Re: [PATCH 24/33] virtio_net: xsk: stop disable tx napi

From: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: "Petr Machata" <petrm@nvidia.com>,
	"Menglong Dong" <imagedong@tencent.com>,
	"Maciej Fijalkowski" <maciej.fijalkowski@intel.com>,
	"Jesper Dangaard Brouer" <hawk@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	netdev@vger.kernel.org,
	"John Fastabend" <john.fastabend@gmail.com>,
	"Björn Töpel" <bjorn@kernel.org>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Eric Dumazet" <edumazet@google.com>,
	"Kuniyuki Iwashima" <kuniyu@amazon.com>,
	"Sebastian Andrzej Siewior" <bigeasy@linutronix.de>,
	"Jonathan Lemon" <jonathan.lemon@gmail.com>,
	"Jakub Kicinski" <kuba@kernel.org>,
	bpf@vger.kernel.org, "Paolo Abeni" <pabeni@redhat.com>,
	virtualization@lists.linux-foundation.org,
	"David S. Miller" <davem@davemloft.net>,
	"Magnus Karlsson" <magnus.karlsson@intel.com>
Subject: Re: [PATCH 24/33] virtio_net: xsk: stop disable tx napi
Date: Fri, 3 Feb 2023 11:24:42 +0800	[thread overview]
Message-ID: <1675394682.9569418-1-xuanzhuo@linux.alibaba.com> (raw)
In-Reply-To: <20230202122445-mutt-send-email-mst@kernel.org>

On Thu, 2 Feb 2023 12:25:59 -0500, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Thu, Feb 02, 2023 at 07:00:49PM +0800, Xuan Zhuo wrote:
> > Since xsk's TX queue is consumed by TX NAPI, if sq is bound to xsk, then
> > we must stop tx napi from being disabled.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > ---
> >  drivers/net/virtio/main.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/virtio/main.c b/drivers/net/virtio/main.c
> > index ed79e750bc6c..232cf151abff 100644
> > --- a/drivers/net/virtio/main.c
> > +++ b/drivers/net/virtio/main.c
> > @@ -2728,8 +2728,15 @@ static int virtnet_set_coalesce(struct net_device *dev,
> >  		return ret;
> >
> >  	if (update_napi) {
> > -		for (i = 0; i < vi->max_queue_pairs; i++)
> > +		for (i = 0; i < vi->max_queue_pairs; i++) {
> > +			/* xsk xmit depend on the tx napi. So if xsk is active,
>
> depends.
>
> > +			 * prevent modifications to tx napi.
> > +			 */
> > +			if (rtnl_dereference(vi->sq[i].xsk.pool))
> > +				continue;
> > +
> >  			vi->sq[i].napi.weight = napi_weight;
>
> I don't get it.
> changing napi weight does not work then.
> why is this ok?

static void skb_xmit_done(struct virtqueue *vq)
{
	struct virtnet_info *vi = vq->vdev->priv;
	struct napi_struct *napi = &vi->sq[vq2txq(vq)].napi;

	/* Suppress further interrupts. */
	virtqueue_disable_cb(vq);

	if (napi->weight)
		virtqueue_napi_schedule(napi, vq);
	else
		/* We were probably waiting for more output buffers. */
		netif_wake_subqueue(vi->dev, vq2txq(vq));
}

If the weight is 0, tx napi will not be triggered again.

Thanks.

>
>
> > +		}
> >  	}
> >
> >  	return ret;
> > --
> > 2.32.0.3.g01195cf9f
>
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