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Message-ID: <aSmT7kMDk7SLqXA5@hyeyoo>
Date: Fri, 28 Nov 2025 21:22:06 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: surenb@...gle.com
Cc: Liam.Howlett@...cle.com, atomlin@...mlin.com, bpf@...r.kernel.org,
cl@...two.org, da.gomez@...nel.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 Fri, Nov 28, 2025 at 08:37:40PM +0900, 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
Ooh, I just realized that I messed up the config and
have only two cores enabled. Will update the numbers after enabling 22 more :)
> 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>
> ---
>
> 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).
>
> 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(-)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index cf443f064a66..937c93d44e8c 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -1149,6 +1149,10 @@ static inline void kvfree_rcu_barrier(void)
> {
> rcu_barrier();
> }
> +static inline void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
> +{
> + rcu_barrier();
> +}
>
> static inline void kfree_rcu_scheduler_running(void) { }
> #else
> @@ -1156,6 +1160,7 @@ void kvfree_rcu_barrier(void);
>
> void kfree_rcu_scheduler_running(void);
> #endif
> +void kvfree_rcu_barrier_on_cache(struct kmem_cache *s);
>
> /**
> * kmalloc_size_roundup - Report allocation bucket size for the given size
> diff --git a/mm/slab.h b/mm/slab.h
> index f730e012553c..e767aa7e91b0 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -422,6 +422,7 @@ static inline bool is_kmalloc_normal(struct kmem_cache *s)
>
> bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj);
> void flush_all_rcu_sheaves(void);
> +void flush_rcu_sheaves_on_cache(struct kmem_cache *s);
>
> #define SLAB_CORE_FLAGS (SLAB_HWCACHE_ALIGN | SLAB_CACHE_DMA | \
> SLAB_CACHE_DMA32 | SLAB_PANIC | \
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 84dfff4f7b1f..dd8a49d6f9cc 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -492,7 +492,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
> return;
>
> /* in-flight kfree_rcu()'s may include objects from our cache */
> - kvfree_rcu_barrier();
> + kvfree_rcu_barrier_on_cache(s);
>
> if (IS_ENABLED(CONFIG_SLUB_RCU_DEBUG) &&
> (s->flags & SLAB_TYPESAFE_BY_RCU)) {
> @@ -2038,25 +2038,13 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
> }
> EXPORT_SYMBOL_GPL(kvfree_call_rcu);
>
> -/**
> - * kvfree_rcu_barrier - Wait until all in-flight kvfree_rcu() complete.
> - *
> - * Note that a single argument of kvfree_rcu() call has a slow path that
> - * triggers synchronize_rcu() following by freeing a pointer. It is done
> - * before the return from the function. Therefore for any single-argument
> - * call that will result in a kfree() to a cache that is to be destroyed
> - * during module exit, it is developer's responsibility to ensure that all
> - * such calls have returned before the call to kmem_cache_destroy().
> - */
> -void kvfree_rcu_barrier(void)
> +static inline void __kvfree_rcu_barrier(void)
> {
> struct kfree_rcu_cpu_work *krwp;
> struct kfree_rcu_cpu *krcp;
> bool queued;
> int i, cpu;
>
> - flush_all_rcu_sheaves();
> -
> /*
> * Firstly we detach objects and queue them over an RCU-batch
> * for all CPUs. Finally queued works are flushed for each CPU.
> @@ -2118,8 +2106,43 @@ void kvfree_rcu_barrier(void)
> }
> }
> }
> +
> +/**
> + * kvfree_rcu_barrier - Wait until all in-flight kvfree_rcu() complete.
> + *
> + * Note that a single argument of kvfree_rcu() call has a slow path that
> + * triggers synchronize_rcu() following by freeing a pointer. It is done
> + * before the return from the function. Therefore for any single-argument
> + * call that will result in a kfree() to a cache that is to be destroyed
> + * during module exit, it is developer's responsibility to ensure that all
> + * such calls have returned before the call to kmem_cache_destroy().
> + */
> +void kvfree_rcu_barrier(void)
> +{
> + flush_all_rcu_sheaves();
> + __kvfree_rcu_barrier();
> +}
> EXPORT_SYMBOL_GPL(kvfree_rcu_barrier);
>
> +/**
> + * kvfree_rcu_barrier_on_cache - Wait for in-flight kvfree_rcu() calls on a
> + * specific slab cache.
> + * @s: slab cache to wait for
> + *
> + * See the description of kvfree_rcu_barrier() for details.
> + */
> +void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
> +{
> + if (s->cpu_sheaves)
> + flush_rcu_sheaves_on_cache(s);
> + /*
> + * TODO: Introduce a version of __kvfree_rcu_barrier() that works
> + * on a specific slab cache.
> + */
> + __kvfree_rcu_barrier();
> +}
> +EXPORT_SYMBOL_GPL(kvfree_rcu_barrier_on_cache);
> +
> static unsigned long
> kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
> {
> @@ -2215,4 +2238,3 @@ void __init kvfree_rcu_init(void)
> }
>
> #endif /* CONFIG_KVFREE_RCU_BATCHED */
> -
> diff --git a/mm/slub.c b/mm/slub.c
> index 785e25a14999..7cec2220712b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4118,42 +4118,47 @@ static void flush_rcu_sheaf(struct work_struct *w)
>
>
> /* needed for kvfree_rcu_barrier() */
> -void flush_all_rcu_sheaves(void)
> +void flush_rcu_sheaves_on_cache(struct kmem_cache *s)
> {
> struct slub_flush_work *sfw;
> - struct kmem_cache *s;
> unsigned int cpu;
>
> - cpus_read_lock();
> - mutex_lock(&slab_mutex);
> + mutex_lock(&flush_lock);
>
> - list_for_each_entry(s, &slab_caches, list) {
> - if (!s->cpu_sheaves)
> - continue;
> + for_each_online_cpu(cpu) {
> + sfw = &per_cpu(slub_flush, cpu);
>
> - mutex_lock(&flush_lock);
> + /*
> + * we don't check if rcu_free sheaf exists - racing
> + * __kfree_rcu_sheaf() might have just removed it.
> + * by executing flush_rcu_sheaf() on the cpu we make
> + * sure the __kfree_rcu_sheaf() finished its call_rcu()
> + */
>
> - for_each_online_cpu(cpu) {
> - sfw = &per_cpu(slub_flush, cpu);
> + INIT_WORK(&sfw->work, flush_rcu_sheaf);
> + sfw->s = s;
> + queue_work_on(cpu, flushwq, &sfw->work);
> + }
>
> - /*
> - * we don't check if rcu_free sheaf exists - racing
> - * __kfree_rcu_sheaf() might have just removed it.
> - * by executing flush_rcu_sheaf() on the cpu we make
> - * sure the __kfree_rcu_sheaf() finished its call_rcu()
> - */
> + for_each_online_cpu(cpu) {
> + sfw = &per_cpu(slub_flush, cpu);
> + flush_work(&sfw->work);
> + }
>
> - INIT_WORK(&sfw->work, flush_rcu_sheaf);
> - sfw->s = s;
> - queue_work_on(cpu, flushwq, &sfw->work);
> - }
> + mutex_unlock(&flush_lock);
> +}
>
> - for_each_online_cpu(cpu) {
> - sfw = &per_cpu(slub_flush, cpu);
> - flush_work(&sfw->work);
> - }
> +void flush_all_rcu_sheaves(void)
> +{
> + struct kmem_cache *s;
> +
> + cpus_read_lock();
> + mutex_lock(&slab_mutex);
>
> - mutex_unlock(&flush_lock);
> + list_for_each_entry(s, &slab_caches, list) {
> + if (!s->cpu_sheaves)
> + continue;
> + flush_rcu_sheaves_on_cache(s);
> }
>
> mutex_unlock(&slab_mutex);
> --
> 2.43.0
>
--
Cheers,
Harry / Hyeonggon
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