Virtualization - Jul 2023 - [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the command waiting loop

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 at redhat.com>
> > > 
> > > This still leaves hung processes, but at least it doesn't pin
the CPU any
> > > more.  Thanks.
> > > Reviewed-by: Shannon Nelson <shannon.nelson at amd.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.
> > 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 at lists.linux-foundation.org
> > > >
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
> >

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 at redhat.com>
>>>>
>>>> This still leaves hung processes, but at least it doesn't
pin the CPU any
>>>> more.  Thanks.
>>>> Reviewed-by: Shannon Nelson <shannon.nelson at amd.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.
>>> 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 at lists.linux-foundation.org
>>>>>
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
>>>
>