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Date: Thu, 15 Dec 2022 04:02:49 -0500 From: "Michael S. Tsirkin" <mst@...hat.com> To: Jason Wang <jasowang@...hat.com> Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, xuanzhuo@...ux.alibaba.com Subject: Re: [PATCH net V2] virtio-net: correctly enable callback during start_xmit On Thu, Dec 15, 2022 at 11:27:19AM +0800, Jason Wang wrote: > Commit a7766ef18b33("virtio_net: disable cb aggressively") enables > virtqueue callback via the following statement: > > do { > ...... > } while (use_napi && kick && > unlikely(!virtqueue_enable_cb_delayed(sq->vq))); > > When NAPI is used and kick is false, the callback won't be enabled > here. And when the virtqueue is about to be full, the tx will be > disabled, but we still don't enable tx interrupt which will cause a TX > hang. This could be observed when using pktgen with burst enabled. > > Fixing this by trying to enable tx interrupt after we disable TX when > we're not using napi or kick is false. > > Fixes: a7766ef18b33 ("virtio_net: disable cb aggressively") > Signed-off-by: Jason Wang <jasowang@...hat.com> > --- > The patch is needed for -stable. > Changes since V1: > - enable tx interrupt after we disable tx > --- > drivers/net/virtio_net.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c > index 86e52454b5b5..dcf3a536d78a 100644 > --- a/drivers/net/virtio_net.c > +++ b/drivers/net/virtio_net.c > @@ -1873,7 +1873,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev) > */ > if (sq->vq->num_free < 2+MAX_SKB_FRAGS) { > netif_stop_subqueue(dev, qnum); > - if (!use_napi && > + if ((!use_napi || !kick) && > unlikely(!virtqueue_enable_cb_delayed(sq->vq))) { > /* More just got used, free them then recheck. */ > free_old_xmit_skbs(sq, false); This will work but the following lines are: if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) { netif_start_subqueue(dev, qnum); virtqueue_disable_cb(sq->vq); } and I thought we are supposed to keep callbacks enabled with napi? One of the ideas of napi is to free on napi callback, not here immediately. I think it is easier to just do a separate branch here. Along the lines of: if (use_napi) { if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) virtqueue_napi_schedule(napi, vq); } else { ... old code ... } also reduces chances of regressions on !napi (which is not well tested) and keeps callbacks off while we free skbs. No? > -- > 2.25.1
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