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

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Message-ID: <CACGkMEvu3hi_D6Y0uqc5GVvdi6TNe-Q1WFie9f=7mVabqO6YvA@mail.gmail.com>
Date: Sun, 8 Oct 2023 13:27:06 +0800
From: Jason Wang <jasowang@...hat.com>
To: Feng Liu <feliu@...dia.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	virtualization@...ts.linux-foundation.org, kuba@...nel.org, 
	Bodong Wang <bodong@...dia.com>
Subject: Re: [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the
 command waiting loop

On Fri, Oct 6, 2023 at 3:35 AM Feng Liu <feliu@...dia.com> wrote:
>
>
>
> On 2023-07-24 a.m.2:46, Michael S. Tsirkin wrote:
> > External email: Use caution opening links or attachments
> >
> >
> > On Fri, Jul 21, 2023 at 10:18:03PM +0200, Maxime Coquelin wrote:
> >>
> >>
> >> On 7/21/23 17:10, Michael S. Tsirkin wrote:
> >>> On Fri, Jul 21, 2023 at 04:58:04PM +0200, Maxime Coquelin wrote:
> >>>>
> >>>>
> >>>> On 7/21/23 16:45, Michael S. Tsirkin wrote:
> >>>>> On Fri, Jul 21, 2023 at 04:37:00PM +0200, Maxime Coquelin wrote:
> >>>>>>
> >>>>>>
> >>>>>> On 7/20/23 23:02, Michael S. Tsirkin wrote:
> >>>>>>> On Thu, Jul 20, 2023 at 01:26:20PM -0700, Shannon Nelson wrote:
> >>>>>>>> On 7/20/23 1:38 AM, 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 on a device whose CVQ might be slow.
> >>>>>>>>>
> >>>>>>>>> Signed-off-by: Jason Wang <jasowang@...hat.com>
> >>>>>>>>
> >>>>>>>> This still leaves hung processes, but at least it doesn't pin the CPU any
> >>>>>>>> more.  Thanks.
> >>>>>>>> Reviewed-by: Shannon Nelson <shannon.nelson@....com>
> >>>>>>>>
> >>>>>>>
> >>>>>>> I'd like to see a full solution
> >>>>>>> 1- block until interrupt
> >>>>>>
> >>>>>> Would it make sense to also have a timeout?
> >>>>>> And when timeout expires, set FAILED bit in device status?
> >>>>>
> >>>>> virtio spec does not set any limits on the timing of vq
> >>>>> processing.
> >>>>
> >>>> Indeed, but I thought the driver could decide it is too long for it.
> >>>>
> >>>> The issue is we keep waiting with rtnl locked, it can quickly make the
> >>>> system unusable.
> >>>
> >>> if this is a problem we should find a way not to keep rtnl
> >>> locked indefinitely.
> >>
> >>  From the tests I have done, I think it is. With OVS, a reconfiguration is
> >> performed when the VDUSE device is added, and when a MLX5 device is
> >> in the same bridge, it ends up doing an ioctl() that tries to take the
> >> rtnl lock. In this configuration, it is not possible to kill OVS because
> >> it is stuck trying to acquire rtnl lock for mlx5 that is held by virtio-
> >> net.
> >
> > So for sure, we can queue up the work and process it later.
> > The somewhat tricky part is limiting the memory consumption.
> >
> >
>
>
> Hi Jason
>
> Excuse me, is there any plan for when will v5 patch series be sent out?
> Will the v5 patches solve the problem of ctrlvq's infinite poll for
> buggy devices?

We agree to harden VDUSE and,

It would be hard if we try to solve it at the virtio-net level, see
the discussions before. It might require support from various layers
(e.g networking core etc).

We can use workqueue etc as a mitigation. If Michael is fine with
this, I can post v5.

Thanks

>
> Thanks
> Feng
>
> >>>
> >>>>>>> 2- still handle surprise removal correctly by waking in that case
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>>>> ---
> >>>>>>>>>       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 9f3b1d6ac33d..e7533f29b219 100644
> >>>>>>>>> --- a/drivers/net/virtio_net.c
> >>>>>>>>> +++ b/drivers/net/virtio_net.c
> >>>>>>>>> @@ -2314,8 +2314,10 @@ 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();
> >>>>>>>>> +       }
> >>>>>>>>>
> >>>>>>>>>              return vi->ctrl->status == VIRTIO_NET_OK;
> >>>>>>>>>       }
> >>>>>>>>> --
> >>>>>>>>> 2.39.3
> >>>>>>>>>
> >>>>>>>>> _______________________________________________
> >>>>>>>>> Virtualization mailing list
> >>>>>>>>> Virtualization@...ts.linux-foundation.org
> >>>>>>>>> https://lists.linuxfoundation.org/mailman/listinfo/virtualization
> >>>>>>>
> >>>>>
> >>>
> >
> > _______________________________________________
> > Virtualization mailing list
> > Virtualization@...ts.linux-foundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/virtualization
>

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