linux-kernel - Re: [PATCH] virtio_net: Warn if insufficient queue length for transmitting

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Date: Mon, 6 May 2024 11:24:25 +0800
From: Jason Wang <jasowang@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Darius Rad <darius@...espec.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, virtualization@...ts.linux.dev, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virtio_net: Warn if insufficient queue length for transmitting

On Wed, May 1, 2024 at 4:07 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Tue, Apr 30, 2024 at 03:35:09PM -0400, Darius Rad wrote:
> > The transmit queue is stopped when the number of free queue entries is less
> > than 2+MAX_SKB_FRAGS, in start_xmit().  If the queue length (QUEUE_NUM_MAX)
> > is less than then this, transmission will immediately trigger a netdev
> > watchdog timeout.  Report this condition earlier and more directly.
> >
> > Signed-off-by: Darius Rad <darius@...espec.com>
> > ---
> >  drivers/net/virtio_net.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 115c3c5414f2..72ee8473b61c 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -4917,6 +4917,9 @@ static int virtnet_probe(struct virtio_device *vdev)
> >                       set_bit(guest_offloads[i], &vi->guest_offloads);
> >       vi->guest_offloads_capable = vi->guest_offloads;
> >
> > +     if (virtqueue_get_vring_size(vi->sq->vq) < 2 + MAX_SKB_FRAGS)
> > +             netdev_warn_once(dev, "not enough queue entries, expect xmit timeout\n");
> > +
>
> How about actually fixing it though? E.g. by linearizing...

Actually, the linearing is only needed for the case when the indirect
descriptor is not supported.

>
> It also bothers me that there's practically
> /proc/sys/net/core/max_skb_frags
> and if that's low then things could actually work.

Probably not as it won't exceed MAX_SKB_FRAGS.

>
> Finally, while originally it was just 17 typically, now it's
> configurable. So it's possible that you change the config to make big
> tcp

Note that virtio-net doesn't fully support big TCP.

> work better and device stops working while it worked fine
> previously.

For this patch, I guess not as we had:

        if (sq->vq->num_free < 2+MAX_SKB_FRAGS)

in the tx path. So it won't even work before this patch.

Thanks

>
>
> >       pr_debug("virtnet: registered device %s with %d RX and TX vq's\n",
> >                dev->name, max_queue_pairs);
> >
> > --
> > 2.39.2
>