linux-kernel - Re: [PATCH net 1/3] virtio-net: make refill work a per receive queue work

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Message-ID: <dd4d01d7-29a8-43b3-bb5b-f50ea384aadb@gmail.com>
Date: Thu, 25 Dec 2025 22:55:36 +0700
From: Bui Quang Minh <minhquangbui99@...il.com>
To: "Michael S. Tsirkin" <mst@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
 Eugenio Pérez <eperezma@...hat.com>,
 Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 John Fastabend <john.fastabend@...il.com>,
 Stanislav Fomichev <sdf@...ichev.me>, virtualization@...ts.linux.dev,
 linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH net 1/3] virtio-net: make refill work a per receive queue
 work

On 12/24/25 23:49, Bui Quang Minh wrote:
> On 12/24/25 08:47, Michael S. Tsirkin wrote:
>> On Wed, Dec 24, 2025 at 09:37:14AM +0800, Xuan Zhuo wrote:
>>> Hi Jason,
>>>
>>> I'm wondering why we even need this refill work. Why not simply let 
>>> NAPI retry
>>> the refill on its next run if the refill fails? That would seem much 
>>> simpler.
>>> This refill work complicates maintenance and often introduces a lot of
>>> concurrency issues and races.
>>>
>>> Thanks.
>> refill work can refill from GFP_KERNEL, napi only from ATOMIC.
>>
>> And if GFP_ATOMIC failed, aggressively retrying might not be a great 
>> idea.
>>
>> Not saying refill work is a great hack, but that is the reason for it.
>
> In case no allocated received buffer and NAPI refill fails, the host 
> will not send any packets. If there is no busy polling loop either, 
> the RX will be stuck. That's also the reason why we need refill work. 
> Is it correct?

I've just looked at mlx5e_napi_poll which is mentioned by Jason. So if 
we want to retry refilling in the next NAPI, we can set a bool (e.g. 
retry_refill) in virtnet_receive, then in virtnet_poll, we don't call 
virtqueue_napi_complete. As a result, our napi poll is still in the 
softirq's poll list, so we don't need a new host packet to trigger 
virtqueue's callback which calls napi_schedule again.
>
> Thanks,
> Quang Minh.
>
>