netdev - Re: [PATCH] virtio_net: suppress cpu stall when free_unused

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Date:   Thu, 27 Apr 2023 16:49:58 +0800
From:   Wenliang Wang <wangwenliang.1995@...edance.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, jasowang@...hat.com,
        davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com
Subject: Re: [PATCH] virtio_net: suppress cpu stall when free_unused_bufs

On 4/27/23 4:23 PM, Michael S. Tsirkin wrote:
> On Thu, Apr 27, 2023 at 04:13:45PM +0800, Xuan Zhuo wrote:
>> On Thu, 27 Apr 2023 04:12:44 -0400, "Michael S. Tsirkin" <mst@...hat.com> wrote:
>>> On Thu, Apr 27, 2023 at 03:13:44PM +0800, Xuan Zhuo wrote:
>>>> On Thu, 27 Apr 2023 15:02:26 +0800, Wenliang Wang <wangwenliang.1995@...edance.com> wrote:
>>>>>
>>>>>
>>>>> On 4/27/23 2:20 PM, Xuan Zhuo wrote:
>>>>>> On Thu, 27 Apr 2023 12:34:33 +0800, Wenliang Wang <wangwenliang.1995@...edance.com> wrote:
>>>>>>> For multi-queue and large rx-ring-size use case, the following error
>>>>>>
>>>>>> Cound you give we one number for example?
>>>>>
>>>>> 128 queues and 16K queue_size is typical.
>>>>>
>>>>>>
>>>>>>> occurred when free_unused_bufs:
>>>>>>> rcu: INFO: rcu_sched self-detected stall on CPU.
>>>>>>>
>>>>>>> Signed-off-by: Wenliang Wang <wangwenliang.1995@...edance.com>
>>>>>>> ---
>>>>>>>    drivers/net/virtio_net.c | 1 +
>>>>>>>    1 file changed, 1 insertion(+)
>>>>>>>
>>>>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>>>>>> index ea1bd4bb326d..21d8382fd2c7 100644
>>>>>>> --- a/drivers/net/virtio_net.c
>>>>>>> +++ b/drivers/net/virtio_net.c
>>>>>>> @@ -3565,6 +3565,7 @@ static void free_unused_bufs(struct virtnet_info *vi)
>>>>>>>    		struct virtqueue *vq = vi->rq[i].vq;
>>>>>>>    		while ((buf = virtqueue_detach_unused_buf(vq)) != NULL)
>>>>>>>    			virtnet_rq_free_unused_buf(vq, buf);
>>>>>>> +		schedule();
>>>>>>
>>>>>> Just for rq?
>>>>>>
>>>>>> Do we need to do the same thing for sq?
>>>>> Rq buffers are pre-allocated, take seconds to free rq unused buffers.
>>>>>
>>>>> Sq unused buffers are much less, so do the same for sq is optional.
>>>>
>>>> I got.
>>>>
>>>> I think we should look for a way, compatible with the less queues or the smaller
>>>> rings. Calling schedule() directly may be not a good way.
>>>>
>>>> Thanks.
>>>
>>> Why isn't it a good way?
>>
>> For the small ring, I don't think it is a good way, maybe we only deal with one
>> buf, then call schedule().
>>
>> We can call the schedule() after processing a certain number of buffers,
>> or check need_resched () first.
>>
>> Thanks.
> 
> 
> Wenliang, does
>              if (need_resched())
>                      schedule();
> fix the issue for you?
> 
Yeah, it works better.
> 
>>
>>
>>>
>>>>
>>>>>
>>>>>>
>>>>>> Thanks.
>>>>>>
>>>>>>
>>>>>>>    	}
>>>>>>>    }
>>>>>>>
>>>>>>> --
>>>>>>> 2.20.1
>>>>>>>
>>>
>