net_sched: sch_fq: avoid touching f->next from fq_gc()

A significant amount of cpu cycles is spent in fq_gc()

When fq_gc() does its lookup in the rb-tree, it needs the
following fields from struct fq_flow :

f->sk       (lookup key in the rb-tree)
f->fq_node  (anchor in the rb-tree)
f->next     (used to determine if the flow is detached)
f->age      (used to determine if the flow is candidate for gc)

This unfortunately spans two cache lines (assuming 64 bytes cache lines)

We can avoid using f->next, if we use the low order bit of f->{age|tail}

This low order bit is 0, if f->tail points to an sk_buff.
We set the low order bit to 1, if the union contains a jiffies value.

Combined with the following patch, this makes sure we only need
to bring into cpu caches one cache line per flow.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_fq.c

@@ -70,14 +70,14 @@ struct fq_flow {
 	struct sk_buff	*head;		/* list of skbs for this flow : first skb */
 	union {
 		struct sk_buff *tail;	/* last skb in the list */
-		unsigned long  age;	/* jiffies when flow was emptied, for gc */
+		unsigned long  age;	/* (jiffies | 1UL) when flow was emptied, for gc */
 	};
 	struct rb_node	fq_node;	/* anchor in fq_root[] trees */
 	struct sock	*sk;
 	int		qlen;		/* number of packets in flow queue */
 	int		credit;
 	u32		socket_hash;	/* sk_hash */
-	struct fq_flow *next;		/* next pointer in RR lists, or &detached */
+	struct fq_flow *next;		/* next pointer in RR lists */
 
 	struct rb_node  rate_node;	/* anchor in q->delayed tree */
 	u64		time_next_packet;
@@ -126,20 +126,25 @@ struct fq_sched_data {
 	struct qdisc_watchdog watchdog;
 };
 
-/* special value to mark a detached flow (not on old/new list) */
-static struct fq_flow detached, throttled;
-
+/*
+ * f->tail and f->age share the same location.
+ * We can use the low order bit to differentiate if this location points
+ * to a sk_buff or contains a jiffies value, if we force this value to be odd.
+ * This assumes f->tail low order bit must be 0 since alignof(struct sk_buff) >= 2
+ */
 static void fq_flow_set_detached(struct fq_flow *f)
 {
-	f->next = &detached;
-	f->age = jiffies;
+	f->age = jiffies | 1UL;
 }
 
 static bool fq_flow_is_detached(const struct fq_flow *f)
 {
-	return f->next == &detached;
+	return !!(f->age & 1UL);
 }
 
+/* special value to mark a throttled flow (not on old/new list) */
+static struct fq_flow throttled;
+
 static bool fq_flow_is_throttled(const struct fq_flow *f)
 {
 	return f->next == &throttled;