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

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Date:   Tue, 06 Sep 2022 12:55:54 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     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 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. 

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?

Thanks!

Paolo