[PATCH RFC v3 3/8] slab: add sheaf support for batching kfree_rcu() operations

[Vlastimil Babka]

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() machinery.

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.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
---
 mm/slab.h        |   2 +
 mm/slab_common.c |  24 ++++++++
 mm/slub.c        | 165 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 189 insertions(+), 2 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index 8daaec53b6ecfc44171191d421adb12e5cba2c58..94e9959e1aefa350d3d74e3f5309fde7a5cf2ec8 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);
+
 /* Legal flag mask for kmem_cache_create(), for various configurations */
 #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 ceeefb287899a82f30ad79b403556001c1860311..9496176770ed47491e01ed78e060a74771d5541e 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1613,6 +1613,27 @@ 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)
 {
@@ -1957,6 +1978,9 @@ 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 fa3a6329713a9f45b189f27d4b1b334b54589c38..83f4395267dccfbc144920baa7d0a85a27fbb1b4 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 */
@@ -442,6 +444,7 @@ struct slab_sheaf {
 		struct rcu_head rcu_head;
 		struct list_head barn_list;
 	};
+	struct kmem_cache *cache;
 	unsigned int size;
 	void *objects[];
 };
@@ -450,6 +453,7 @@ struct slub_percpu_sheaves {
 	localtry_lock_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;
 };
 
@@ -2461,6 +2465,8 @@ 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;
@@ -2585,6 +2591,24 @@ 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);
+
+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
@@ -2597,7 +2621,7 @@ 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;
 
 	localtry_lock(&s->cpu_sheaves->lock);
 	pcs = this_cpu_ptr(s->cpu_sheaves);
@@ -2605,6 +2629,9 @@ static void pcs_flush_all(struct kmem_cache *s)
 	spare = pcs->spare;
 	pcs->spare = NULL;
 
+	rcu_free = pcs->rcu_free;
+	pcs->rcu_free = NULL;
+
 	localtry_unlock(&s->cpu_sheaves->lock);
 
 	if (spare) {
@@ -2612,6 +2639,9 @@ 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);
 }
 
@@ -2628,6 +2658,11 @@ 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)
@@ -2644,6 +2679,7 @@ static void pcs_destroy(struct kmem_cache *s)
 			continue;
 
 		WARN_ON(pcs->spare);
+		WARN_ON(pcs->rcu_free);
 
 		if (!WARN_ON(pcs->main->size)) {
 			free_empty_sheaf(s, pcs->main);
@@ -3707,7 +3743,7 @@ 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);
@@ -5240,6 +5276,122 @@ bool free_to_pcs(struct kmem_cache *s, void *object)
 	return true;
 }
 
+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, false))) {
+			p[i] = p[--sheaf->size];
+			continue;
+		}
+
+		i++;
+	}
+}
+
+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;
+
+	__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;
+
+	if (!barn_put_full_sheaf(barn, sheaf, false))
+		return;
+
+	sheaf_flush_unused(s, sheaf);
+
+empty:
+	if (!barn_put_empty_sheaf(barn, sheaf, false))
+		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 (!localtry_trylock(&s->cpu_sheaves->lock))
+		goto fail;
+
+	pcs = this_cpu_ptr(s->cpu_sheaves);
+
+	if (unlikely(!pcs->rcu_free)) {
+
+		struct slab_sheaf *empty;
+
+		empty = barn_get_empty_sheaf(pcs->barn);
+
+		if (empty) {
+			pcs->rcu_free = empty;
+			goto do_free;
+		}
+
+		localtry_unlock(&s->cpu_sheaves->lock);
+
+		empty = alloc_empty_sheaf(s, GFP_NOWAIT);
+
+		if (!empty)
+			goto fail;
+
+		if (!localtry_trylock(&s->cpu_sheaves->lock))
+			goto fail;
+
+		pcs = this_cpu_ptr(s->cpu_sheaves);
+
+		if (unlikely(pcs->rcu_free))
+			barn_put_empty_sheaf(pcs->barn, empty, true);
+		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)) {
+		localtry_unlock(&s->cpu_sheaves->lock);
+		stat(s, FREE_RCU_SHEAF);
+		return true;
+	}
+
+	pcs->rcu_free = NULL;
+	localtry_unlock(&s->cpu_sheaves->lock);
+
+	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
@@ -6569,6 +6721,11 @@ 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)
@@ -7974,6 +8131,8 @@ 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);
@@ -8069,6 +8228,8 @@ 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,

-- 
2.48.1