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

[Harry Yoo <harry.yoo@oracle.com>]

On Thu, Dec 04, 2025 at 02:05:07PM -0800, Suren Baghdasaryan wrote:
> 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);

Oops, nice catch!
Interestingly this didn't break CONFIG_KVFREE_RCU_BATCHED=n builds...

I'll send V3 shortly.

> Other than that LGTM

Thanks!

> >  /**
> >   * 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