Virtualization - Dec 2022 - [PATCH net V2] virtio-net: correctly enable callback during start_xmit

On Thu, Dec 15, 2022 at 5:02 PM Michael S. Tsirkin <mst at redhat.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 at redhat.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.
> 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).
>                 } 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
>
> No?
>
>
> > --
> > 2.25.1
>

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 at
redhat.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 at redhat.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.
> > 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.
> >                 } 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
> >