Date:   Tue, 2 Feb 2021 09:37:46 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Wei Wang <weiwan@...gle.com>, David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net] virtio-net: suppress bad irq warning for tx napi

On Mon, Feb 1, 2021 at 10:09 PM Jason Wang <jasowang@...hat.com> wrote:
>
>
> On 2021/1/29 上午8:21, Wei Wang wrote:
> > With the implementation of napi-tx in virtio driver, we clean tx
> > descriptors from rx napi handler, for the purpose of reducing tx
> > complete interrupts. But this could introduce a race where tx complete
> > interrupt has been raised, but the handler found there is no work to do
> > because we have done the work in the previous rx interrupt handler.
> > This could lead to the following warning msg:
> > [ 3588.010778] irq 38: nobody cared (try booting with the
> > "irqpoll" option)
> > [ 3588.017938] CPU: 4 PID: 0 Comm: swapper/4 Not tainted
> > 5.3.0-19-generic #20~18.04.2-Ubuntu
> > [ 3588.017940] Call Trace:
> > [ 3588.017942]  <IRQ>
> > [ 3588.017951]  dump_stack+0x63/0x85
> > [ 3588.017953]  __report_bad_irq+0x35/0xc0
> > [ 3588.017955]  note_interrupt+0x24b/0x2a0
> > [ 3588.017956]  handle_irq_event_percpu+0x54/0x80
> > [ 3588.017957]  handle_irq_event+0x3b/0x60
> > [ 3588.017958]  handle_edge_irq+0x83/0x1a0
> > [ 3588.017961]  handle_irq+0x20/0x30
> > [ 3588.017964]  do_IRQ+0x50/0xe0
> > [ 3588.017966]  common_interrupt+0xf/0xf
> > [ 3588.017966]  </IRQ>
> > [ 3588.017989] handlers:
> > [ 3588.020374] [<000000001b9f1da8>] vring_interrupt
> > [ 3588.025099] Disabling IRQ #38
> >
> > This patch adds a new param to struct vring_virtqueue, and we set it for
> > tx virtqueues if napi-tx is enabled, to suppress the warning in such
> > case.
> >
> > Fixes: 7b0411ef4aa6 ("virtio-net: clean tx descriptors from rx napi")
> > Reported-by: Rick Jones <jonesrick@...gle.com>
> > Signed-off-by: Wei Wang <weiwan@...gle.com>
> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>
>
> Please use get_maintainer.pl to make sure Michael and me were cced.

Will do. Sorry about that. I suggested just the virtualization list, my bad.

>
> > ---
> >   drivers/net/virtio_net.c     | 19 ++++++++++++++-----
> >   drivers/virtio/virtio_ring.c | 16 ++++++++++++++++
> >   include/linux/virtio.h       |  2 ++
> >   3 files changed, 32 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 508408fbe78f..e9a3f30864e8 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -1303,13 +1303,22 @@ static void virtnet_napi_tx_enable(struct virtnet_info *vi,
> >               return;
> >       }
> >
> > +     /* With napi_tx enabled, free_old_xmit_skbs() could be called from
> > +      * rx napi handler. Set work_steal to suppress bad irq warning for
> > +      * IRQ_NONE case from tx complete interrupt handler.
> > +      */
> > +     virtqueue_set_work_steal(vq, true);
> > +
> >       return virtnet_napi_enable(vq, napi);
>
>
> Do we need to force the ordering between steal set and napi enable?

The warning only occurs after one hundred spurious interrupts, so not
really.

>
> >   }
> >
> > -static void virtnet_napi_tx_disable(struct napi_struct *napi)
> > +static void virtnet_napi_tx_disable(struct virtqueue *vq,
> > +                                 struct napi_struct *napi)
> >   {
> > -     if (napi->weight)
> > +     if (napi->weight) {
> >               napi_disable(napi);
> > +             virtqueue_set_work_steal(vq, false);
> > +     }
> >   }
> >
> >   static void refill_work(struct work_struct *work)
> > @@ -1835,7 +1844,7 @@ static int virtnet_close(struct net_device *dev)
> >       for (i = 0; i < vi->max_queue_pairs; i++) {
> >               xdp_rxq_info_unreg(&vi->rq[i].xdp_rxq);
> >               napi_disable(&vi->rq[i].napi);
> > -             virtnet_napi_tx_disable(&vi->sq[i].napi);
> > +             virtnet_napi_tx_disable(vi->sq[i].vq, &vi->sq[i].napi);
> >       }
> >
> >       return 0;
> > @@ -2315,7 +2324,7 @@ static void virtnet_freeze_down(struct virtio_device *vdev)
> >       if (netif_running(vi->dev)) {
> >               for (i = 0; i < vi->max_queue_pairs; i++) {
> >                       napi_disable(&vi->rq[i].napi);
> > -                     virtnet_napi_tx_disable(&vi->sq[i].napi);
> > +                     virtnet_napi_tx_disable(vi->sq[i].vq, &vi->sq[i].napi);
> >               }
> >       }
> >   }
> > @@ -2440,7 +2449,7 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
> >       if (netif_running(dev)) {
> >               for (i = 0; i < vi->max_queue_pairs; i++) {
> >                       napi_disable(&vi->rq[i].napi);
> > -                     virtnet_napi_tx_disable(&vi->sq[i].napi);
> > +                     virtnet_napi_tx_disable(vi->sq[i].vq, &vi->sq[i].napi);
> >               }
> >       }
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 71e16b53e9c1..f7c5d697c302 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -105,6 +105,9 @@ struct vring_virtqueue {
> >       /* Host publishes avail event idx */
> >       bool event;
> >
> > +     /* Tx side napi work could be done from rx side. */
> > +     bool work_steal;
>
>
> So vring_vritqueue is a general structure, let's avoid mentioning
> network specific stuffs here. And we need a better name like
> "no_interrupt_check"?
>
> And we need a separate patch for virtio core changes.

Ack. Will change.

>
> > +
> >       /* Head of free buffer list. */
> >       unsigned int free_head;
> >       /* Number we've added since last sync. */
> > @@ -1604,6 +1607,7 @@ static struct virtqueue *vring_create_virtqueue_packed(
> >       vq->notify = notify;
> >       vq->weak_barriers = weak_barriers;
> >       vq->broken = false;
> > +     vq->work_steal = false;
> >       vq->last_used_idx = 0;
> >       vq->num_added = 0;
> >       vq->packed_ring = true;
> > @@ -2038,6 +2042,9 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
> >
> >       if (!more_used(vq)) {
> >               pr_debug("virtqueue interrupt with no work for %p\n", vq);
>
>
> Do we still need to keep this warning?

Come to think of it, I would say no, in this case.

>
>
> > +             if (vq->work_steal)
> > +                     return IRQ_HANDLED;
>
>
> So I wonder instead of doing trick like this, maybe it's time to unify
> TX/RX NAPI with the help of[1] (virtio-net use queue pairs).
>
> Thanks
>
> [1] https://lkml.org/lkml/2014/12/25/169

Interesting idea. It does sound like a good fit for this model. The
patch in the Fixes line proved effective at suppressing unnecessary TX
interrupts when processing in RX interrupt handler. So not sure how
much will help in practice. Might be a nice project to evaluate
separate for net-next at some point.

Thanks for the review!

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