linux-kernel - Re: [PATCH net-next v4 2/2] virtio-net: add cond

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Message-ID: <CACGkMEvbm1LmwpiOzE0mCt6YKHsDy5zYv9fdLhcKBPaPOzLmpA@mail.gmail.com>
Date:   Mon, 31 Jul 2023 14:30:04 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Maxime Coquelin <maxime.coquelin@...hat.com>,
        Shannon Nelson <shannon.nelson@....com>,
        xuanzhuo@...ux.alibaba.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, davem@...emloft.net
Subject: Re: [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the
 command waiting loop

On Thu, Jul 27, 2023 at 5:46 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Thu, Jul 27, 2023 at 04:59:33PM +0800, Jason Wang wrote:
> > > They really shouldn't - any NIC that takes forever to
> > > program will create issues in the networking stack.
> >
> > Unfortunately, it's not rare as the device/cvq could be implemented
> > via firmware or software.
>
> Currently that mean one either has sane firmware with a scheduler that
> can meet deadlines, or loses ability to report errors back.
>
> > > But if they do they can always set this flag too.
> >
> > This may have false negatives and may confuse the management.
> >
> > Maybe we can extend the networking core to allow some device specific
> > configurations to be done with device specific lock without rtnl. For
> > example, split the set_channels to
> >
> > pre_set_channels
> > set_channels
> > post_set_channels
> >
> > The device specific part could be done in pre and post without a rtnl lock?
> >
> > Thanks
>
>
> Would the benefit be that errors can be reported to userspace then?
> Then maybe.  I think you will have to show how this works for at least
> one card besides virtio.

Even for virtio, this seems not easy, as e.g the
virtnet_send_command() and netif_set_real_num_tx_queues() need to
appear to be atomic to the networking core.

I wonder if we can re-consider the way of a timeout here and choose a
sane value as a start.

Thanks

>
>
> --
> MST
>