change, or be removed, without prior
> notice.
> + *
> + * Zero-copy un-put objects in a mempool cache.
> + *
> + * @param cache
> + *   A pointer to the mempool cache.
> + * @param n
> + *   The number of objects not put in the mempool cache after calling
> + *   rte_mempool_cache_zc_put_bulk().
> + */
> +__rte_experimental
> +static __rte_always_inline void
> +rte_mempool_cache_zc_put_rewind(struct rte_mempool_cache *cache,
> +		unsigned int n)
> +{
> +	RTE_ASSERT(cache != NULL);
> +	RTE_ASSERT(n <= cache->len);
> +
> +	rte_mempool_trace_cache_zc_put_rewind(cache, n);
> +
> +	cache->len -= n;
> +
> +	RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_objs, (int)-n); }
> +
> +/**
> + * @warning
> + * @b EXPERIMENTAL: This API may change, or be removed, without prior
> notice.
> + *
> + * Zero-copy get objects from a mempool cache backed by the specified
> mempool.
> + *
> + * @param cache
> + *   A pointer to the mempool cache.
> + * @param mp
> + *   A pointer to the mempool.
> + * @param n
> + *   The number of objects to be made available for extraction from the
> mempool cache.
> + * @return
> + *   The pointer to the objects in the mempool cache.
> + *   NULL on error; i.e. the cache + the pool does not contain 'n' objects.
> + *   With rte_errno set to the error code of the mempool dequeue function,
> + *   or EINVAL if the request itself is too big for the cache, i.e.
> + *   exceeds the cache flush threshold.
> + */
> +__rte_experimental
> +static __rte_always_inline void *
> +rte_mempool_cache_zc_get_bulk(struct rte_mempool_cache *cache,
> +		struct rte_mempool *mp,
> +		unsigned int n)
> +{
> +	unsigned int len, size;
> +
> +	RTE_ASSERT(cache != NULL);
> +	RTE_ASSERT(mp != NULL);
> +
> +	rte_mempool_trace_cache_zc_get_bulk(cache, mp, n);
> +
> +	len = cache->len;
> +	size = cache->size;
> +
> +	if (n <= len) {
> +		/* The request can be satisfied from the cache as is. */
> +		len -= n;
> +	} else if (likely(n <= size)) {
> +		/*
> +		 * The request itself can be satisfied from the cache.
> +		 * But first, the cache must be filled from the backend;
> +		 * fetch size + requested - len objects.
> +		 */
> +		int ret;
> +
> +		ret = rte_mempool_ops_dequeue_bulk(mp, &cache-
> >objs[len], size + n - len);
> +		if (unlikely(ret < 0)) {
> +			/*
> +			 * We are buffer constrained.
> +			 * Do not fill the cache, just satisfy the request.
> +			 */
> +			ret = rte_mempool_ops_dequeue_bulk(mp, &cache-
> >objs[len], n - len);
> +			if (unlikely(ret < 0)) {
> +				/* Unable to satisfy the request. */
> +
> +				RTE_MEMPOOL_STAT_ADD(mp,
> get_fail_bulk, 1);
> +				RTE_MEMPOOL_STAT_ADD(mp,
> get_fail_objs, n);
> +
> +				rte_errno = -ret;
> +				return NULL;
> +			}
> +
> +			len = 0;
> +		} else {
> +			len = size;
> +		}
> +	} else {
> +		/* The request itself is too big for the cache. */
> +		rte_errno = EINVAL;
> +		return NULL;
> +	}
> +
> +	cache->len = len;
> +
> +	RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_bulk, 1);
> +	RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_objs, n);
> +
> +	return &cache->objs[len];
> +}
> +
>  /**
>   * @internal Put several objects back in the mempool; used internally.
>   * @param mp
> @@ -1364,32 +1557,25 @@ rte_mempool_do_generic_put(struct
> rte_mempool *mp, void * const *obj_table,  {
>  	void **cache_objs;
> 
> -	/* No cache provided */
> -	if (unlikely(cache == NULL))
> -		goto driver_enqueue;
> +	/* No cache provided? */
> +	if (unlikely(cache == NULL)) {
> +		/* Increment stats now, adding in mempool always succeeds.
> */
> +		RTE_MEMPOOL_STAT_ADD(mp, put_bulk, 1);
> +		RTE_MEMPOOL_STAT_ADD(mp, put_objs, n);
> 
> -	/* increment stat now, adding in mempool always success */
> -	RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_bulk, 1);
> -	RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_objs, n);
> +		goto driver_enqueue;
> +	}
> 
> -	/* The request itself is too big for the cache */
> -	if (unlikely(n > cache->flushthresh))
> -		goto driver_enqueue_stats_incremented;
> +	/* Prepare to add the objects to the cache. */
> +	cache_objs = __rte_mempool_cache_zc_put_bulk(cache, mp, n);
> 
> -	/*
> -	 * 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.
> -	 */
> +	/* The request itself is too big for the cache? */
> +	if (unlikely(cache_objs == NULL)) {
> +		/* 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);
> 
> -	if (cache->len + n <= cache->flushthresh) {
> -		cache_objs = &cache->objs[cache->len];
> -		cache->len += n;
> -	} else {
> -		cache_objs = &cache->objs[0];
> -		rte_mempool_ops_enqueue_bulk(mp, cache_objs, cache-
> >len);
> -		cache->len = n;
> +		goto driver_enqueue;
>  	}
> 
>  	/* Add the objects to the cache. */
> @@ -1399,13 +1585,7 @@ rte_mempool_do_generic_put(struct
> rte_mempool *mp, void * const *obj_table,
> 
>  driver_enqueue:
> 
> -	/* increment stat now, adding in mempool always success */
> -	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);  }
> 
> diff --git a/lib/mempool/rte_mempool_trace_fp.h
> b/lib/mempool/rte_mempool_trace_fp.h
> index ed060e887c..14666457f7 100644
> --- a/lib/mempool/rte_mempool_trace_fp.h
> +++ b/lib/mempool/rte_mempool_trace_fp.h
> @@ -109,6 +109,29 @@ RTE_TRACE_POINT_FP(
>  	rte_trace_point_emit_ptr(mempool);
>  )
> 
> +RTE_TRACE_POINT_FP(
> +	rte_mempool_trace_cache_zc_put_bulk,
> +	RTE_TRACE_POINT_ARGS(void *cache, void *mempool, uint32_t
> nb_objs),
> +	rte_trace_point_emit_ptr(cache);
> +	rte_trace_point_emit_ptr(mempool);
> +	rte_trace_point_emit_u32(nb_objs);
> +)
> +
> +RTE_TRACE_POINT_FP(
> +	rte_mempool_trace_cache_zc_put_rewind,
> +	RTE_TRACE_POINT_ARGS(void *cache, uint32_t nb_objs),
> +	rte_trace_point_emit_ptr(cache);
> +	rte_trace_point_emit_u32(nb_objs);
> +)
> +
> +RTE_TRACE_POINT_FP(
> +	rte_mempool_trace_cache_zc_get_bulk,
> +	RTE_TRACE_POINT_ARGS(void *cache, void *mempool, uint32_t
> nb_objs),
> +	rte_trace_point_emit_ptr(cache);
> +	rte_trace_point_emit_ptr(mempool);
> +	rte_trace_point_emit_u32(nb_objs);
> +)
> +
>  #ifdef __cplusplus
>  }
>  #endif
> diff --git a/lib/mempool/version.map b/lib/mempool/version.map index
> b67d7aace7..1383ae6db2 100644
> --- a/lib/mempool/version.map
> +++ b/lib/mempool/version.map
> @@ -63,6 +63,11 @@ EXPERIMENTAL {
>  	__rte_mempool_trace_ops_alloc;
>  	__rte_mempool_trace_ops_free;
>  	__rte_mempool_trace_set_ops_byname;
> +
> +	# added in 23.03
> +	__rte_mempool_trace_cache_zc_put_bulk;
> +	__rte_mempool_trace_cache_zc_put_rewind;
> +	__rte_mempool_trace_cache_zc_get_bulk;
>  };
> 
>  INTERNAL {
> --
> 2.17.1