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

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Message-ID: <66ab87ca67229_2441da294a5@willemb.c.googlers.com.notmuch>
Date: Thu, 01 Aug 2024 09:04:10 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Randy Li <ayaka@...lik.info>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org, 
 jasowang@...hat.com, 
 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

Randy Li wrote:
> 
> On 2024/8/1 05:57, Willem de Bruijn wrote:
> > nits:
> >
> > - INDX->INDEX. It's correct in the code
> > - prefix networking patches with the target tree: PATCH net-next
> I see.
> >
> > Randy Li wrote:
> >> On 2024/7/31 22:12, Willem de Bruijn wrote:
> >>> Randy Li wrote:
> >>>> We need the queue index in qdisc mapping rule. There is no way to
> >>>> fetch that.
> >>> In which command exactly?
> >> That is for sch_multiq, here is an example
> >>
> >> tc qdisc add dev  tun0 root handle 1: multiq
> >>
> >> tc filter add dev tun0 parent 1: protocol ip prio 1 u32 match ip dst
> >> 172.16.10.1 action skbedit queue_mapping 0
> >> tc filter add dev tun0 parent 1: protocol ip prio 1 u32 match ip dst
> >> 172.16.10.20 action skbedit queue_mapping 1
> >>
> >> tc filter add dev tun0 parent 1: protocol ip prio 1 u32 match ip dst
> >> 172.16.10.10 action skbedit queue_mapping 2
> > If using an IFF_MULTI_QUEUE tun device, packets are automatically
> > load balanced across the multiple queues, in tun_select_queue.
> >
> > If you want more explicit queue selection than by rxhash, tun
> > supports TUNSETSTEERINGEBPF.
> 
> I know this eBPF thing. But I am newbie to eBPF as well I didn't figure 
> out how to config eBPF dynamically.

Lack of experience with an existing interface is insufficient reason
to introduce another interface, of course.

> Besides, I think I still need to know which queue is the target in eBPF.

See SKF_AD_QUEUE for classic BPF and __sk_buff queue_mapping for eBPF.

> >> The purpose here is taking advantage of the multiple threads. For the
> >> the server side(gateway of the tunnel's subnet), usually a different
> >> peer would invoked a different encryption/decryption key pair, it would
> >> be better to handle each in its own thread. Or the application would
> >> need to implement a dispatcher here.
> > A thread in which context? Or do you mean queue?
> The thread in the userspace. Each thread responds for a queue.
> >
> >> I am newbie to the tc(8), I verified the command above with a tun type
> >> multiple threads demo. But I don't know how to drop the unwanted ingress
> >> filter here, the queue 0 may be a little broken.
> > Not opposed to exposing the queue index if there is a need. Not sure
> > yet that there is.
> >
> > Also, since for an IFF_MULTI_QUEUE the queue_id is just assigned
> > iteratively, it can also be inferred without an explicit call.
> 
> I don't think there would be sequence lock in creating multiple queue. 
> Unless application uses an explicitly lock itself.
> 
> While that did makes a problem when a queue would be disabled. It would 
> swap the last queue index with that queue, leading to fetch the queue 
> index calling again, also it would request an update for the qdisc flow 
> rule.
> 
> Could I submit a ***new*** PATCH which would peak a hole, also it 
> applies for re-enabling the queue.
> 
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index 1d06c560c5e6..5473a0fca2e1 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -3115,6 +3115,10 @@ static long __tun_chr_ioctl(struct file *file, 
> > unsigned int cmd,
> >           if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
> >               return -EPERM;
> >           return open_related_ns(&net->ns, get_net_ns);
> > +    } else if (cmd == TUNGETQUEUEINDEX) {
> > +        if (tfile->detached)
> > +            return -EINVAL;
> > +        return put_user(tfile->queue_index, (unsigned int __user*)argp);
> > Unless you're certain that these fields can be read without RTNL, move
> > below rtnl_lock() statement.
> > Would fix in v2. 
> I was trying to not hold the global lock or long period, that is why I 
> didn't made v2 yesterday.
> 
> When I wrote this,  I saw ioctl() TUNSETQUEUE->tun_attach() above. Is 
> the rtnl_lock() scope the lighting lock here?
>