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

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Message-ID: <CACGkMEuTfKQCHqJFgtG1-HBn--fdGSKdPqF_=p078WbiGN=J=w@mail.gmail.com>
Date: Mon, 29 Dec 2025 10:57:09 +0800
From: Jason Wang <jasowang@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Xuan Zhuo <xuanzhuo@...ux.alibaba.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, 
	Bui Quang Minh <minhquangbui99@...il.com>
Subject: Re: [PATCH net 1/3] virtio-net: make refill work a per receive queue work

On Fri, Dec 26, 2025 at 3:37 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Fri, Dec 26, 2025 at 09:31:26AM +0800, Jason Wang wrote:
> > On Fri, Dec 26, 2025 at 12:27 AM Michael S. Tsirkin <mst@...hat.com> wrote:
> > >
> > > On Thu, Dec 25, 2025 at 03:33:29PM +0800, Jason Wang wrote:
> > > > On Wed, Dec 24, 2025 at 9:48 AM Michael S. Tsirkin <mst@...hat.com> 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.
> > > >
> > > > Btw, I see some drivers are doing things as Xuan said. E.g
> > > > mlx5e_napi_poll() did:
> > > >
> > > > busy |= INDIRECT_CALL_2(rq->post_wqes,
> > > >                                 mlx5e_post_rx_mpwqes,
> > > >                                 mlx5e_post_rx_wqes,
> > > >
> > > > ...
> > > >
> > > > if (busy) {
> > > >          if (likely(mlx5e_channel_no_affinity_change(c))) {
> > > >                 work_done = budget;
> > > >                 goto out;
> > > > ...
> > >
> > >
> > > is busy a GFP_ATOMIC allocation failure?
> >
> > Yes, and I think the logic here is to fallback to ksoftirqd if the
> > allocation fails too much.
> >
> > Thanks
>
>
> True. I just don't know if this works better or worse than the
> current design, but it is certainly simpler and we never actually
> worried about the performance of the current one.
>
>
> So you know, let's roll with this approach.
>
> I do however ask that some testing is done on the patch forcing these OOM
> situations just to see if we are missing something obvious.
>
>
> the beauty is the patch can be very small:
> 1. patch 1 do not schedule refill ever, just retrigger napi
> 2. remove all the now dead code
>
> this way patch 1 will be small and backportable to stable.

I fully agree here.

Thanks