Re: [PATCH] mempool: fix get objects from mempool with cache

[Olivier Matz <olivier.matz@6wind.com>]

Hi Morten,

Few comments below.

On Fri, Jan 14, 2022 at 05:36:50PM +0100, Morten Brørup wrote:
> 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.
> 
> This patch fixes 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.
> 
> 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 ring.
> 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 ring were returned ahead of the objects from the cache. This is
> bad for CPUs with a small L1 cache, which benefit from having the hot
> objects first in the returned array. (This is also the reason why
> the function returns the objects in reverse order.)
> Now, all code paths first return objects from the cache, subsequently
> from the ring.
> 
> 3. If the cache could not be backfilled, the function would attempt
> to get all the requested objects from the ring (instead of only the
> number of requested objects minus the objects available in the ring),
> 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 ring fails, only
> the remaining requested objects are retrieved from the ring.

This is the only point I'd consider to be a fix. The problem, from the
user perspective, is that a get() can fail despite there are enough
objects in cache + common pool.

To be honnest, I feel a bit uncomfortable to have such a list of
problems solved in one commit, even if I understand that they are part
of the same code rework.

Ideally, this fix should be a separate commit. What do you think of
having this simple patch for this fix, and then do the
optimizations/rework in another commit?

  --- a/lib/mempool/rte_mempool.h
  +++ b/lib/mempool/rte_mempool.h
  @@ -1484,7 +1484,22 @@ rte_mempool_do_generic_get(struct rte_mempool *mp, void **obj_table,
                           * the ring directly. If that fails, we are truly out of
                           * buffers.
                           */
  -                       goto ring_dequeue;
  +                       req = n - cache->len;
  +                       ret = rte_mempool_ops_dequeue_bulk(mp, obj_table, req);
  +                       if (ret < 0) {
  +                               RTE_MEMPOOL_STAT_ADD(mp, get_fail_bulk, 1);
  +                               RTE_MEMPOOL_STAT_ADD(mp, get_fail_objs, n);
  +                               return ret;
  +                       }
  +                       obj_table += req;
  +                       len = cache->len;
  +                       while (len > 0)
  +                               *obj_table++ = cache_objs[--len];
  +                       cache->len = 0;
  +                       RTE_MEMPOOL_STAT_ADD(mp, get_success_bulk, 1);
  +                       RTE_MEMPOOL_STAT_ADD(mp, get_success_objs, n);
  +
  +                       return 0;
                  }
   
                  cache->len += req;

The title of this commit could then be more precise to describe
the solved issue.

> 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.
> And in the code path where the cache was backfilled first, numbers were
> added and subtracted from the cache length; now this code path simply
> sets the cache length to its final value.
> 
> 5. Some comments were not correct anymore.
> The comments have been updated.
> Most importanly, the description of the succesful return value was
> inaccurate. Success only returns 0, not >= 0.
> 
> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> ---
>  lib/mempool/rte_mempool.h | 81 ++++++++++++++++++++++++++++-----------
>  1 file changed, 59 insertions(+), 22 deletions(-)
> 
> diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h
> index 1e7a3c1527..88f1b8b7ab 100644
> --- a/lib/mempool/rte_mempool.h
> +++ b/lib/mempool/rte_mempool.h
> @@ -1443,6 +1443,10 @@ rte_mempool_put(struct rte_mempool *mp, void *obj)
>  
>  /**
>   * @internal Get several objects from the mempool; used internally.
> + *
> + * If cache is enabled, objects are returned from the cache in Last In First
> + * Out (LIFO) order for the benefit of CPUs with small L1 cache.
> + *
>   * @param mp
>   *   A pointer to the mempool structure.
>   * @param obj_table
> @@ -1452,7 +1456,7 @@ rte_mempool_put(struct rte_mempool *mp, void *obj)
>   * @param cache
>   *   A pointer to a mempool cache structure. May be NULL if not needed.
>   * @return
> - *   - >=0: Success; number of objects supplied.
> + *   - 0: Success; got n objects.
>   *   - <0: Error; code of ring dequeue function.
>   */
>  static __rte_always_inline int

I think that part should be in a separate commit too. This is a
documentation fix, which is easily backportable (and should be
backported) (Fixes: af75078fece3 ("first public release")).

> @@ -1463,38 +1467,71 @@ rte_mempool_do_generic_get(struct rte_mempool *mp, void **obj_table,
>  	uint32_t index, len;
>  	void **cache_objs;
>  
> -	/* No cache provided or cannot be satisfied from cache */
> -	if (unlikely(cache == NULL || n >= cache->size))
> +	/* No cache provided or if get would overflow mem allocated for cache */
> +	if (unlikely(cache == NULL || n > RTE_MEMPOOL_CACHE_MAX_SIZE))
>  		goto ring_dequeue;
>  
> -	cache_objs = cache->objs;
> +	cache_objs = &cache->objs[cache->len];
> +
> +	if (n <= cache->len) {
> +		/* The entire request can be satisfied from the cache. */
> +		cache->len -= n;
> +		for (index = 0; index < n; 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);
> +		RTE_MEMPOOL_STAT_ADD(mp, get_success_bulk, 1);
> +		RTE_MEMPOOL_STAT_ADD(mp, get_success_objs, n);
>  
> -		/* 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);
> +		return 0;
> +	}
> +
> +	/* Satisfy the first part of the request by depleting the cache. */
> +	len = cache->len;
> +	for (index = 0; index < len; index++)
> +		*obj_table++ = *--cache_objs;
> +
> +	/* Number of objects remaining to satisfy the request. */
> +	len = n - len;
> +
> +	/* Fill the cache from the ring; fetch size + remaining objects. */
> +	ret = rte_mempool_ops_dequeue_bulk(mp, cache->objs,
> +			cache->size + len);
> +	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 ring.
> +		 */
> +		ret = rte_mempool_ops_dequeue_bulk(mp, obj_table, len);
>  		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.
> +			 * That also failed.
> +			 * No furter action is required to roll the first
> +			 * part of the request back into the cache, as both
> +			 * cache->len and the objects in the cache are intact.
>  			 */
> -			goto ring_dequeue;
> +			RTE_MEMPOOL_STAT_ADD(mp, get_fail_bulk, 1);
> +			RTE_MEMPOOL_STAT_ADD(mp, get_fail_objs, n);
> +
> +			return ret;
>  		}
>  
> -		cache->len += req;
> +		/* Commit that the cache was emptied. */
> +		cache->len = 0;
> +
> +		RTE_MEMPOOL_STAT_ADD(mp, get_success_bulk, 1);
> +		RTE_MEMPOOL_STAT_ADD(mp, get_success_objs, n);
> +
> +		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];
> +	cache_objs = &cache->objs[cache->size + len];
>  
> -	cache->len -= n;
> +	/* Satisfy the remaining part of the request from the filled cache. */
> +	cache->len = cache->size;
> +	for (index = 0; index < len; index++)
> +		*obj_table++ = *--cache_objs;
>  
>  	RTE_MEMPOOL_STAT_ADD(mp, get_success_bulk, 1);
>  	RTE_MEMPOOL_STAT_ADD(mp, get_success_objs, n);
> @@ -1503,7 +1540,7 @@ rte_mempool_do_generic_get(struct rte_mempool *mp, void **obj_table,
>  
>  ring_dequeue:
>  
> -	/* get remaining objects from ring */
> +	/* Get the objects from the ring. */
>  	ret = rte_mempool_ops_dequeue_bulk(mp, obj_table, n);
>  
>  	if (ret < 0) {

About the code itself, it is more readable now, and probably more
efficient. Did you notice any performance change in mempool perf
autotests ?

Thanks,
Olivier