netdev - Re: [PATCH] net: tuntap: add ioctl() TUNGETQUEUEINDX to fetch queue index

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Message-ID: <66ba421ee77f4_48f70294e@willemb.c.googlers.com.notmuch>
Date: Mon, 12 Aug 2024 13:10:54 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jason Wang <jasowang@...hat.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: ayaka <ayaka@...lik.info>, 
 netdev@...r.kernel.org, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: tuntap: add ioctl() TUNGETQUEUEINDX to fetch queue
 index

Jason Wang wrote:
> On Fri, Aug 9, 2024 at 10:55 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > ayaka wrote:
> > >
> > > Sent from my iPad
> >
> > Try to avoid ^^^
> >
> 
> [...]
> 
> > > 2. Does such a hash operation happen to every packet passing through?
> >
> > For packets with a local socket, the computation is cached in the
> > socket.
> >
> > For these tunnel packets, see tun_automq_select_queue. Specifically,
> > the call to __skb_get_hash_symmetric.
> >
> > I'm actually not entirely sure why tun has this, rather than defer
> > to netdev_pick_tx, which call skb_tx_hash.
> 
> Not sure I get the question, but it needs to use a consistent hash to
> match the flows stored before.

This is a bit tangential to Randy's original thread, but I would like
to understand this part a bit better, if you don't mind.

Tun automq calls __skb_get_hash_symmetric instead of the
non-symmetrical skb_get_hash of netdev_pick_tx. That makes sense.

Also, netdev_pick_tx tries other things first, like XPS.

Why does automq have to be stateful, keeping a table. Rather than
always computing symmetrical_hash % reciprocal_scale(txq, numqueues)
directly, as is does when the flow is not found?

Just curious, thanks.

> >
> > > 3. Is rxhash based on the flow tracking record in the tun driver?
> > > Those CPU overhead may demolish the benefit of the multiple queues and filters in the kernel solution.
> >
> > Keyword is "may". Avoid premature optimization in favor of data.
> >
> > > Also the flow tracking has a limited to 4096 or 1024, for a IPv4 /24 subnet, if everyone opened 16 websites, are we run out of memory before some entries expired?
> > >
> > > I want to  seek there is a modern way to implement VPN in Linux after so many features has been introduced to Linux. So far, I don’t find a proper way to make any advantage here than other platforms.
> 
> I think I need to understand how we could define "modern" here.
> 
> Btw, I vaguely remember there are some new vpn projects that try to
> use vhost-net to accelerate.
> 
> E.g https://gitlab.com/openconnect/openconnect
> 
> Thanks
>