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

[Jason Wang]

On Sat, Jul 22, 2023 at 4:18 AM Maxime Coquelin
<maxime.coquelin@xxxxxxxxxx> 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@xxxxxxxxxx>
> >>>>>>
> >>>>>> This still leaves hung processes, but at least it doesn't pin the CPU any
> >>>>>> more.  Thanks.
> >>>>>> Reviewed-by: Shannon Nelson <shannon.nelson@xxxxxxx>
> >>>>>>
> >>>>>
> >>>>> I'd like to see a full solution
> >>>>> 1- block until interrupt

I remember in previous versions, you worried about the extra MSI
vector. (Maybe I was wrong).

> >>>>
> >>>> 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.

Any ideas on this direction? Simply dropping rtnl during the busy loop
will result in a lot of races. This seems to require non-trivial
changes in the networking core.

>
>  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.

Yeah, basically, any RTNL users would be blocked forever.

And the infinite loop has other side effects like it blocks the freezer to work.

To summarize, there are three issues

1) busy polling
2) breaks freezer
3) hold RTNL during the loop

Solving 3 may help somehow for 2 e.g some pm routine e.g wireguard or
even virtnet_restore() itself may try to hold the lock.

>
> >
> >>>>> 2- still handle surprise removal correctly by waking in that case

This is basically what version 1 did?

https://lore.kernel.org/lkml/6026e801-6fda-fee9-a69b-d06a80368621@xxxxxxxxxx/t/

Thanks

> >>>>>
> >>>>>
> >>>>>
> >>>>>>> ---
> >>>>>>>      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@xxxxxxxxxxxxxxxxxxxxxxxxxx
> >>>>>>> https://lists.linuxfoundation.org/mailman/listinfo/virtualization
> >>>>>
> >>>
> >
>