On Wed, Sep 03, 2025 at 02:59:46PM +0200, Vlastimil Babka wrote: > Extend the sheaf infrastructure for more efficient kfree_rcu() handling. > For caches with sheaves, on each cpu maintain a rcu_free sheaf in > addition to main and spare sheaves. > > kfree_rcu() operations will try to put objects on this sheaf. Once full, > the sheaf is detached and submitted to call_rcu() with a handler that > will try to put it in the barn, or flush to slab pages using bulk free, > when the barn is full. Then a new empty sheaf must be obtained to put > more objects there. > > It's possible that no free sheaves are available to use for a new > rcu_free sheaf, and the allocation in kfree_rcu() context can only use > GFP_NOWAIT and thus may fail. In that case, fall back to the existing > kfree_rcu() implementation. > > Expected advantages: > - batching the kfree_rcu() operations, that could eventually replace the > existing batching > - sheaves can be reused for allocations via barn instead of being > flushed to slabs, which is more efficient > - this includes cases where only some cpus are allowed to process rcu > callbacks (Android) > > Possible disadvantage: > - objects might be waiting for more than their grace period (it is > determined by the last object freed into the sheaf), increasing memory > usage - but the existing batching does that too. > > Only implement this for CONFIG_KVFREE_RCU_BATCHED as the tiny > implementation favors smaller memory footprint over performance. > > Add CONFIG_SLUB_STATS counters free_rcu_sheaf and free_rcu_sheaf_fail to > count how many kfree_rcu() used the rcu_free sheaf successfully and how > many had to fall back to the existing implementation. > > Reviewed-by: Harry Yoo <[email protected]> > Reviewed-by: Suren Baghdasaryan <[email protected]> > Signed-off-by: Vlastimil Babka <[email protected]> > --- > mm/slab.h | 2 + > mm/slab_common.c | 24 +++++++ > mm/slub.c | 192 > ++++++++++++++++++++++++++++++++++++++++++++++++++++++- > 3 files changed, 216 insertions(+), 2 deletions(-) > > diff --git a/mm/slab.h b/mm/slab.h > index > 206987ce44a4d053ebe3b5e50784d2dd23822cd1..f1866f2d9b211bb0d7f24644b80ef4b50a7c3d24 > 100644 > --- a/mm/slab.h > +++ b/mm/slab.h > @@ -435,6 +435,8 @@ static inline bool is_kmalloc_normal(struct kmem_cache *s) > return !(s->flags & (SLAB_CACHE_DMA|SLAB_ACCOUNT|SLAB_RECLAIM_ACCOUNT)); > } > > +bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj); > + > #define SLAB_CORE_FLAGS (SLAB_HWCACHE_ALIGN | SLAB_CACHE_DMA | \ > SLAB_CACHE_DMA32 | SLAB_PANIC | \ > SLAB_TYPESAFE_BY_RCU | SLAB_DEBUG_OBJECTS | \ > diff --git a/mm/slab_common.c b/mm/slab_common.c > index > e2b197e47866c30acdbd1fee4159f262a751c5a7..2d806e02568532a1000fd3912db6978e945dcfa8 > 100644 > --- a/mm/slab_common.c > +++ b/mm/slab_common.c > @@ -1608,6 +1608,27 @@ static void kfree_rcu_work(struct work_struct *work) > kvfree_rcu_list(head); > } > > +static bool kfree_rcu_sheaf(void *obj) > +{ > + struct kmem_cache *s; > + struct folio *folio; > + struct slab *slab; > + > + if (is_vmalloc_addr(obj)) > + return false; > + > + folio = virt_to_folio(obj); > + if (unlikely(!folio_test_slab(folio))) > + return false; > + > + slab = folio_slab(folio); > + s = slab->slab_cache; > + if (s->cpu_sheaves) > + return __kfree_rcu_sheaf(s, obj); > + > + return false; > +} > + > static bool > need_offload_krc(struct kfree_rcu_cpu *krcp) > { > @@ -1952,6 +1973,9 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr) > if (!head) > might_sleep(); > > + if (kfree_rcu_sheaf(ptr)) > + return; > + Uh.. I have some concerns about this.
This patch introduces a new path which is a collision to the existing kvfree_rcu() logic. It implements some batching which we already have. - kvfree_rcu_barrier() does not know about "sheaf" path. Am i missing something? How do you guarantee that kvfree_rcu_barrier() flushes sheafs? If it is part of kvfree_rcu() it has to care about this. - we do not allocate in kvfree_rcu() path because of PREEMMPT_RT, i.e. kvfree_rcu() is supposed it can be called from the non-sleeping contexts. - call_rcu() can be slow, therefore we do not use it in the kvfree_rcu(). IMO, it is worth to reuse existing logic in the kvfree_rcu(). I can help with it when i have more cycles as part of my RCU work. -- Uladzislau Rezki
