Re: [PATCH net] virtio-net: suppress bad irq warning for tx napi

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Wed, Feb 3, 2021 at 10:06 PM Jason Wang <jasowang@redhat.com> wrote:
>
>
> On 2021/2/4 上午2:28, Willem de Bruijn wrote:
> > On Wed, Feb 3, 2021 at 12:33 AM Jason Wang <jasowang@redhat.com> wrote:
> >>
> >> On 2021/2/2 下午10:37, Willem de Bruijn wrote:
> >>> On Mon, Feb 1, 2021 at 10:09 PM Jason Wang <jasowang@redhat.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@google.com>
> >>>>> Signed-off-by: Wei Wang <weiwan@google.com>
> >>>>> Signed-off-by: Willem de Bruijn <willemb@google.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.
> >>
> >> Ok, so it looks like a hint. Then I wonder how much value do we need to
> >> introduce helper like virtqueue_set_work_steal() that allows the caller
> >> to toggle. How about disable the check forever during virtqueue
> >> initialization?
> > Yes, that is even simpler.
> >
> > We still need the helper, as the internal variables of vring_virtqueue
> > are not accessible from virtio-net. An earlier patch added the
> > variable to virtqueue itself, but I think it belongs in
> > vring_virtqueue. And the helper is not a lot of code.
>
>
> It's better to do this before the allocating the irq. But it looks not
> easy unless we extend find_vqs().

Can you elaborate why that is better? At virtnet_open the interrupts
are not firing either.

I have no preference. Just curious, especially if it complicates the patch.