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

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Message-ID: <1334058300.3126.99.camel@edumazet-glaptop>
Date:	Tue, 10 Apr 2012 13:45:00 +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 14:25 +0300, Michael S. Tsirkin wrote:
> On Tue, Apr 10, 2012 at 12:04:19PM +0200, Eric Dumazet wrote:
> > On Tue, 2012-04-10 at 12:31 +0300, Michael S. Tsirkin wrote:
> > 
> > > True. Still this is the only interface we have for controlling
> > > the internal queue length so it seems safe to assume someone
> > > is using it for this purpose.
> > > 
> > 
> > So to workaround a problem in tun, you want to hack net/core/dev.c :(
> 
> Sorry about being unclear, I'm just saying that your patch assumes
> tx_queue_len == 0 since you set it that way at device init but we can't
> rely on this as existing users might have changed that value.
> One way to fix would be a patch at the bottom: then we
> can leave tun to treat tx_queue_len like it always did.


> ----
> 
> We don't want a queue for tun since it can stall forever, but userspace
> might tweak it's tx_queue_len as a way to control RX queue depth,
> and we don't want to break userspace. Use a private flag to disable queue.
> 
> Warning: untested.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> 
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index 27883d1..644ca53 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -695,7 +692,7 @@ static void attach_one_default_qdisc(struct net_device *dev,
>  {
>  	struct Qdisc *qdisc = &noqueue_qdisc;
>  
> -	if (dev->tx_queue_len) {
> +	if (dev->tx_queue_len && !(dev->priv_flags & IFF_TX_CAN_STALL)) {
>  		qdisc = qdisc_create_dflt(dev_queue,
>  					  &pfifo_fast_ops, TC_H_ROOT);
>  		if (!qdisc) {


Thing is this function is called before userspace can tweak tx_queue_len

So if you create a vlan device (this sets tx_queue_len to 0), no qdisc
is attached.

If later userspace changes tx_queue_len to this device, qdisc wont
automatically be created/attached.

Really, tx_queue_len is private to net/sched layer, it should not be
used by tun device to control a receive queue limit.

Please try to not hack net/sched or net/core for your needs.

Its not because tun abused tx_queue_len in the past we must keep this
hack forever.

In ethernet drivers, TX ring size is controlled by ethtool -g

Why tun driver would use another way ?



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