bpf: cpumap use ptr_ring_consume_batched

Move ptr_ring dequeue outside loop, that allocate SKBs and calls network
stack, as these operations that can take some time. The ptr_ring is a
communication channel between CPUs, where we want to reduce/limit any
cacheline bouncing.

Do a concentrated bulk dequeue via ptr_ring_consume_batched, to shorten the
period and times the remote cacheline in ptr_ring is read

Batch size 8 is both to (1) limit BH-disable period, and (2) consume one
cacheline on 64-bit archs. After reducing the BH-disable section further
then we can consider changing this, while still thinking about L1 cacheline
size being active.
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/cpumap.c

@@ -240,6 +240,8 @@ static void put_cpu_map_entry(struct bpf_cpu_map_entry *rcpu)
 	}
 }
 
+#define CPUMAP_BATCH 8
+
 static int cpu_map_kthread_run(void *data)
 {
 	struct bpf_cpu_map_entry *rcpu = data;
@@ -252,8 +254,9 @@ static int cpu_map_kthread_run(void *data)
 	 * kthread_stop signal until queue is empty.
 	 */
 	while (!kthread_should_stop() || !__ptr_ring_empty(rcpu->queue)) {
-		unsigned int processed = 0, drops = 0, sched = 0;
-		struct xdp_frame *xdpf;
+		unsigned int drops = 0, sched = 0;
+		void *frames[CPUMAP_BATCH];
+		int i, n;
 
 		/* Release CPU reschedule checks */
 		if (__ptr_ring_empty(rcpu->queue)) {
@@ -269,14 +272,16 @@ static int cpu_map_kthread_run(void *data)
 			sched = cond_resched();
 		}
 
-		/* Process packets in rcpu->queue */
-		local_bh_disable();
 		/*
 		 * The bpf_cpu_map_entry is single consumer, with this
 		 * kthread CPU pinned. Lockless access to ptr_ring
 		 * consume side valid as no-resize allowed of queue.
 		 */
-		while ((xdpf = __ptr_ring_consume(rcpu->queue))) {
+		n = ptr_ring_consume_batched(rcpu->queue, frames, CPUMAP_BATCH);
+
+		local_bh_disable();
+		for (i = 0; i < n; i++) {
+			struct xdp_frame *xdpf = frames[i];
 			struct sk_buff *skb;
 			int ret;
 
@@ -290,13 +295,9 @@ static int cpu_map_kthread_run(void *data)
 			ret = netif_receive_skb_core(skb);
 			if (ret == NET_RX_DROP)
 				drops++;
-
-			/* Limit BH-disable period */
-			if (++processed == 8)
-				break;
 		}
 		/* Feedback loop via tracepoint */
-		trace_xdp_cpumap_kthread(rcpu->map_id, processed, drops, sched);
+		trace_xdp_cpumap_kthread(rcpu->map_id, n, drops, sched);
 
 		local_bh_enable(); /* resched point, may call do_softirq() */
 	}