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

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Date:   Wed, 10 Feb 2021 16:13:06 -0800
From:   Wei Wang <weiwan@...gle.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Jason Wang <jasowang@...hat.com>,
        David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net] virtio-net: suppress bad irq warning for tx napi

On Wed, Feb 10, 2021 at 1:14 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Tue, Feb 09, 2021 at 10:00:22AM -0800, Wei Wang wrote:
> > On Tue, Feb 9, 2021 at 6:58 AM Willem de Bruijn
> > <willemdebruijn.kernel@...il.com> wrote:
> > >
> > > > >>> I have no preference. Just curious, especially if it complicates the patch.
> > > > >>>
> > > > >> My understanding is that. It's probably ok for net. But we probably need
> > > > >> to document the assumptions to make sure it was not abused in other drivers.
> > > > >>
> > > > >> Introduce new parameters for find_vqs() can help to eliminate the subtle
> > > > >> stuffs but I agree it looks like a overkill.
> > > > >>
> > > > >> (Btw, I forget the numbers but wonder how much difference if we simple
> > > > >> remove the free_old_xmits() from the rx NAPI path?)
> > > > > The committed patchset did not record those numbers, but I found them
> > > > > in an earlier iteration:
> > > > >
> > > > >    [PATCH net-next 0/3] virtio-net tx napi
> > > > >    https://lists.openwall.net/netdev/2017/04/02/55
> > > > >
> > > > > It did seem to significantly reduce compute cycles ("Gcyc") at the
> > > > > time. For instance:
> > > > >
> > > > >      TCP_RR Latency (us):
> > > > >      1x:
> > > > >        p50              24       24       21
> > > > >        p99              27       27       27
> > > > >        Gcycles         299      432      308
> > > > >
> > > > > I'm concerned that removing it now may cause a regression report in a
> > > > > few months. That is higher risk than the spurious interrupt warning
> > > > > that was only reported after years of use.
> > > >
> > > >
> > > > Right.
> > > >
> > > > So if Michael is fine with this approach, I'm ok with it. But we
> > > > probably need to a TODO to invent the interrupt handlers that can be
> > > > used for more than one virtqueues. When MSI-X is enabled, the interrupt
> > > > handler (vring_interrup()) assumes the interrupt is used by a single
> > > > virtqueue.
> > >
> > > Thanks.
> > >
> > > The approach to schedule tx-napi from virtnet_poll_cleantx instead of
> > > cleaning directly in this rx-napi function was not effective at
> > > suppressing the warning, I understand.
> >
> > Correct. I tried the approach to schedule tx napi instead of directly
> > do free_old_xmit_skbs() in virtnet_poll_cleantx(). But the warning
> > still happens.
>
> Two questions here: is the device using packed or split vqs?
> And is event index enabled?
>

The device is indeed using split vqs with event index enabled.

> I think one issue is that at the moment with split and event index we
> don't actually disable events at all.
>
You mean we don't disable 'interrupts' right?
What is the reason for that?

> static void virtqueue_disable_cb_split(struct virtqueue *_vq)
> {
>         struct vring_virtqueue *vq = to_vvq(_vq);
>
>         if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
>                 vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
>                 if (!vq->event)
>                         vq->split.vring.avail->flags =
>                                 cpu_to_virtio16(_vq->vdev,
>                                                 vq->split.avail_flags_shadow);
>         }
> }
>
> Can you try your napi patch + disable event index?
>

Thanks for the suggestion.
I've run the reproducer with  napi patch + disable event index, and so
far, I did not see the warning getting triggered. Will keep it running
for a bit longer.

>
> --
> MST
>