netdev - Re: [PATCH v3] net: batch skb dequeueing from softnet input_pkt

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Date:	Thu, 22 Apr 2010 01:05:49 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Tom Herbert <therbert@...gle.com>, jamal <hadi@...erus.ca>
Subject: Re: [PATCH v3] net: batch skb dequeueing from softnet
 input_pkt_queue

Le mercredi 14 avril 2010 à 17:52 +0800, Changli Gao a écrit :
> batch skb dequeueing from softnet input_pkt_queue
> 
> batch skb dequeueing from softnet input_pkt_queue to reduce potential lock
> contention and irq disabling/enabling.
> 
> Signed-off-by: Changli Gao <xiaosuo@...il.com>
> ----

lock contention _is_ a problem, Jamal tests can show it.

irq disabling/enabling is not, and force to use stop_machine() killer.

I suggest something very simple, like a small buffer (16 slots), so that
process_backlog() can batch 16 buffers at once.

Following patch not tested, but its late here and I need to sleep ;)

This is a RFC, not for inclusion, and based on current net-next-2.6 tree

[RFC] net: introduce a batch mode in process_backlog()

We see a lock contention on input_pkt_queue.lock in RPS benches.

As suggested by Changli Gao, we can batch several skbs at once in
process_backlog(), so that we dirty input_pkt_queue less often.

I chose to batch at most 16 skbs per round, and place them in
softnet_data zone where flush_backlog() can find them and eventually
free this skbs at device dismantle.

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
 include/linux/netdevice.h |    2 +
 net/core/dev.c            |   48 +++++++++++++++++++++++++++---------
 2 files changed, 38 insertions(+), 12 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 3c5ed5f..16da8db 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1383,11 +1383,13 @@ static inline int unregister_gifconf(unsigned int family)
 /*
  * Incoming packets are placed on per-cpu queues
  */
+#define SD_BATCH_SZ 16
 struct softnet_data {
 	struct Qdisc		*output_queue;
 	struct list_head	poll_list;
 	struct sk_buff		*completion_queue;
 
+	struct sk_buff		*batch[SD_BATCH_SZ]; /* process_backlog() & flush_backlog() */
 #ifdef CONFIG_RPS
 	struct softnet_data	*rps_ipi_list;
 
diff --git a/net/core/dev.c b/net/core/dev.c
index e904c47..2673ce0 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2932,6 +2932,7 @@ static void flush_backlog(void *arg)
 	struct net_device *dev = arg;
 	struct softnet_data *sd = &__get_cpu_var(softnet_data);
 	struct sk_buff *skb, *tmp;
+	int i;
 
 	rps_lock(sd);
 	skb_queue_walk_safe(&sd->input_pkt_queue, skb, tmp)
@@ -2941,6 +2942,13 @@ static void flush_backlog(void *arg)
 			input_queue_head_incr(sd);
 		}
 	rps_unlock(sd);
+	for (i = 0; i < ARRAY_SIZE(sd->batch); i++) {
+		skb = sd->batch[i];
+		if (skb && skb->dev == dev) {
+			kfree_skb(skb);
+			sd->batch[i] = NULL;
+		}
+	}
 }
 
 static int napi_gro_complete(struct sk_buff *skb)
@@ -3245,29 +3253,47 @@ EXPORT_SYMBOL(napi_gro_frags);
 
 static int process_backlog(struct napi_struct *napi, int quota)
 {
-	int work = 0;
+	int i, n, lim, work = 0;
 	struct softnet_data *sd = &__get_cpu_var(softnet_data);
+	struct sk_buff *skb;
 
 	napi->weight = weight_p;
+	local_irq_disable();
+
 	do {
-		struct sk_buff *skb;
+		lim = quota - work;
+		if (lim > ARRAY_SIZE(sd->batch))
+			lim = ARRAY_SIZE(sd->batch);
+		/* batch at most 16 buffers */
 
-		local_irq_disable();
 		rps_lock(sd);
-		skb = __skb_dequeue(&sd->input_pkt_queue);
-		if (!skb) {
+		for (n = 0; n < lim; n++) {
+			sd->batch[n] = __skb_dequeue(&sd->input_pkt_queue);
+			if (!sd->batch[n])
+				break;
+		}
+		if (!sd->input_pkt_queue.qlen) {
 			__napi_complete(napi);
-			rps_unlock(sd);
-			local_irq_enable();
-			break;
+			quota = 0;
 		}
-		input_queue_head_incr(sd);
 		rps_unlock(sd);
-		local_irq_enable();
 
-		__netif_receive_skb(skb);
-	} while (++work < quota);
+		/* Now process our batch */
+		for (i = 0; i < n; i++) {
+			skb = sd->batch[i];
+			/* flush_backlog() might have stolen this skb */
+			input_queue_head_incr(sd);
+			if (likely(skb)) {
+				sd->batch[i] = NULL;
+				local_irq_enable();
+				__netif_receive_skb(skb);
+				local_irq_disable();
+			}
+		}
+		work += n;
+	} while (work < quota);
 
+	local_irq_enable();
 	return work;
 }
 


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