netdev - Re: [PATCH net] virtio-net: add cond_resched() to the command waiting loop

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Date:   Wed, 7 Sep 2022 10:09:52 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Gautam Dawar <gautam.dawar@...inx.com>,
        davem <davem@...emloft.net>
Subject: Re: [PATCH net] virtio-net: add cond_resched() to the command waiting loop

On Tue, Sep 6, 2022 at 6:56 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Mon, 2022-09-05 at 15:49 +0800, Jason Wang wrote:
> > On Mon, Sep 5, 2022 at 3:15 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > >
> > > On Mon, Sep 05, 2022 at 12:53:41PM +0800, Jason Wang wrote:
> > > > Adding cond_resched() to the command waiting loop for a better
> > > > co-operation with the scheduler. This allows to give CPU a breath to
> > > > run other task(workqueue) instead of busy looping when preemption is
> > > > not allowed.
> > > >
> > > > What's more important. This is a must for some vDPA parent to work
> > > > since control virtqueue is emulated via a workqueue for those parents.
> > > >
> > > > Fixes: bda324fd037a ("vdpasim: control virtqueue support")
> > >
> > > That's a weird commit to fix. so it fixes the simulator?
> >
> > Yes, since the simulator is using a workqueue to handle control virtueue.
>
> Uhmm... touching a driver for a simulator's sake looks a little weird.

Simulator is not the only one that is using a workqueue (but should be
the first).

I can see  that the mlx5 vDPA driver is using a workqueue as well (see
mlx5_vdpa_kick_vq()).

And in the case of VDUSE, it needs to wait for the response from the
userspace, this means cond_resched() is probably a must for the case
like UP.

>
> Additionally, if the bug is vdpasim, I think it's better to try to
> solve it there, if possible.
>
> Looking at vdpasim_net_work() and vdpasim_blk_work() it looks like
> neither needs a process context, so perhaps you could rework it to run
> the work_fn() directly from vdpasim_kick_vq(), at least for the control
> virtqueue?

It's possible (but require some rework on the simulator core). But
considering we have other similar use cases, it looks better to solve
it in the virtio-net driver.

Additionally, this may have better behaviour when using for the buggy
hardware (e.g the control virtqueue takes too long to respond). We may
consider switching to use interrupt/sleep in the future (but not
suitable for -net).

Thanks

>
> Thanks!
>
> Paolo
>