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

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Message-ID: <20251225112636-mutt-send-email-mst@kernel.org>
Date: Thu, 25 Dec 2025 11:27:09 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Bui Quang Minh <minhquangbui99@...il.com>
Cc: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	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 Thu, Dec 25, 2025 at 10:55:36PM +0700, Bui Quang Minh wrote:
> 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.
> > 
>


yes yes.
but aggressively retrying GFP_ATOMIC until it works is not the thing to
do.

-- 
MST