Re: [PATCH v4] virtio_ring: interrupt disable flag updated to vq even with event_triggered is set

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 29, 2023 at 07:45:01PM +0800, Xuan Zhuo wrote:
> On Wed, 29 Mar 2023 18:23:00 +0800, Albert Huang <huangjie.albert@bytedance.com> wrote:
> > From: "huangjie.albert" <huangjie.albert@bytedance.com>
> >
> > in virtio_net, if we disable the napi_tx, when we triger a tx interrupt,
> > the 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.
> >
> > 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 bring more interruptions.
> >
> > To summarize:
> > 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
> >
> > To fix:
> > update VRING_AVAIL_F_NO_INTERRUPT or VRING_PACKED_EVENT_FLAG_DISABLE to vq
> > when we call virtqueue_disable_cb even the event_triggered is set to true.
> >
> > Tested with iperf:
> > iperf3 tcp stream:
> > vm1 -----------------> vm2
> > vm2 just receives tcp data stream from vm1, and sends the ack to vm1,
> > there are many tx interrupts in vm2.
> > but without event_triggered there are just a few tx interrupts.
> >
> > v2->v3:
> > -update the interrupt disable flag even with the event_triggered is set,
> > -instead of checking whether event_triggered is set in
> > -virtqueue_get_buf_ctx_{packed/split}, will cause the drivers  which have
> > -not called virtqueue_{enable/disable}_cb to miss notifications.
> >
> > v3->v4:
> > -remove change for
> > -"if (vq->packed.event_flags_shadow != VRING_PACKED_EVENT_FLAG_DISABLE)"
> > -in virtqueue_disable_cb_packed
> >
> > Fixes: 8d622d21d248 ("virtio: fix up virtio_disable_cb")
> 
> Hi Michael,
> 
> I want to know what the purpose of this patch("virtio: fix up virtio_disable_cb")
> is. I re-review this patch. I didn't understand what the purpose of this
> patches. Does it reduce one write to vring_used_event(&vq->split.vring) ?
> 
> Thanks.

Are you asking why I applied 8d622d21d248 ("virtio: fix up
virtio_disable_cb")?

It was a prerequisite to fixing interrupt storms we saw in the field previously.

See Message-ID: <20210413011508-mutt-send-email-mst@kernel.org>
for the bug report and Message-ID: <20210413054733.36363-1-mst@redhat.com>
for the original patchset fixing it.


> 
> > Signed-off-by: huangjie.albert <huangjie.albert@bytedance.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > ---
> >  drivers/virtio/virtio_ring.c | 22 ++++++++++++++++------
> >  1 file changed, 16 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index cbeeea1b0439..ec7ab8e04846 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -931,6 +931,14 @@ static void virtqueue_disable_cb_split(struct virtqueue *_vq)
> >
> >  	if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
> >  		vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> > +
> > +		/*
> > +		 * If device triggered an event already it won't trigger one again:
> > +		 * no need to disable.
> > +		 */
> > +		if (vq->event_triggered)
> > +			return;
> > +
> >  		if (vq->event)
> >  			/* TODO: this is a hack. Figure out a cleaner value to write. */
> >  			vring_used_event(&vq->split.vring) = 0x0;
> > @@ -1761,6 +1769,14 @@ static void virtqueue_disable_cb_packed(struct virtqueue *_vq)
> >
> >  	if (vq->packed.event_flags_shadow != VRING_PACKED_EVENT_FLAG_DISABLE) {
> >  		vq->packed.event_flags_shadow = VRING_PACKED_EVENT_FLAG_DISABLE;
> > +
> > +		/*
> > +		 * If device triggered an event already it won't trigger one again:
> > +		 * no need to disable.
> > +		 */
> > +		if (vq->event_triggered)
> > +			return;
> > +
> >  		vq->packed.vring.driver->flags =
> >  			cpu_to_le16(vq->packed.event_flags_shadow);
> >  	}
> > @@ -2462,12 +2478,6 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
> >  {
> >  	struct vring_virtqueue *vq = to_vvq(_vq);
> >
> > -	/* 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
> > --
> > 2.20.1
> >

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