Subject: Re: [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the command waiting loop

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 11 Aug 2023 06:12:10 -0400

On Fri, Aug 11, 2023 at 05:54:15PM +0800, Jason Wang wrote:
> On Fri, Aug 11, 2023 at 5:51 PM Michael S. Tsirkin <[email-masked]> wrote:
> >
> > On Fri, Aug 11, 2023 at 05:43:25PM +0800, Jason Wang wrote:
> > > On Fri, Aug 11, 2023 at 5:21 PM Michael S. Tsirkin <[email-masked]> wrote:
> > > >
> > > > On Fri, Aug 11, 2023 at 05:18:51PM +0800, Jason Wang wrote:
> > > > > On Fri, Aug 11, 2023 at 1:42 PM Michael S. Tsirkin <[email-masked]> wrote:
> > > > > >
> > > > > > On Fri, Aug 11, 2023 at 10:23:15AM +0800, Jason Wang wrote:
> > > > > > > On Fri, Aug 11, 2023 at 3:41 AM Michael S. Tsirkin <[email-masked]> wrote:
> > > > > > > >
> > > > > > > > On Tue, Aug 08, 2023 at 10:30:56AM +0800, Jason Wang wrote:
> > > > > > > > > On Mon, Jul 31, 2023 at 2:30 PM Jason Wang <[email-masked]> wrote:
> > > > > > > > > >
> > > > > > > > > > On Thu, Jul 27, 2023 at 5:46 PM Michael S. Tsirkin <[email-masked]> 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.
> > > > > > > > >
> > > > > > > > > Michael, any more input on this?
> > > > > > > > >
> > > > > > > > > Thanks
> > > > > > > >
> > > > > > > > I think this is just mission creep. We are trying to fix
> > > > > > > > vduse - let's do that for starters.
> > > > > > > >
> > > > > > > > Recovering from firmware timeouts is far from trivial and
> > > > > > > > just assuming that just because it timed out it will not
> > > > > > > > access memory is just as likely to cause memory corruption
> > > > > > > > with worse results than an infinite spin.
> > > > > > >
> > > > > > > Yes, this might require support not only in the driver
> > > > > > >
> > > > > > > >
> > > > > > > > I propose we fix this for vduse and assume hardware/firmware
> > > > > > > > is well behaved.
> > > > > > >
> > > > > > > One major case is the re-connection, in that case it might take
> > > > > > > whatever longer that the kernel virito-net driver expects.
> > > > > > > So we can have a timeout in VDUSE and trap CVQ then VDUSE can return
> > > > > > > and fail early?
> > > > > >
> > > > > > Ugh more mission creep. not at all my point. vduse should cache
> > > > > > values in the driver,
> > > > >
> > > > > What do you mean by values here? The cvq command?
> > > > >
> > > > > Thanks
> > > >
> > > > The card status generally.
> > >
> > > Just to make sure I understand here. The CVQ needs to be processed by
> > > the userspace now. How could we cache the status?
> > >
> > > Thanks
> >
> > vduse will have to process it in kernel.
> 
> Right, that's my understanding (trap CVQ).
> 
> Thanks

oh this what you mean by trap. ok. I don't see a new
for a timeout then though.

> >
> > > >
> > > > > > until someone manages to change
> > > > > > net core to be more friendly to userspace devices.
> > > > > >
> > > > > > >
> > > > > > > > Or maybe not well behaved firmware will
> > > > > > > > set the flag losing error reporting ability.
> > > > > > >
> > > > > > > This might be hard since it means not only the set but also the get is
> > > > > > > unreliable.
> > > > > > >
> > > > > > > Thanks
> > > > > >
> > > > > > /me shrugs
> > > > > >
> > > > > >
> > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > > > >
> > > > > > > > > > Thanks
> > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > > > --
> > > > > > > > > > > MST
> > > > > > > > > > >
> > > > > > > >
> > > > > >
> > > >
> >