optimise local CPU skb_attempt_defer_free

Optimise the case when an skb comes to skb_attempt_defer_free() on the same CPU it was allocated on. The patch 1 enables skb caches and gives frags a chance to hit the page pool's fast path. CPU bound benchmarking with perfect skb_attempt_defer_free() gives around 1% of extra throughput.

v4: SKB_DROP_REASON_NOT_SPECIFIED -> SKB_CONSUMED
v3: rebased, no changes otherwise
v2: pass @napi_safe=true by using __napi_kfree_skb()

Pavel Begunkov (2):
net: cache for same cpu skb_attempt_defer_free
net: use SKB_CONSUMED in skb_attempt_defer_free()

 net/core/skbuff.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

2.44.0

Optimise skb_attempt_defer_free() when run by the same CPU the skb was allocated on. Instead of __kfree_skb() -> kmem_cache_free() we can disable softirqs and put the buffer into cpu local caches.

CPU bound TCP ping pong style benchmarking (i.e. netbench) showed a 1% throughput increase (392.2 -> 396.4 Krps). Cross checking with profiles, the total CPU share of skb_attempt_defer_free() dropped by 0.6%. Note, I'd expect the win doubled with rx only benchmarks, as the optimisation is for the receive path, but the test spends >55% of CPU doing writes.

---
 net/core/skbuff.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

index 21cd01641f4c..62b07ed3af98 100644
@@ -6974,6 +6974,19 @@ void __skb_ext_put(struct skb_ext *ext)
 EXPORT_SYMBOL(__skb_ext_put);
 #endif /* CONFIG_SKB_EXTENSIONS */
 
+static void kfree_skb_napi_cache(struct sk_buff *skb)
+{
+	/* if SKB is a clone, don't handle this case */
+	if (skb->fclone != SKB_FCLONE_UNAVAILABLE) {
+		__kfree_skb(skb);
+		return;
+	}
+
+	local_bh_disable();
+	__napi_kfree_skb(skb, SKB_DROP_REASON_NOT_SPECIFIED);
+	local_bh_enable();
+}
+
 /**
  * skb_attempt_defer_free - queue skb for remote freeing
  * @skb: buffer

@@ -6992,7 +7005,7 @@ void skb_attempt_defer_free(struct sk_buff *skb)
 	if (WARN_ON_ONCE(cpu >= nr_cpu_ids) ||
 	    !cpu_online(cpu) ||
 	    cpu == raw_smp_processor_id()) {
-nodefer:	__kfree_skb(skb);
+nodefer:	kfree_skb_napi_cache(skb);
 		return;
 	}
 

2.44.0

skb_attempt_defer_free() is used to free already processed skbs, so pass SKB_CONSUMED as the reason in kfree_skb_napi_cache().

---
 net/core/skbuff.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 62b07ed3af98..dd266f44aaff 100644
@@ -6983,7 +6983,7 @@ static void kfree_skb_napi_cache(struct sk_buff *skb)
 	}
 
 	local_bh_disable();
-	__napi_kfree_skb(skb, SKB_DROP_REASON_NOT_SPECIFIED);
+	__napi_kfree_skb(skb, SKB_CONSUMED);
 	local_bh_enable();
 }
 

2.44.0