Messages in this thread | | | Date | Tue, 10 Apr 2012 15:41:51 +0300 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCH] net: orphan queued skbs if device tx can stall |
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On Tue, Apr 10, 2012 at 01:45:00PM +0200, Eric Dumazet wrote: > 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@redhat.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.
True. But there's another place where this can happen - after dev_change_net_namespace, no? This calls dev_shutdown.
> 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 ? >
I think it's a bad interface too but it's in a userspace ABI now so I suspect we are stuck with it for now. We can try deprecating but we can't just drop it.
-- MST
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