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From:   Jason Wang <jasowang@redhat.com>
Date:   Mon, 5 Sep 2022 15:49:32 +0800
Message-ID: <CACGkMEtjQ0Jfok-gcRW+kuinsua2X0TscyTNfBJoXHny0Yob+g@mail.gmail.com>
Subject: Re: [PATCH net] virtio-net: add cond_resched() to the command waiting loop
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     davem <davem@davemloft.net>, Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Gautam Dawar <gautam.dawar@xilinx.com>,
        virtualization <virtualization@lists.linux-foundation.org>,
        netdev <netdev@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Mon, Sep 5, 2022 at 3:15 PM Michael S. Tsirkin <mst@redhat.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.

>
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > ---
> >  drivers/net/virtio_net.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index ece00b84e3a7..169368365d6a 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -2000,8 +2000,10 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
> >        * into the hypervisor, so the request should be handled immediately.
> >        */
> >       while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> > -            !virtqueue_is_broken(vi->cvq))
> > +            !virtqueue_is_broken(vi->cvq)) {
> > +             cond_resched();
> >               cpu_relax();
> > +     }
>
> with cond_resched do we still need cpu_relax?

I think so, it's possible that cond_sched() just doesn't trigger scheduling.

Thanks

>
> >       return vi->ctrl->status == VIRTIO_NET_OK;
> >  }
> > --
> > 2.25.1
>