netdev - Re: [PATCH 5/5] mac80211: set NETIF_F_LLTX when using intermediate tx queues

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Date:   Tue, 23 Apr 2019 14:41:33 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Arend Van Spriel <arend.vanspriel@...adcom.com>,
        Felix Fietkau <nbd@....name>, linux-wireless@...r.kernel.org,
        Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH 5/5] mac80211: set NETIF_F_LLTX when using intermediate
 tx queues

On Wed, 2019-04-17 at 10:17 +0100, Toke Høiland-Jørgensen wrote:
> Herbert Xu <herbert@...dor.apana.org.au> writes:
> 
> > On Tue, Apr 16, 2019 at 02:18:36PM +0100, Toke Høiland-Jørgensen wrote:
> > > 
> > > > The congestion control happens at two levels. You are right that the
> > > > socket buffer acts as one limit. However, other applications may also
> > > > rely on the TX queue being full as the throttle (by setting a
> > > > sufficiently large socket buffer size).
> > > 
> > > Do you happen to have an example of an application that does this that
> > > could be used for testing? :)
> > 
> > Have a look at
> > 
> > commit 6ce9e7b5fe3195d1ae6e3a0753d4ddcac5cd699e
> > Author: Eric Dumazet <eric.dumazet@...il.com>
> > Date:   Wed Sep 2 18:05:33 2009 -0700
> > 
> >     ip: Report qdisc packet drops
> > 
> > You should be able to do a UDP flood while setting IP_RECVERR to
> > detect the packet drop due to a full queue which AFAICS will never
> > happen with the current mac80211 setup.
> 
> Also, looking at udp.c, it seems it uses net_xmit_errno() - which means
> that returning NET_XMIT_CN has the same effect as NET_XMIT_SUCCESS when
> propagated back to userspace? Which would kinda defeat the point of
> going to the trouble of propagating up the return code (the mac80211
> queue will never drop the most recently enqueued packet)...

I guess there might be value in returning NET_XMIT_CN anyway, but I
think you're right in that we can never return anything but
NET_XMIT_SUCCESS or NET_XMIT_CN since we never drop this new packet,
just older ones.

Which, btw, is exactly the same with net/sched/sch_fq_codel.c, AFAICT?

johannes