netdev - Re: Tun congestion/BQL

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Date:   Thu, 11 Apr 2019 12:16:06 +0300
From:   David Woodhouse <dwmw2@...radead.org>
To:     Jason Wang <jasowang@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        netdev@...r.kernel.org
Cc:     "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: Tun congestion/BQL

On Thu, 2019-04-11 at 17:04 +0800, Jason Wang wrote:
> Btw, forget to mention, I modify your patch to use 
> netif_stop/wake_subqueue() instead.

Hm...


--- /usr/src/debug/kernel-5.0.fc29/linux-5.0.5-
200.fc29.x86_64/drivers/net/tun.c2019-03-03 23:21:29.000000000 +0000
+++ /home/fedora/tun/tun.c	2019-04-11 09:11:20.781683956 +0000
@@ -1118,8 +1118,14 @@ static netdev_tx_t tun_net_xmit(struct s
 
 	nf_reset(skb);
 
-	if (ptr_ring_produce(&tfile->tx_ring, skb))
+	if (ptr_ring_produce(&tfile->tx_ring, skb)) {
+		netif_stop_subqueue(tun->dev, txq);
 		goto drop;
+	}
+
+	if (ptr_ring_full(&tfile->tx_ring)) {
+		netif_stop_subqueue(tun->dev, txq);
+	}
 
 	/* Notify and wake up reader process */
 	if (tfile->flags & TUN_FASYNC)
@@ -2229,6 +2235,8 @@ static ssize_t tun_do_read(struct tun_st
 			consume_skb(skb);
 	}
 
+	netif_wake_subqueue(tun->dev, tfile->queue_index);
+
 	return ret;
 }
 

That gives me

Socket  Message  Elapsed      Messages                
Size    Size     Time         Okay Errors   Throughput
bytes   bytes    secs            #      #   10^6bits/sec

212992    1400   10.90     2277474      0    2340.95
212992           10.90     1174728           1207.47

Without the first netif_stop_subqueue() call in the case where we
actually *do* drop the packet (which AFAICT should never happen?) it
looks like this:

Socket  Message  Elapsed      Messages                
Size    Size     Time         Okay Errors   Throughput
bytes   bytes    secs            #      #   10^6bits/sec

212992    1400   10.00     7675563      0    8596.61
212992           10.00     1441186           1614.13

And without the patch at all, it was still faster when I just let it
drop packets:

Socket  Message  Elapsed      Messages                
Size    Size     Time         Okay Errors   Throughput
bytes   bytes    secs            #      #   10^6bits/sec

212992    1400   10.00     8143709      0    9120.93
212992           10.00     1545035           1730.44



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