netdev - Re: Tun congestion/BQL

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Date:   Thu, 11 Apr 2019 11:56:06 +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 15:17 +0800, Jason Wang wrote:
> > > Ideally we want to react when the queue starts building rather than when
> > > it starts getting full; by pushing back on upper layers (or, if
> > > forwarding, dropping packets to signal congestion).
> > 
> > This is precisely what my first accidental if (!ptr_ring_empty())
> > variant was doing, right? :)
> 
> 
> But I give a try on your ptr_ring_full() patch on VM, looks like it 
> works (single flow), no packets were dropped by TAP anymore. How many 
> flows did you use?

Hm, I thought I was only using one. This is just a simple case of
userspace opening /dev/net/tun, TUNSETIFF, and reading/writing.

But if I was stopping the *wrong* queue that might explain things.

This is a persistent tun device.

> 
> > 
> > > In practice, this means tuning the TX ring to the *minimum* size it can
> > > be without starving (this is basically what BQL does for Ethernet), and
> > > keeping packets queued in the qdisc layer instead, where it can be
> > > managed...
> > 
> > I was going to add BQL (as $SUBJECT may have caused you to infer) but
> > trivially adding the netdev_sent_queue() in tun_net_xmit() and
> > netdev_completed_queue() for xdp vs. skb in tun_do_read() was tripping
> > the BUG in dql_completed().
> 
> 
> Something like https://lists.openwall.net/netdev/2012/11/12/6767 ?

Fairly much.

Except again I was being lazy for the proof-of-concept, ignoring 'txq'
and just using netdev_sent_queue() etc.


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