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

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Date:   Fri, 3 Feb 2023 04:29:41 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Menglong Dong <imagedong@...cent.com>,
        Kuniyuki Iwashima <kuniyu@...zon.com>,
        Petr Machata <petrm@...dia.com>,
        virtualization@...ts.linux-foundation.org, bpf@...r.kernel.org
Subject: Re: [PATCH 24/33] virtio_net: xsk: stop disable tx napi

On Fri, Feb 03, 2023 at 04:49:16PM +0800, Xuan Zhuo wrote:
> On Fri, 3 Feb 2023 03:33:41 -0500, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> > On Fri, Feb 03, 2023 at 11:24:42AM +0800, Xuan Zhuo wrote:
> > > On Thu, 2 Feb 2023 12:25:59 -0500, "Michael S. Tsirkin" <mst@...hat.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@...ux.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.
> >
> > This needs more thought then. First ignoring what user is requesting is
> > not nice.
> 
> Maybe we should return an error.

maybe


> >Second what if napi is first disabled and then xsk enabled?
> 
> 
> static int virtnet_xsk_pool_enable(struct net_device *dev,
> 				   struct xsk_buff_pool *pool,
> 				   u16 qid)
> {
> 	struct virtnet_info *vi = netdev_priv(dev);
> 	struct receive_queue *rq;
> 	struct send_queue *sq;
> 	int err;
> 
> 	if (qid >= vi->curr_queue_pairs)
> 		return -EINVAL;
> 
> 	sq = &vi->sq[qid];
> 	rq = &vi->rq[qid];
> 
> 	/* xsk zerocopy depend on the tx napi.
> 	 *
> 	 * All xsk packets are actually consumed and sent out from the xsk tx
> 	 * queue under the tx napi mechanism.
> 	 */
> ->	if (!sq->napi.weight)
> 		return -EPERM;
> 
> Thanks.
> 
> 
> >
> >
> > > >
> > > >
> > > > > +		}
> > > > >  	}
> > > > >
> > > > >  	return ret;
> > > > > --
> > > > > 2.32.0.3.g01195cf9f
> > > >
> >