DPDK22.11RC1 meson test eventdev_selftest_sw failed//RE: [PATCH v4] mempool: fix get objects from mempool with cache

DPDK22.11RC1 meson test eventdev_selftest_sw failed//RE: [PATCH v4] mempool: fix get objects from mempool with cache

[Li, WeiyuanX]

Hi Andrew Rybchenko,

This patch is merged into dpdk22.11.0-rc1 we execute meson test driver/eventdev_selftest_sw  failed.
Could you please have a look at it, also submitted a Bugzilla ticket: https://bugs.dpdk.org/show_bug.cgi?id=1101

Regards,
Li, Weiyuan

> -----Original Message-----
> From: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
> Sent: Friday, October 7, 2022 6:45 PM
> To: Matz, Olivier <olivier.matz@6wind.com>
> Cc: dev@dpdk.org; Morten Brørup <mb@smartsharesystems.com>
> Subject: [PATCH v4] mempool: fix get objects from mempool with cache
> 
> From: Morten Brørup <mb@smartsharesystems.com>
> 
> 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.
> 
> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
> Reviewed-by: Morten Brørup <mb@smartsharesystems.com>
> ---
> v4 changes (Andrew Rybchenko)
>  - Avoid usage of misleading ring, since other mempool drivers
>    exist, use term backend
>  - Avoid term ring in goto label, use driver_dequeue as a label name
>  - Add likely() to cache != NULL in driver dequeue, just for symmetry
>  - Highlight that remaining objects are deqeueued from the driver
> 
> v3 changes (Andrew Rybchenko)
>  - Always get first objects from the cache even if request is bigger
>    than cache size. Remove one corresponding condition from the path
>    when request is fully served from cache.
>  - Simplify code to avoid duplication:
>     - Get objects directly from backend in single place only.
>     - Share code which gets from the cache first regardless if
>       everythihg is obtained from the cache or just the first part.
>  - Rollback cache length in unlikely failure branch to avoid cache
>    vs NULL check in success branch.
> 
> v2 changes
> - Do not modify description of return value. This belongs in a separate doc fix.
> - Elaborate even more on which bugs the modifications fix.
> 
>  lib/mempool/rte_mempool.h | 80 +++++++++++++++++++++++++-----------
> ---