On Tue, Apr 13, 2021 at 10:27:16AM -0400, Willem de Bruijn wrote: > On Tue, Apr 13, 2021 at 1:06 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > > > On Mon, Apr 24, 2017 at 01:49:30PM -0400, Willem de Bruijn wrote: > > > From: Willem de Bruijn <willemb@xxxxxxxxxx> > > > > > > Tx napi mode increases the rate of transmit interrupts. Suppress some > > > by masking interrupts while more packets are expected. The interrupts > > > will be reenabled before the last packet is sent. > > > > > > This optimization reduces the througput drop with tx napi for > > > unidirectional flows such as UDP_STREAM that do not benefit from > > > cleaning tx completions in the the receive napi handler. > > > > > > Signed-off-by: Willem de Bruijn <willemb@xxxxxxxxxx> > > > --- > > > 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 9dd978f34c1f..003143835766 100644 > > > --- a/drivers/net/virtio_net.c > > > +++ b/drivers/net/virtio_net.c > > > @@ -1200,6 +1200,9 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev) > > > /* Free up any pending old buffers before queueing new ones. */ > > > free_old_xmit_skbs(sq); > > > > > > + if (use_napi && kick) > > > + virtqueue_enable_cb_delayed(sq->vq); > > > + > > > /* timestamp packet in software */ > > > skb_tx_timestamp(skb); > > > > > > I have been poking at this code today and I noticed that is > > actually does enable cb where the commit log says masking interrupts. > > I think the reason is that with even index previously disable cb > > actually did nothing while virtqueue_enable_cb_delayed pushed > > the event index out some more. > > And this likely explains why it does not work well for packed, > > where virtqueue_enable_cb_delayed is same as virtqueue_enable_cb. > > > > Right? Or did I miss something? > > This was definitely based on the split queue with event index handling. > > When you say does not work well for packed, you mean that with packed > mode we see the consequences of the race condition when accessing vq > without holding __netif_tx_lock, in a way that I did not notice with > split queue with event index, right? I mean curretly packed does not seem to show same performance gains as a micro-benchmark. Could be due to enabling interrupts more aggressively there. > Thanks for looking into this and proposing fixes for this issue and the > known other spurious tx interrupt issue. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
