From: Alexei Starovoitov <ast@xxxxxxxxxx>
SLAB_TYPESAFE_BY_RCU makes kmem_caches non mergeable and slows down
kmem_cache_destroy. All bpf_mem_cache are safe to share across different maps
and programs. Convert SLAB_TYPESAFE_BY_RCU to batched call_rcu. This change
solves the memory consumption issue, avoids kmem_cache_destroy latency and
keeps bpf hash map performance the same.
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
---
kernel/bpf/memalloc.c | 64 +++++++++++++++++++++++++++++++++++++++++--
kernel/bpf/syscall.c | 5 +++-
2 files changed, 65 insertions(+), 4 deletions(-)
diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
index 22b729914afe..d765a5cb24b4 100644
--- a/kernel/bpf/memalloc.c
+++ b/kernel/bpf/memalloc.c
@@ -100,6 +100,11 @@ struct bpf_mem_cache {
/* count of objects in free_llist */
int free_cnt;
int low_watermark, high_watermark, batch;
+
+ struct rcu_head rcu;
+ struct llist_head free_by_rcu;
+ struct llist_head waiting_for_gp;
+ atomic_t call_rcu_in_progress;
};
struct bpf_mem_caches {
@@ -188,6 +193,45 @@ static void free_one(struct bpf_mem_cache *c, void *obj)
kfree(obj);
}
+static void __free_rcu(struct rcu_head *head)
+{
+ struct bpf_mem_cache *c = container_of(head, struct bpf_mem_cache, rcu);
+ struct llist_node *llnode = llist_del_all(&c->waiting_for_gp);
+ struct llist_node *pos, *t;
+
+ llist_for_each_safe(pos, t, llnode)
+ free_one(c, pos);
+ atomic_set(&c->call_rcu_in_progress, 0);
+}
+
+static void enque_to_free(struct bpf_mem_cache *c, void *obj)
+{
+ struct llist_node *llnode = obj;
+
+ /* bpf_mem_cache is a per-cpu object. Freeing happens in irq_work.
+ * Nothing races to add to free_by_rcu list.
+ */
+ __llist_add(llnode, &c->free_by_rcu);
+}
+
+static void do_call_rcu(struct bpf_mem_cache *c)
+{
+ struct llist_node *llnode, *t;
+
+ if (atomic_xchg(&c->call_rcu_in_progress, 1))
+ return;
+
+ WARN_ON_ONCE(!llist_empty(&c->waiting_for_gp));
+ llist_for_each_safe(llnode, t, __llist_del_all(&c->free_by_rcu))
+ /* There is no concurrent __llist_add(waiting_for_gp) access.
+ * It doesn't race with llist_del_all either.
+ * But there could be two concurrent llist_del_all(waiting_for_gp):
+ * from __free_rcu() and from drain_mem_cache().
+ */
+ __llist_add(llnode, &c->waiting_for_gp);
+ call_rcu(&c->rcu, __free_rcu);
+}
+
static void free_bulk(struct bpf_mem_cache *c)
{
struct llist_node *llnode, *t;
@@ -207,12 +251,13 @@ static void free_bulk(struct bpf_mem_cache *c)
local_dec(&c->active);
if (IS_ENABLED(CONFIG_PREEMPT_RT))
local_irq_restore(flags);
- free_one(c, llnode);
+ enque_to_free(c, llnode);
} while (cnt > (c->high_watermark + c->low_watermark) / 2);
/* and drain free_llist_extra */
llist_for_each_safe(llnode, t, llist_del_all(&c->free_llist_extra))
- free_one(c, llnode);
+ enque_to_free(c, llnode);
+ do_call_rcu(c);
}
static void bpf_mem_refill(struct irq_work *work)
@@ -298,7 +343,7 @@ int bpf_mem_alloc_init(struct bpf_mem_alloc *ma, int size)
return -ENOMEM;
size += LLIST_NODE_SZ; /* room for llist_node */
snprintf(buf, sizeof(buf), "bpf-%u", size);
- kmem_cache = kmem_cache_create(buf, size, 8, SLAB_TYPESAFE_BY_RCU, NULL);
+ kmem_cache = kmem_cache_create(buf, size, 8, 0, NULL);
if (!kmem_cache) {
free_percpu(pc);
return -ENOMEM;
@@ -340,6 +385,15 @@ static void drain_mem_cache(struct bpf_mem_cache *c)
{
struct llist_node *llnode, *t;
+ /* The caller has done rcu_barrier() and no progs are using this
+ * bpf_mem_cache, but htab_map_free() called bpf_mem_cache_free() for
+ * all remaining elements and they can be in free_by_rcu or in
+ * waiting_for_gp lists, so drain those lists now.
+ */
+ llist_for_each_safe(llnode, t, __llist_del_all(&c->free_by_rcu))
+ free_one(c, llnode);
+ llist_for_each_safe(llnode, t, llist_del_all(&c->waiting_for_gp))
+ free_one(c, llnode);
llist_for_each_safe(llnode, t, llist_del_all(&c->free_llist))
free_one(c, llnode);
llist_for_each_safe(llnode, t, llist_del_all(&c->free_llist_extra))
@@ -361,6 +415,10 @@ void bpf_mem_alloc_destroy(struct bpf_mem_alloc *ma)
kmem_cache_destroy(c->kmem_cache);
if (c->objcg)
obj_cgroup_put(c->objcg);
+ /* c->waiting_for_gp list was drained, but __free_rcu might
+ * still execute. Wait for it now before we free 'c'.
+ */
+ rcu_barrier();
free_percpu(ma->cache);
ma->cache = NULL;
}
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index a4d40d98428a..850270a72350 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -638,7 +638,10 @@ static void __bpf_map_put(struct bpf_map *map, bool do_idr_lock)
bpf_map_free_id(map, do_idr_lock);
btf_put(map->btf);
INIT_WORK(&map->work, bpf_map_free_deferred);
- schedule_work(&map->work);
+ /* Avoid spawning kworkers, since they all might contend
+ * for the same mutex like slab_mutex.
+ */
+ queue_work(system_unbound_wq, &map->work);
}
}
--
2.30.2
