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

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Date:   Wed, 3 Feb 2021 13:28:11 -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 Wed, Feb 3, 2021 at 12:33 AM Jason Wang <jasowang@...hat.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@...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.
>
>
> 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.