net-sysfs: add backlog len and CPU id to softnet data

Currently the backlog status in not exposed to user-space.
Since long backlogs (or simply not empty ones) can be a
source of noticeable latency, -RT deployments need some way
to inspect it.

Additionally, there isn't a direct match between 'softnet_stat'
lines and the related CPU - sd for offline CPUs are not dumped -
so this patch also includes the CPU id into such entry.
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net-procfs.c

@@ -116,6 +116,12 @@ static int dev_seq_show(struct seq_file *seq, void *v)
 	return 0;
 }
 
+static u32 softnet_backlog_len(struct softnet_data *sd)
+{
+	return skb_queue_len_lockless(&sd->input_pkt_queue) +
+	       skb_queue_len_lockless(&sd->process_queue);
+}
+
 static struct softnet_data *softnet_get_online(loff_t *pos)
 {
 	struct softnet_data *sd = NULL;
@@ -159,12 +165,17 @@ static int softnet_seq_show(struct seq_file *seq, void *v)
 	rcu_read_unlock();
 #endif
 
+	/* the index is the CPU id owing this sd. Since offline CPUs are not
+	 * displayed, it would be othrwise not trivial for the user-space
+	 * mapping the data a specific CPU
+	 */
 	seq_printf(seq,
-		   "%08x %08x %08x %08x %08x %08x %08x %08x %08x %08x %08x\n",
+		   "%08x %08x %08x %08x %08x %08x %08x %08x %08x %08x %08x %08x %08x\n",
 		   sd->processed, sd->dropped, sd->time_squeeze, 0,
 		   0, 0, 0, 0, /* was fastroute */
 		   0,	/* was cpu_collision */
-		   sd->received_rps, flow_limit_count);
+		   sd->received_rps, flow_limit_count,
+		   softnet_backlog_len(sd), (int)seq->index);
 	return 0;
 }