linux-kernel - Re: [PATCH V1] mm/slab: introduce kvfree_rcu_barrier_on

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Message-ID: <1f19b775-d670-40ef-9147-2dcdce62b56e@kernel.org>
Date: Fri, 28 Nov 2025 13:38:34 +0100
From: Daniel Gomez <da.gomez@...nel.org>
To: Harry Yoo <harry.yoo@...cle.com>, surenb@...gle.com
Cc: Liam.Howlett@...cle.com, atomlin@...mlin.com, bpf@...r.kernel.org,
 cl@...two.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-modules@...r.kernel.org, lucas.demarchi@...el.com,
 maple-tree@...ts.infradead.org, mcgrof@...nel.org, petr.pavlu@...e.com,
 rcu@...r.kernel.org, rientjes@...gle.com, roman.gushchin@...ux.dev,
 samitolvanen@...gle.com, sidhartha.kumar@...cle.com, urezki@...il.com,
 vbabka@...e.cz, jonathanh@...dia.com
Subject: Re: [PATCH V1] mm/slab: introduce kvfree_rcu_barrier_on_cache() for
 cache destruction



On 28/11/2025 12.37, Harry Yoo wrote:
> Currently, kvfree_rcu_barrier() flushes RCU sheaves across all slab
> caches when a cache is destroyed. This is unnecessary when destroying
> a slab cache; only the RCU sheaves belonging to the cache being destroyed
> need to be flushed.
> 
> As suggested by Vlastimil Babka, introduce a weaker form of
> kvfree_rcu_barrier() that operates on a specific slab cache and call it
> on cache destruction.
> 
> The performance benefit is evaluated on a 12 core 24 threads AMD Ryzen
> 5900X machine (1 socket), by loading slub_kunit module.
> 
> Before:
>   Total calls: 19
>   Average latency (us): 8529
>   Total time (us): 162069
> 
> After:
>   Total calls: 19
>   Average latency (us): 3804
>   Total time (us): 72287
> 
> Link: https://lore.kernel.org/linux-mm/0406562e-2066-4cf8-9902-b2b0616dd742@kernel.org
> Link: https://lore.kernel.org/linux-mm/e988eff6-1287-425e-a06c-805af5bbf262@nvidia.com
> Link: https://lore.kernel.org/linux-mm/1bda09da-93be-4737-aef0-d47f8c5c9301@suse.cz
> Suggested-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Harry Yoo <harry.yoo@...cle.com>
> ---

Thanks Harry for the patch,

A quick test on a different machine from the one I originally used to report
this shows a decrease from 214s to 100s.

LGTM,

Tested-by: Daniel Gomez <da.gomez@...sung.com>

> 
> Not sure if the regression is worse on the reporters' machines due to
> higher core count (or because some cores were busy doing other things,
> dunno).

FWIW, CI modules run on an 8 core VM. Depending on the host CPU, this made the
absolute number different but equivalent performance degradation.

> 
> Hopefully this will reduce the time to complete tests,
> and Suren could add his patch on top of this ;)
> 
>  include/linux/slab.h |  5 ++++
>  mm/slab.h            |  1 +
>  mm/slab_common.c     | 52 +++++++++++++++++++++++++++++------------
>  mm/slub.c            | 55 ++++++++++++++++++++++++--------------------
>  4 files changed, 73 insertions(+), 40 deletions(-)