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Wed, 23 Jul 2025 13:35:03 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id aCjYNwflgGh0IwAAD6G6ig (envelope-from ); Wed, 23 Jul 2025 13:35:03 +0000 From: Vlastimil Babka Date: Wed, 23 Jul 2025 15:34:35 +0200 Subject: [PATCH v5 02/14] slab: add sheaf support for batching kfree_rcu() operations MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20250723-slub-percpu-caches-v5-2-b792cd830f5d@suse.cz> References: <20250723-slub-percpu-caches-v5-0-b792cd830f5d@suse.cz> In-Reply-To: <20250723-slub-percpu-caches-v5-0-b792cd830f5d@suse.cz> To: Suren Baghdasaryan , "Liam R. Howlett" , Christoph Lameter , David Rientjes Cc: Roman Gushchin , Harry Yoo , Uladzislau Rezki , linux-mm@kvack.org, linux-kernel@vger.kernel.org, rcu@vger.kernel.org, maple-tree@lists.infradead.org, vbabka@suse.cz X-Mailer: b4 0.14.2 X-Rspamd-Action: no action X-Rspam-User: X-Rspamd-Queue-Id: 55DDE14000D X-Rspamd-Server: rspam06 X-Stat-Signature: fq9rh4kepyo4odjebozfpksf7ii4yd7f X-HE-Tag: 1753277717-394399 X-HE-Meta: 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 ozE9SeMk Rk27/CG3syya1SNCueoxqVXjZnOq57IjEYHxyn5tvstM4jC2FKCgC7LTEUMruUcWxDRP1+k+7YmtI33DMIenvGkUFv++cZviyVg2zucR0in2lkghDdN6+XrNrhQ6QHw9bZUjrIU8otE3maSjxdxpvYa0TaXR4OXzapMxO32B8v8G90rEb1mV3rFwbHjhm+Ag+pShuiho0uCw78fZCGWg/lXY33o0GYQL5xJQ6OjfSkVUICKrYx0qzpIw0ACUU/wjqe09UOwtOAWuy9tIkBsfLLciJisLfWuE+gw3LKE/P/O1uLKyxuVVsODsn66GP8in7MZigt9eNE7R8EAs/ELyxuksL7A== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 Reviewed-by: Suren Baghdasaryan Signed-off-by: Vlastimil Babka --- mm/slab.h | 2 + mm/slab_common.c | 24 +++++++ mm/slub.c | 193 +++++++++++++++++++++++++++++++++++++++++++++++++++++-- 3 files changed, 214 insertions(+), 5 deletions(-) diff --git a/mm/slab.h b/mm/slab.h index 1980330c2fcb4a4613a7e4f7efc78b349993fd89..44c9b70eaabbd87c06fb39b79dfb791d515acbde 100644 --- a/mm/slab.h +++ b/mm/slab.h @@ -459,6 +459,8 @@ 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; + // Queue the object but don't yet schedule the batch. if (debug_rcu_head_queue(ptr)) { // Probable double kfree_rcu(), just leak. diff --git a/mm/slub.c b/mm/slub.c index 6543aaade60b0adaab232b2256d65c1042c62e1c..f6d86cd3983533784583f1df6add186c4a74cd97 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -350,6 +350,8 @@ enum stat_item { ALLOC_FASTPATH, /* Allocation from cpu slab */ ALLOC_SLOWPATH, /* Allocation by getting a new cpu slab */ FREE_PCS, /* Free to percpu sheaf */ + FREE_RCU_SHEAF, /* Free to rcu_free sheaf */ + FREE_RCU_SHEAF_FAIL, /* Failed to free to a rcu_free sheaf */ FREE_FASTPATH, /* Free to cpu slab */ FREE_SLOWPATH, /* Freeing not to cpu slab */ FREE_FROZEN, /* Freeing to frozen slab */ @@ -444,6 +446,7 @@ struct slab_sheaf { struct rcu_head rcu_head; struct list_head barn_list; }; + struct kmem_cache *cache; unsigned int size; void *objects[]; }; @@ -452,6 +455,7 @@ struct slub_percpu_sheaves { local_trylock_t lock; struct slab_sheaf *main; /* never NULL when unlocked */ struct slab_sheaf *spare; /* empty or full, may be NULL */ + struct slab_sheaf *rcu_free; /* for batching kfree_rcu() */ struct node_barn *barn; }; @@ -2490,6 +2494,8 @@ static struct slab_sheaf *alloc_empty_sheaf(struct kmem_cache *s, gfp_t gfp) if (unlikely(!sheaf)) return NULL; + sheaf->cache = s; + stat(s, SHEAF_ALLOC); return sheaf; @@ -2614,6 +2620,43 @@ static void sheaf_flush_unused(struct kmem_cache *s, struct slab_sheaf *sheaf) sheaf->size = 0; } +static void __rcu_free_sheaf_prepare(struct kmem_cache *s, + struct slab_sheaf *sheaf) +{ + bool init = slab_want_init_on_free(s); + void **p = &sheaf->objects[0]; + unsigned int i = 0; + + while (i < sheaf->size) { + struct slab *slab = virt_to_slab(p[i]); + + memcg_slab_free_hook(s, slab, p + i, 1); + alloc_tagging_slab_free_hook(s, slab, p + i, 1); + + if (unlikely(!slab_free_hook(s, p[i], init, true))) { + p[i] = p[--sheaf->size]; + continue; + } + + i++; + } +} + +static void rcu_free_sheaf_nobarn(struct rcu_head *head) +{ + struct slab_sheaf *sheaf; + struct kmem_cache *s; + + sheaf = container_of(head, struct slab_sheaf, rcu_head); + s = sheaf->cache; + + __rcu_free_sheaf_prepare(s, sheaf); + + sheaf_flush_unused(s, sheaf); + + free_empty_sheaf(s, sheaf); +} + /* * Caller needs to make sure migration is disabled in order to fully flush * single cpu's sheaves @@ -2626,7 +2669,7 @@ static void sheaf_flush_unused(struct kmem_cache *s, struct slab_sheaf *sheaf) static void pcs_flush_all(struct kmem_cache *s) { struct slub_percpu_sheaves *pcs; - struct slab_sheaf *spare; + struct slab_sheaf *spare, *rcu_free; local_lock(&s->cpu_sheaves->lock); pcs = this_cpu_ptr(s->cpu_sheaves); @@ -2634,6 +2677,9 @@ static void pcs_flush_all(struct kmem_cache *s) spare = pcs->spare; pcs->spare = NULL; + rcu_free = pcs->rcu_free; + pcs->rcu_free = NULL; + local_unlock(&s->cpu_sheaves->lock); if (spare) { @@ -2641,6 +2687,9 @@ static void pcs_flush_all(struct kmem_cache *s) free_empty_sheaf(s, spare); } + if (rcu_free) + call_rcu(&rcu_free->rcu_head, rcu_free_sheaf_nobarn); + sheaf_flush_main(s); } @@ -2657,6 +2706,11 @@ static void __pcs_flush_all_cpu(struct kmem_cache *s, unsigned int cpu) free_empty_sheaf(s, pcs->spare); pcs->spare = NULL; } + + if (pcs->rcu_free) { + call_rcu(&pcs->rcu_free->rcu_head, rcu_free_sheaf_nobarn); + pcs->rcu_free = NULL; + } } static void pcs_destroy(struct kmem_cache *s) @@ -2682,6 +2736,7 @@ static void pcs_destroy(struct kmem_cache *s) */ WARN_ON(pcs->spare); + WARN_ON(pcs->rcu_free); if (!WARN_ON(pcs->main->size)) { free_empty_sheaf(s, pcs->main); @@ -3742,7 +3797,7 @@ static bool has_pcs_used(int cpu, struct kmem_cache *s) pcs = per_cpu_ptr(s->cpu_sheaves, cpu); - return (pcs->spare || pcs->main->size); + return (pcs->spare || pcs->rcu_free || pcs->main->size); } static void pcs_flush_all(struct kmem_cache *s); @@ -5339,6 +5394,127 @@ bool free_to_pcs(struct kmem_cache *s, void *object) return true; } +static void rcu_free_sheaf(struct rcu_head *head) +{ + struct slab_sheaf *sheaf; + struct node_barn *barn; + struct kmem_cache *s; + + sheaf = container_of(head, struct slab_sheaf, rcu_head); + + s = sheaf->cache; + + /* + * This may remove some objects due to slab_free_hook() returning false, + * so that the sheaf might no longer be completely full. But it's easier + * to handle it as full (unless it became completely empty), as the code + * handles it fine. The only downside is that sheaf will serve fewer + * allocations when reused. It only happens due to debugging, which is a + * performance hit anyway. + */ + __rcu_free_sheaf_prepare(s, sheaf); + + barn = get_node(s, numa_mem_id())->barn; + + /* due to slab_free_hook() */ + if (unlikely(sheaf->size == 0)) + goto empty; + + /* + * Checking nr_full/nr_empty outside lock avoids contention in case the + * barn is at the respective limit. Due to the race we might go over the + * limit but that should be rare and harmless. + */ + + if (data_race(barn->nr_full) < MAX_FULL_SHEAVES) { + stat(s, BARN_PUT); + barn_put_full_sheaf(barn, sheaf); + return; + } + + stat(s, BARN_PUT_FAIL); + sheaf_flush_unused(s, sheaf); + +empty: + if (data_race(barn->nr_empty) < MAX_EMPTY_SHEAVES) { + barn_put_empty_sheaf(barn, sheaf); + return; + } + + free_empty_sheaf(s, sheaf); +} + +bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj) +{ + struct slub_percpu_sheaves *pcs; + struct slab_sheaf *rcu_sheaf; + + if (!local_trylock(&s->cpu_sheaves->lock)) + goto fail; + + pcs = this_cpu_ptr(s->cpu_sheaves); + + if (unlikely(!pcs->rcu_free)) { + + struct slab_sheaf *empty; + + if (pcs->spare && pcs->spare->size == 0) { + pcs->rcu_free = pcs->spare; + pcs->spare = NULL; + goto do_free; + } + + empty = barn_get_empty_sheaf(pcs->barn); + + if (empty) { + pcs->rcu_free = empty; + goto do_free; + } + + local_unlock(&s->cpu_sheaves->lock); + + empty = alloc_empty_sheaf(s, GFP_NOWAIT); + + if (!empty) + goto fail; + + if (!local_trylock(&s->cpu_sheaves->lock)) { + barn_put_empty_sheaf(pcs->barn, empty); + goto fail; + } + + pcs = this_cpu_ptr(s->cpu_sheaves); + + if (unlikely(pcs->rcu_free)) + barn_put_empty_sheaf(pcs->barn, empty); + else + pcs->rcu_free = empty; + } + +do_free: + + rcu_sheaf = pcs->rcu_free; + + rcu_sheaf->objects[rcu_sheaf->size++] = obj; + + if (likely(rcu_sheaf->size < s->sheaf_capacity)) + rcu_sheaf = NULL; + else + pcs->rcu_free = NULL; + + local_unlock(&s->cpu_sheaves->lock); + + if (rcu_sheaf) + call_rcu(&rcu_sheaf->rcu_head, rcu_free_sheaf); + + stat(s, FREE_RCU_SHEAF); + return true; + +fail: + stat(s, FREE_RCU_SHEAF_FAIL); + return false; +} + /* * Bulk free objects to the percpu sheaves. * Unlike free_to_pcs() this includes the calls to all necessary hooks @@ -5348,10 +5524,8 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p) { struct slub_percpu_sheaves *pcs; struct slab_sheaf *main, *empty; + bool init = slab_want_init_on_free(s); unsigned int batch, i = 0; - bool init; - - init = slab_want_init_on_free(s); while (i < size) { struct slab *slab = virt_to_slab(p[i]); @@ -6838,6 +7012,11 @@ int __kmem_cache_shutdown(struct kmem_cache *s) struct kmem_cache_node *n; flush_all_cpus_locked(s); + + /* we might have rcu sheaves in flight */ + if (s->cpu_sheaves) + rcu_barrier(); + /* Attempt to free all objects */ for_each_kmem_cache_node(s, node, n) { if (n->barn) @@ -8251,6 +8430,8 @@ STAT_ATTR(ALLOC_PCS, alloc_cpu_sheaf); STAT_ATTR(ALLOC_FASTPATH, alloc_fastpath); STAT_ATTR(ALLOC_SLOWPATH, alloc_slowpath); STAT_ATTR(FREE_PCS, free_cpu_sheaf); +STAT_ATTR(FREE_RCU_SHEAF, free_rcu_sheaf); +STAT_ATTR(FREE_RCU_SHEAF_FAIL, free_rcu_sheaf_fail); STAT_ATTR(FREE_FASTPATH, free_fastpath); STAT_ATTR(FREE_SLOWPATH, free_slowpath); STAT_ATTR(FREE_FROZEN, free_frozen); @@ -8349,6 +8530,8 @@ static struct attribute *slab_attrs[] = { &alloc_fastpath_attr.attr, &alloc_slowpath_attr.attr, &free_cpu_sheaf_attr.attr, + &free_rcu_sheaf_attr.attr, + &free_rcu_sheaf_fail_attr.attr, &free_fastpath_attr.attr, &free_slowpath_attr.attr, &free_frozen_attr.attr, -- 2.50.1