netdev - Re: [PATCH] [PATCH net-next] tun: fix multiqueue rx

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Date:   Fri, 16 Nov 2018 13:45:49 -0700
From:   Matt Cover <werekraken@...il.com>
To:     mst@...hat.com
Cc:     davem@...emloft.net, jasowang@...hat.com, brouer@...hat.com,
        Eric Dumazet <edumazet@...gle.com>, sd@...asysnail.net,
        netdev@...r.kernel.org, matthew.cover@...ckpath.com
Subject: Re: [PATCH] [PATCH net-next] tun: fix multiqueue rx

On Fri, Nov 16, 2018 at 1:10 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Fri, Nov 16, 2018 at 12:00:15AM -0700, Matthew Cover wrote:
> > When writing packets to a descriptor associated with a combined queue, the
> > packets should end up on that queue.
> >
> > Before this change all packets written to any descriptor associated with a
> > tap interface end up on rx-0, even when the descriptor is associated with a
> > different queue.
> >
> > The rx traffic can be generated by either of the following.
> >   1. a simple tap program which spins up multiple queues and writes packets
> >      to each of the file descriptors
> >   2. tx from a qemu vm with a tap multiqueue netdev
> >
> > The queue for rx traffic can be observed by either of the following (done
> > on the hypervisor in the qemu case).
> >   1. a simple netmap program which opens and reads from per-queue
> >      descriptors
> >   2. configuring RPS and doing per-cpu captures with rxtxcpu
> >
> > Alternatively, if you printk() the return value of skb_get_rx_queue() just
> > before each instance of netif_receive_skb() in tun.c, you will get 65535
> > for every skb.
> >
> > Calling skb_record_rx_queue() to set the rx queue to the queue_index fixes
> > the association between descriptor and rx queue.
> >
> > Signed-off-by: Matthew Cover <matthew.cover@...ckpath.com>
>
> Acked-by: Michael S. Tsirkin <mst@...hat.com>
>
> stable material?
>

Yes, I believe so.

The documentation below I think justifies classifying this as a fix.
https://github.com/torvalds/linux/blob/v4.19/Documentation/networking/tuntap.txt#L111

> > ---
> >  drivers/net/tun.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index a65779c6d72f..ce8620f3ea5e 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -1536,6 +1536,7 @@ static void tun_rx_batched(struct tun_struct *tun, struct tun_file *tfile,
> >
> >       if (!rx_batched || (!more && skb_queue_empty(queue))) {
> >               local_bh_disable();
> > +             skb_record_rx_queue(skb, tfile->queue_index);
> >               netif_receive_skb(skb);
> >               local_bh_enable();
> >               return;
> > @@ -1555,8 +1556,11 @@ static void tun_rx_batched(struct tun_struct *tun, struct tun_file *tfile,
> >               struct sk_buff *nskb;
> >
> >               local_bh_disable();
> > -             while ((nskb = __skb_dequeue(&process_queue)))
> > +             while ((nskb = __skb_dequeue(&process_queue))) {
> > +                     skb_record_rx_queue(nskb, tfile->queue_index);
> >                       netif_receive_skb(nskb);
> > +             }
> > +             skb_record_rx_queue(skb, tfile->queue_index);
> >               netif_receive_skb(skb);
> >               local_bh_enable();
> >       }
> > @@ -2452,6 +2456,7 @@ static int tun_xdp_one(struct tun_struct *tun,
> >           !tfile->detached)
> >               rxhash = __skb_get_hash_symmetric(skb);
> >
> > +     skb_record_rx_queue(skb, tfile->queue_index);
> >       netif_receive_skb(skb);
> >
> >       stats = get_cpu_ptr(tun->pcpu_stats);
> > --
> > 2.15.2 (Apple Git-101.1)