nfp: bpf: add short busy wait for FW replies

Scheduling out and in for every FW message can slow us down
unnecessarily.  Our experiments show that even under heavy load
the FW responds to 99.9% messages within 200 us.  Add a short
busy wait before entering the wait queue.

Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

Author: Jakub Kicinski

drivers/net/ethernet/netronome/nfp/bpf/cmsg.c

@@ -157,7 +157,14 @@ nfp_bpf_cmsg_wait_reply(struct nfp_app_bpf *bpf, enum nfp_bpf_cmsg_type type,
 			int tag)
 {
 	struct sk_buff *skb;
-	int err;
+	int i, err;
+
+	for (i = 0; i < 50; i++) {
+		udelay(4);
+		skb = nfp_bpf_reply(bpf, tag);
+		if (skb)
+			return skb;
+	}
 
 	err = wait_event_interruptible_timeout(bpf->cmsg_wq,
 					       skb = nfp_bpf_reply(bpf, tag),