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

[Maxime Coquelin <maxime.coquelin@redhat.com>]

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