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Date:   Mon, 8 May 2023 11:12:03 +0800
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Subject: Re: [PATCH v4] virtio_net: suppress cpu stall when free_unused_bufs
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Wenliang Wang <wangwenliang.1995@bytedance.com>,
        davem@davemloft.net, edumazet@google.com, kuba@kernel.org,
        pabeni@redhat.com, zhengqi.arch@bytedance.com,
        willemdebruijn.kernel@gmail.com,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, xuanzhuo@linux.alibaba.com
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From:   Jason Wang <jasowang@redhat.com>
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在 2023/5/7 21:34, Michael S. Tsirkin 写道:
> On Fri, May 05, 2023 at 11:28:25AM +0800, Jason Wang wrote:
>> On Thu, May 4, 2023 at 10:27 AM Wenliang Wang
>> <wangwenliang.1995@bytedance.com> wrote:
>>> For multi-queue and large ring-size use case, the following error
>>> occurred when free_unused_bufs:
>>> rcu: INFO: rcu_sched self-detected stall on CPU.
>>>
>>> Fixes: 986a4f4d452d ("virtio_net: multiqueue support")
>>> Signed-off-by: Wenliang Wang <wangwenliang.1995@bytedance.com>
>>> ---
>>> v2:
>>> -add need_resched check.
>>> -apply same logic to sq.
>>> v3:
>>> -use cond_resched instead.
>>> v4:
>>> -add fixes tag
>>> ---
>>>   drivers/net/virtio_net.c | 2 ++
>>>   1 file changed, 2 insertions(+)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index 8d8038538fc4..a12ae26db0e2 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -3560,12 +3560,14 @@ static void free_unused_bufs(struct virtnet_info *vi)
>>>                  struct virtqueue *vq = vi->sq[i].vq;
>>>                  while ((buf = virtqueue_detach_unused_buf(vq)) != NULL)
>>>                          virtnet_sq_free_unused_buf(vq, buf);
>>> +               cond_resched();
>> Does this really address the case when the virtqueue is very large?
>>
>> Thanks
>
> it does in that a very large queue is still just 64k in size.
> we might however have 64k of these queues.


Ok, but we have other similar loops especially the refill, I think we 
may need cond_resched() there as well.

Thanks


>
>>>          }
>>>
>>>          for (i = 0; i < vi->max_queue_pairs; i++) {
>>>                  struct virtqueue *vq = vi->rq[i].vq;
>>>                  while ((buf = virtqueue_detach_unused_buf(vq)) != NULL)
>>>                          virtnet_rq_free_unused_buf(vq, buf);
>>> +               cond_resched();
>>>          }
>>>   }
>>>
>>> --
>>> 2.20.1
>>>