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

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Date: Fri, 21 Jul 2023 16:37:00 +0200
From: Maxime Coquelin <maxime.coquelin@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>,
 Shannon Nelson <shannon.nelson@....com>
Cc: Jason Wang <jasowang@...hat.com>, xuanzhuo@...ux.alibaba.com,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 virtualization@...ts.linux-foundation.org, edumazet@...gle.com,
 kuba@...nel.org, pabeni@...hat.com, davem@...emloft.net
Subject: Re: [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the
 command waiting loop

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?

> 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
> 

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