[PATCH bpf-next v1 5/7] selftests/bpf: libarena: Refill local caches in batches

[Emil Tsalapatis <emil@etsalapatis.com>]

Cache hits avoid the central lock, but every cache miss still asks the
buddy allocator for one object. A sequence of misses therefore repeats
the same locking and metadata work.

Allocate eight objects with buddy_alloc_bulk() on a local-cache miss.
Return one object directly and retain the other seven in the per-CPU LIFO
array. This amortizes buddy locking and metadata updates over subsequent
allocations. Skip the refill in NMI context, where the backend allocator
cannot safely take its lock.

Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
---
 .../selftests/bpf/libarena/src/buddy.bpf.c    | 44 +++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/tools/testing/selftests/bpf/libarena/src/buddy.bpf.c b/tools/testing/selftests/bpf/libarena/src/buddy.bpf.c
index 85d670b8762b..3b760b1fc66e 100644
--- a/tools/testing/selftests/bpf/libarena/src/buddy.bpf.c
+++ b/tools/testing/selftests/bpf/libarena/src/buddy.bpf.c
@@ -185,6 +185,43 @@ int buddy_cache_free(struct buddy __arena *buddy, void __arena *addr)
 	return ret;
 }
 
+static void __arena *buddy_cache_refill(struct buddy __arena *buddy,
+					size_t size, int order)
+{
+	struct buddy_cache_state __arena *state = &buddy->cache_state;
+	struct buddy_cache_order __arena *slots;
+	struct buddy_cache_ctx ctx;
+	void __arena *address = NULL;
+	int ret;
+
+	DISABLE_CACHE_IF_ASAN(NULL);
+
+	if (order >= BUDDY_CACHE_NUM_ORDERS)
+		return NULL;
+
+	if (buddy_cache_enter(state, &ctx))
+		return NULL;
+
+	/* A free may have populated the cache between the miss and refill. */
+	address = buddy_cache_pop(ctx.cache, order);
+	if (address)
+		goto done;
+
+	slots = &ctx.cache->orders[order];
+	ret = buddy_alloc_bulk(buddy, size, arena_fls(BUDDY_CACHE_BATCH_SIZE) - 1,
+				(void __arena * __arena *)slots->objects);
+	if (ret)
+		goto done;
+
+	address = slots->objects[BUDDY_CACHE_BATCH_SIZE - 1];
+	WRITE_ONCE(slots->elems, BUDDY_CACHE_BATCH_SIZE - 1);
+
+done:
+	buddy_cache_exit(&ctx);
+
+	return address;
+}
+
 enum {
 	BUDDY_POISONED = (s8)0xef,
 
@@ -1052,6 +1089,13 @@ void __arena *buddy_alloc(struct buddy __arena *buddy, size_t size)
 	if (address)
 		return address;
 
+	if (bpf_in_nmi())
+		return NULL;
+
+	address = buddy_cache_refill(buddy, size, order);
+	if (address)
+		return address;
+
 	return buddy_alloc_memory(buddy, size, order, 0);
 }
 
-- 
2.52.0