Date:   Fri, 23 Dec 2022 14:29:43 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        xuanzhuo@...ux.alibaba.com
Subject: Re: [PATCH net V2] virtio-net: correctly enable callback during start_xmit

On Fri, Dec 16, 2022 at 11:43 AM Jason Wang <jasowang@...hat.com> wrote:
>
> On Thu, Dec 15, 2022 at 5:35 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> >
> > On Thu, Dec 15, 2022 at 05:15:43PM +0800, Jason Wang wrote:
> > > On Thu, Dec 15, 2022 at 5:02 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > >
> > > > On Thu, Dec 15, 2022 at 11:27:19AM +0800, Jason Wang wrote:
> > > > > Commit a7766ef18b33("virtio_net: disable cb aggressively") enables
> > > > > virtqueue callback via the following statement:
> > > > >
> > > > >         do {
> > > > >            ......
> > > > >       } while (use_napi && kick &&
> > > > >                unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
> > > > >
> > > > > When NAPI is used and kick is false, the callback won't be enabled
> > > > > here. And when the virtqueue is about to be full, the tx will be
> > > > > disabled, but we still don't enable tx interrupt which will cause a TX
> > > > > hang. This could be observed when using pktgen with burst enabled.
> > > > >
> > > > > Fixing this by trying to enable tx interrupt after we disable TX when
> > > > > we're not using napi or kick is false.
> > > > >
> > > > > Fixes: a7766ef18b33 ("virtio_net: disable cb aggressively")
> > > > > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > > > > ---
> > > > > The patch is needed for -stable.
> > > > > Changes since V1:
> > > > > - enable tx interrupt after we disable tx
> > > > > ---
> > > > >  drivers/net/virtio_net.c | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > > index 86e52454b5b5..dcf3a536d78a 100644
> > > > > --- a/drivers/net/virtio_net.c
> > > > > +++ b/drivers/net/virtio_net.c
> > > > > @@ -1873,7 +1873,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> > > > >        */
> > > > >       if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
> > > > >               netif_stop_subqueue(dev, qnum);
> > > > > -             if (!use_napi &&
> > > > > +             if ((!use_napi || !kick) &&
> > > > >                   unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
> > > > >                       /* More just got used, free them then recheck. */
> > > > >                       free_old_xmit_skbs(sq, false);
> > > >
> > > > This will work but the following lines are:
> > > >
> > > >                        if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
> > > >                                 netif_start_subqueue(dev, qnum);
> > > >                                 virtqueue_disable_cb(sq->vq);
> > > >                         }
> > > >
> > > >
> > > > and I thought we are supposed to keep callbacks enabled with napi?
> > >
> > > This seems to be the opposite logic of commit a7766ef18b33 that
> > > disables callbacks for NAPI.
> > >
> > > It said:
> > >
> > >     There are currently two cases where we poll TX vq not in response to a
> > >     callback: start xmit and rx napi.  We currently do this with callbacks
> > >     enabled which can cause extra interrupts from the card.  Used not to be
> > >     a big issue as we run with interrupts disabled but that is no longer the
> > >     case, and in some cases the rate of spurious interrupts is so high
> > >     linux detects this and actually kills the interrupt.
> > >
> > > My undersatnding is that it tries to disable callbacks on TX.
> >
> > I think we want to disable callbacks while polling, yes. here we are not
> > polling, and I think we want a callback because otherwise nothing will
> > orphan skbs and a socket can be blocked, not transmitting anything - a
> > deadlock.
>
> I'm not sure how I got here, did you mean a partial revert of
> a7766ef18b33 (the part that disables TX callbacks on start_xmit)?

Michael, any idea on this?

Thanks

>
> Btw, I plan to remove non NAPI mode completely, since it was disabled
> by default for years and we don't see any complaint, then we may have
> modern features like BQL and better TCP performance. In that sense we
> may simply keep tx callback open as most of modern NIC did.
>
> >
> > > > One of the ideas of napi is to free on napi callback, not here
> > > > immediately.
> > > >
> > > > I think it is easier to just do a separate branch here. Along the
> > > > lines of:
> > > >
> > > >                 if (use_napi) {
> > > >                         if (unlikely(!virtqueue_enable_cb_delayed(sq->vq)))
> > > >                                 virtqueue_napi_schedule(napi, vq);
> > >
> > > This seems to be a new logic and it causes some delay in processing TX
> > > (unnecessary NAPI).
> >
> > That's good, we overloaded the queue so we are already going
> > too fast, deferring tx so queue has chance to drain
> > will allow better batching in the qdisc.
>
> I meant, compare to
>
> 1) schedule NAPI and poll TX
>
> The current code did
>
> 2) poll TX immediately
>
> 2) seems faster?
>
> Thanks
>
> >
> > > >                 } else {
> > > >                         ... old code ...
> > > >                 }
> > > >
> > > > also reduces chances of regressions on !napi (which is not well tested)
> > > > and keeps callbacks off while we free skbs.
> > >
> > > I think my patch doesn't change the logic of !napi? (It checks !napi || kick).
> > >
> > > Thanks
> >
> > I agree it doesn't seem to as written.
> >
> > > >
> > > > No?
> > > >
> > > >
> > > > > --
> > > > > 2.25.1
> > > >
> >

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