Re: [External] Re: [PATCH] virtio_ring: Suppress tx interrupt when napi_tx disable

Michael S. Tsirkin
2023-03-24T09:07:00Z

On Fri, Mar 24, 2023 at 03:37:04PM +0800, Jason Wang wrote:
> On Fri, Mar 24, 2023 at 3:00 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Fri, Mar 24, 2023 at 02:47:02PM +0800, Jason Wang wrote:
> > > On Fri, Mar 24, 2023 at 2:42 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > >
> > > > On Fri, Mar 24, 2023 at 02:32:40PM +0800, Jason Wang wrote:
> > > > > On Fri, Mar 24, 2023 at 1:59 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > > > >
> > > > > > On Fri, Mar 24, 2023 at 11:41:12AM +0800, Jason Wang wrote:
> > > > > > > On Thu, Mar 23, 2023 at 4:01 PM 黄杰 <huangjie.albert@bytedance.com> wrote:
> > > > > > > >
> > > > > > > > Jason Wang <jasowang@redhat.com> 于2023年3月22日周三 10:37写道：
> > > > > > > > >
> > > > > > > > > On Tue, Mar 21, 2023 at 5:00 PM Albert Huang
> > > > > > > > > <huangjie.albert@bytedance.com> wrote:
> > > > > > > > > >
> > > > > > > > > > From: "huangjie.albert" <huangjie.albert@bytedance.com>
> > > > > > > > > >
> > > > > > > > > > fix commit 8d622d21d248 ("virtio: fix up virtio_disable_cb")
> > > > > > > > > >
> > > > > > > > > > if we disable the napi_tx. when we triger a tx interrupt, the
> > > > > > > > >
> > > > > > > > > typo should be "trigger"
> > > > > > > > >
> > > > > > > >
> > > > > > > > OK, thanks for this. I will correct it in the next version
> > > > > > > >
> > > > > > > > > > vq->event_triggered will be set to true. It will no longer be
> > > > > > > > > > set to false. Unless we explicitly call virtqueue_enable_cb_delayed
> > > > > > > > > > or virtqueue_enable_cb_prepare
> > > > > > > > > >
> > > > > > > > > > if we disable the napi_tx, It will only be called when the tx ring
> > > > > > > > > > buffer is relatively small:
> > > > > > > > > > virtio_net->start_xmit:
> > > > > > > > > > if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
> > > > > > > > > > netif_stop_subqueue(dev, qnum);
> > > > > > > > > > if (!use_napi &&
> > > > > > > > > > unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
> > > > > > > > > > /* More just got used, free them then recheck. */
> > > > > > > > > > free_old_xmit_skbs(sq, false);
> > > > > > > > > > if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
> > > > > > > > > > netif_start_subqueue(dev, qnum);
> > > > > > > > > > virtqueue_disable_cb(sq->vq);
> > > > > > > > > > }
> > > > > > > > >
> > > > > > > > > The code example here is out of date, make sure your tree has this:
> > > > > > > >
> > > > > > > > also, I will correct it in the next version，this is from kernel 5.15.
> > > > > > > >
> > > > > > > > >
> > > > > > > > > commit d71ebe8114b4bf622804b810f5e274069060a174
> > > > > > > > > Author: Jason Wang <jasowang@redhat.com>
> > > > > > > > > Date: Tue Jan 17 11:47:07 2023 +0800
> > > > > > > > >
> > > > > > > > > virtio-net: correctly enable callback during start_xmit
> > > > > > > > >
> > > > > > > > > > }
> > > > > > > > > > }
> > > > > > > > > > Because event_triggered is true.Therefore, VRING_AVAIL_F_NO_INTERRUPT or
> > > > > > > > > > VRING_PACKED_EVENT_FLAG_DISABLE will not be set.So we update
> > > > > > > > > > vring_used_event(&vq->split.vring) or vq->packed.vring.driver->off_wrap
> > > > > > > > > > every time we call virtqueue_get_buf_ctx.This will bring more interruptions.
> > > > > > > > >
> > > > > > > > > Can you please post how to test with the performance numbers?
> > > > > > > > >
> > > > > > > >
> > > > > > > > iperf3 tcp stream:
> > > > > > > > vm1 -----------------> vm2
> > > > > > > > vm2 just receive tcp data stream from vm1, and send the ack to vm1,
> > > > > > > > there are so
> > > > > > > > many tx interruptions in vm2.
> > > > > > > >
> > > > > > > > but without event_triggered there are just a few tx interruptions.
> > > > > > > >
> > > > > > > > > >
> > > > > > > > > > if event_triggered is set to true, do not update vring_used_event(&vq->split.vring)
> > > > > > > > > > or vq->packed.vring.driver->off_wrap
> > > > > > > > > >
> > > > > > > > > > Signed-off-by: huangjie.albert <huangjie.albert@bytedance.com>
> > > > > > > > > > ---
> > > > > > > > > > drivers/virtio/virtio_ring.c | 6 ++++--
> > > > > > > > > > 1 file changed, 4 insertions(+), 2 deletions(-)
> > > > > > > > > >
> > > > > > > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > > > > > > > > index 307e139cb11d..f486cccadbeb 100644
> > > > > > > > > > --- a/drivers/virtio/virtio_ring.c
> > > > > > > > > > +++ b/drivers/virtio/virtio_ring.c
> > > > > > > > > > @@ -795,7 +795,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
> > > > > > > > > > /* If we expect an interrupt for the next entry, tell host
> > > > > > > > > > * by writing event index and flush out the write before
> > > > > > > > > > * the read in the next get_buf call. */
> > > > > > > > > > - if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT))
> > > > > > > > > > + if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)
> > > > > > > > > > + && (vq->event_triggered == false))
> > > > > > > > >
> > > > > > > > > I'm not sure this can work, when event_triggered is true it means
> > > > > > > > > we've got an interrupt, in this case if we want another interrupt for
> > > > > > > > > the next entry, we should update used_event otherwise we will lose
> > > > > > > > > that interrupt?
> > > > > > > > >
> > > > > > > > > Thanks
> > > > > > > >
> > > > > > > > Normally, if we receive an interrupt, we should disable the interrupt
> > > > > > > > in the interrupt callback handler.
> > > > > > >
> > > > > > > So the problem is:
> > > > > > >
> > > > > > > 1) event_triggered was set to true in vring_interrupt()
> > > > > > >
> > > > > > > 2) after this nothing will happen for virtqueue_disable_cb() so
> > > > > > > VRING_AVAIL_F_NO_INTERRUPT is not set in avail_flags_shadow
> > > > > > > 3) virtqueue_get_buf_ctx_split() will still think the cb is enabled
> > > > > > > then it tries to publish new event
> > > > > >
> > > > > > Oh. Good point! I think when I wrote up
> > > > > > 8d622d21d248 ("virtio: fix up virtio_disable_cb")
> > > > > > I missed this corner case.
> > > > > >
> > > > > >
> > > > > >
> > > > > > > This makes me think about whether or not we really need
> > > > > > > event_triggered. The assumption in the virtqueue_disable_cb() seems
> > > > > > > wrong:
> > > > > > >
> > > > > > > /* If device triggered an event already it won't trigger one again:
> > > > > > > * no need to disable.
> > > > > > > */
> > > > > > > if (vq->event_triggered)
> > > > > > > return;
> > > > > > >
> > > > > > > This is wrong if there's no event index support.
> > > > > >
> > > > > >
> > > > > > I don't get it. how does this get triggered?
> > > > > >
> > > > > > You are talking about device without event index?
> > > > > > Here's code from vring_interrupt():
> > > > > >
> > > > > > /* Just a hint for performance: so it's ok that this can be racy! */
> > > > > > if (vq->event)
> > > > > > vq->event_triggered = true;
> > > > >
> > > > > But we have the following in virtqueue_disable_cb():
> > > > >
> > > > > /* If device triggered an event already it won't trigger one again:
> > > > > * no need to disable.
> > > > > */
> > > > > if (vq->event_triggered)
> > > > > return;
> > > > >
> > > > > if (vq->packed_ring)
> > > > > virtqueue_disable_cb_packed(_vq);
> > > > > else
> > > > > virtqueue_disable_cb_split(_vq);
> > > > >
> > > > > This means, without an event index, we don't set avail flags. So the
> > > > > interrupt is not disabled actually in this case.
> > > > >
> > > > > Thanks
> > > >
> > > > Only if event_triggered is true, which without event index it never is.
> > >
> > > I'm not sure I will get here. I meant for example the commit
> > > suppresses the effort of skb_xmit_done():
> > >
> > > 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);
> > >
> > > The virtqueue_disable_cb() doesn't disable further interrupts when the
> > > event index is not there.
> > >
> > > Thanks
> >
> > Check what can set event_triggered, you will see.
>
> Set to truth by vring_interrupt()

vring_interrupt only sets it to true if vq->event is true

> Set to false by virtqueue_init(), virtqueue_enable_cb_prepare(),
> virtqueue_enable_cb_delayed()
>
> Assuming NAPI TX is enabled and the device doesn't support event index.
>
> 1) driver sends packets 1-10
> 2) the start_xmit() for the last packet will call
> virtqueue_enable_cb_delayed() which set event_triggered = false
> 3) 1st packet were sent, vring_interrupt set event_triggered = true
> 4) skb_xmit_done() won't disable virtqueue_disable_cb() in this case
> 5) so we will get the interrupts for 2nd to 10th packet
>
> Anything I missed here?

3 does not happen if event index is off.

> Note the comment said it's used for event index:
>
> /* Hint for event idx: already triggered no need to disable. */
> bool event_triggered;
>
> I guess what you meant is that if we don't publish a new event, we
> will get at most 1 interrupt for everything $queue_size used buffers.
> But this is not the case without event index. Btw, it may supress the
> effort of:
>
> vring_used_event(&vq->split.vring) = 0x0;
>
> Thanks

Because it's not necessary then.

> >
> >
> >
> > > >
> > > > > >
> > > > > >
> > > > > >
> > > > > >
> > > > > > > And the
> > > > > > > event_triggered is somehow duplicated with the
> > > > > > > VRING_AVAIL_F_NO_INTERRUPT in the case of event index. The correct fix
> > > > > > > might be:
> > > > > > >
> > > > > > > 1) remove event_triggered
> > > > > > > 2) set VRING_AVAIL_F_NO_INTERRUPT in avail_flags_shadow in
> > > > > > > vring_interrrupt if event index is supported
> > > > > > >
> > > > > > > ?
> > > > > > >
> > > > > > > Thanks
> > > > > >
> > > > > > I am not sure all this is right and I'd rather we focused
> > > > > > performance/correctness and cleanups separately.
> > > > > >
> > > > > >
> > > > > >
> > > > > >
> > > > > > >
> > > > > > > > But because of the introduction of event_triggered, here,
> > > > > > > > virtqueue_get_buf_ctx_split cannot be recognized
> > > > > > > > that the interrupt has been turned off.
> > > > > > > >
> > > > > > > > if we want another interrupt for the next entry, We should probably
> > > > > > > > call virtqueue_enable_cb？
> > > > > > > >
> > > > > > > > Thanks
> > > > > > > >
> > > > > > > > >
> > > > > > > > > > virtio_store_mb(vq->weak_barriers,
> > > > > > > > > > &vring_used_event(&vq->split.vring),
> > > > > > > > > > cpu_to_virtio16(_vq->vdev, vq->last_used_idx));
> > > > > > > > > > @@ -1529,7 +1530,8 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> > > > > > > > > > * by writing event index and flush out the write before
> > > > > > > > > > * the read in the next get_buf call.
> > > > > > > > > > */
> > > > > > > > > > - if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC)
> > > > > > > > > > + if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC
> > > > > > > > > > + && (vq->event_triggered == false))
> > > > > > > > > > virtio_store_mb(vq->weak_barriers,
> > > > > > > > > > &vq->packed.vring.driver->off_wrap,
> > > > > > > > > > cpu_to_le16(vq->last_used_idx));
> > > > > > > > > > --
> > > > > > > > > > 2.31.1
> > > > > > > > > >
> > > > > > > > >
> > > > > > > >
> > > > > >
> > > >
> >