Re: [PATCH V2] mm/slab: introduce kvfree_rcu_barrier_on_cache() for cache destruction

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Dec 2, 2025 at 2:16 AM Harry Yoo <harry.yoo@oracle.com> wrote:
>
> Currently, kvfree_rcu_barrier() flushes RCU sheaves across all slab
> caches when a cache is destroyed. This is unnecessary; 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.
>
> Factor out flush_rcu_sheaves_on_cache() from flush_all_rcu_sheaves() and
> call it from flush_all_rcu_sheaves() and kvfree_rcu_barrier_on_cache().
>
> Call kvfree_rcu_barrier_on_cache() instead of kvfree_rcu_barrier() 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): 18127
>   Total time (us): 344414
>
> After:
>   Total calls: 19
>   Average latency (us): 10066
>   Total time (us): 191264
>
> Two performance regression have been reported:
>   - stress module loader test's runtime increases by 50-60% (Daniel)
>   - internal graphics test's runtime on Tegra23 increases by 35% (Jon)
>
> They are fixed by this change.
>
> Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> Fixes: ec66e0d59952 ("slab: add sheaf support for batching kfree_rcu() operations")
> Cc: <stable@vger.kernel.org>
> Link: https://lore.kernel.org/linux-mm/1bda09da-93be-4737-aef0-d47f8c5c9301@suse.cz
> Reported-and-tested-by: Daniel Gomez <da.gomez@samsung.com>
> Closes: https://lore.kernel.org/linux-mm/0406562e-2066-4cf8-9902-b2b0616dd742@kernel.org
> Reported-and-tested-by: Jon Hunter <jonathanh@nvidia.com>
> Closes: https://lore.kernel.org/linux-mm/e988eff6-1287-425e-a06c-805af5bbf262@nvidia.com
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>
> No code change, added proper tags and updated changelog.
>
>  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);

Should the above line be before the #endif? I was expecting something like this:

#ifndef CONFIG_KVFREE_RCU_BATCHED
...
static inline void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
{
    rcu_barrier();
}
#else
...
void kvfree_rcu_barrier_on_cache(struct kmem_cache *s);
#endif

but when I apply this patch on mm-new I get this:

#ifndef CONFIG_KVFREE_RCU_BATCHED
...
static inline void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
{
    rcu_barrier();
}
#else
...
#endif
void kvfree_rcu_barrier_on_cache(struct kmem_cache *s);

Other than that LGTM

>
>  /**
>   * 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
>