linux-kernel - Re: [PATCH] net: orphan queued skbs if device tx can stall

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Date:	Tue, 10 Apr 2012 10:55:00 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Jamal Hadi Salim <hadi@...erus.ca>,
	Stephen Hemminger <shemminger@...tta.com>,
	Jason Wang <jasowang@...hat.com>,
	Neil Horman <nhorman@...driver.com>,
	Jiri Pirko <jpirko@...hat.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Michał Mirosław <mirq-linux@...e.qmqm.pl>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Herbert Xu <herbert@...dor.hengli.com.au>
Subject: Re: [PATCH] net: orphan queued skbs if device tx can stall

On Tue, 2012-04-10 at 11:41 +0300, Michael S. Tsirkin wrote:
> On Tue, Apr 10, 2012 at 09:55:58AM +0200, Eric Dumazet wrote:

> > In your case I would just not use qdisc at all, like other virtual
> > devices.
> 
> I think that if we do this, this also disables gso
> for the device, doesn't it?

Not at all, thats unrelated.

> If true that would be a problem as this would
> hurt performance of virtualized setups a lot.

In fact, removing qdisc layer will help a lot, removing a contention
point.

Anyway, with a 500 packet limit in TUN queue itself, qdisc layer should
be always empty. Whats the point storing more than 500 packets for a
device ? Thats a latency killer.

> 
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index bb8c72c..fd8c7f0 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -396,7 +396,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
> >  	    sk_filter(tun->socket.sk, skb))
> >  		goto drop;
> >  
> > -	if (skb_queue_len(&tun->socket.sk->sk_receive_queue) >= dev->tx_queue_len) {
> > +	if (skb_queue_len(&tun->socket.sk->sk_receive_queue) >= TUN_READQ_SIZE) {
> >  		if (!(tun->flags & TUN_ONE_QUEUE)) {
> >  			/* Normal queueing mode. */
> >  			/* Packet scheduler handles dropping of further packets. */
> 
> tx_queue_len is controllable by SIOCSIFTXQLEN
> so we'll need to override SIOCSIFTXQLEN somehow
> to avoid breaking userspace that actually uses SIOCSIFTXQLEN, right?

Right now, you control with this tx_queue_len both the qdisc limit (if
pfifo_fast default) and the receive_queue in TUN.

That doesnt seem right to me, and more a hack/side effect.

Maybe you want to introduce a new setting, only controling receive queue
limit, and use tx_queue_len for its original meaning.

Then, setting tx_queue_len to 0 permits to remove qdisc layer, as any
other netdevice.



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