netdev - [PATCH net-next 2/4] net_sched: fq_codel: cache skb->truesize into skb->cb

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Date:	Tue, 21 Jun 2016 23:16:50 -0700
From:	Eric Dumazet <edumazet@...gle.com>
To:	"David S . Miller" <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH net-next 2/4] net_sched: fq_codel: cache skb->truesize into skb->cb

Now we defer skb drops, it makes sense to keep a copy
of skb->truesize in struct codel_skb_cb to avoid one
cache line miss per dropped skb in fq_codel_drop(),
to reduce latencies a bit further.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/net/codel_qdisc.h | 1 +
 net/sched/sch_fq_codel.c  | 7 ++++---
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/net/codel_qdisc.h b/include/net/codel_qdisc.h
index 8144d9cd2908..098630f83a55 100644
--- a/include/net/codel_qdisc.h
+++ b/include/net/codel_qdisc.h
@@ -52,6 +52,7 @@
 /* Qdiscs using codel plugin must use codel_skb_cb in their own cb[] */
 struct codel_skb_cb {
 	codel_time_t enqueue_time;
+	unsigned int mem_usage;
 };
 
 static struct codel_skb_cb *get_codel_cb(const struct sk_buff *skb)
diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c
index f715195459c9..a5ea0e9b6be4 100644
--- a/net/sched/sch_fq_codel.c
+++ b/net/sched/sch_fq_codel.c
@@ -172,7 +172,7 @@ static unsigned int fq_codel_drop(struct Qdisc *sch, unsigned int max_packets,
 	do {
 		skb = dequeue_head(flow);
 		len += qdisc_pkt_len(skb);
-		mem += skb->truesize;
+		mem += get_codel_cb(skb)->mem_usage;
 		__qdisc_drop(skb, to_free);
 	} while (++i < max_packets && len < threshold);
 
@@ -216,7 +216,8 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		flow->deficit = q->quantum;
 		flow->dropped = 0;
 	}
-	q->memory_usage += skb->truesize;
+	get_codel_cb(skb)->mem_usage = skb->truesize;
+	q->memory_usage += get_codel_cb(skb)->mem_usage;
 	memory_limited = q->memory_usage > q->memory_limit;
 	if (++sch->q.qlen <= sch->limit && !memory_limited)
 		return NET_XMIT_SUCCESS;
@@ -267,7 +268,7 @@ static struct sk_buff *dequeue_func(struct codel_vars *vars, void *ctx)
 	if (flow->head) {
 		skb = dequeue_head(flow);
 		q->backlogs[flow - q->flows] -= qdisc_pkt_len(skb);
-		q->memory_usage -= skb->truesize;
+		q->memory_usage -= get_codel_cb(skb)->mem_usage;
 		sch->q.qlen--;
 		sch->qstats.backlog -= qdisc_pkt_len(skb);
 	}
-- 
2.8.0.rc3.226.g39d4020