[PATCH v2] virtio_net: disable cb when NAPI is busy-polled

[PATCH v2] virtio_net: disable cb when NAPI is busy-polled

[Longjun Tang]

From: Longjun Tang <tanglongjun@kylinos.cn>

When busy-poll is active, napi_schedule_prep() returns false in
virtqueue_napi_schedule(), so virtqueue_disable_cb() is skipped.
The device may keep firing irqs until reaches virtqueue_napi_complete().
Under load (received == budget), it will lead to a large number
of spurious interrupts.

Fix it by disabling the callback at the virtnet_poll() entry. This keeps
the callback off while we poll and re-enable by virtqueue_napi_complete()
when going idle.

Signed-off-by: Longjun Tang <tanglongjun@kylinos.cn>

---
V1 -> V2: Remain agnostic to busy polling
---
 drivers/net/virtio_net.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index f4adcfee7a80..0a11f2b32500 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -3008,6 +3008,11 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
 	unsigned int xdp_xmit = 0;
 	bool napi_complete;
 
+	/* Keep callbacks suppressed for the duration of this poll,
+	 * busy-poll need.
+	 */
+	virtqueue_disable_cb(rq->vq);
+
 	virtnet_poll_cleantx(rq, budget);
 
 	received = virtnet_receive(rq, budget, &xdp_xmit);
-- 
2.43.0

Re: [PATCH v2] virtio_net: disable cb when NAPI is busy-polled

[Eric Dumazet]

On Tue, Jun 23, 2026 at 2:19 AM Longjun Tang <lange_tang@163.com> wrote:
>
> From: Longjun Tang <tanglongjun@kylinos.cn>
>
> When busy-poll is active, napi_schedule_prep() returns false in
> virtqueue_napi_schedule(), so virtqueue_disable_cb() is skipped.
> The device may keep firing irqs until reaches virtqueue_napi_complete().
> Under load (received == budget), it will lead to a large number
> of spurious interrupts.
>
> Fix it by disabling the callback at the virtnet_poll() entry. This keeps
> the callback off while we poll and re-enable by virtqueue_napi_complete()
> when going idle.
>
> Signed-off-by: Longjun Tang <tanglongjun@kylinos.cn>
>

I added netdev@ to get more attention from networking napi polling experts,

Please add a Fixes: tag as this will ease code review.

My rough guess is:

Fixes: ceef438d613f ("virtio_net: remove custom busy_poll")

Thanks.

> ---
> V1 -> V2: Remain agnostic to busy polling
> ---
>  drivers/net/virtio_net.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index f4adcfee7a80..0a11f2b32500 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3008,6 +3008,11 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
>         unsigned int xdp_xmit = 0;
>         bool napi_complete;
>
> +       /* Keep callbacks suppressed for the duration of this poll,
> +        * busy-poll need.
> +        */
> +       virtqueue_disable_cb(rq->vq);
> +
>         virtnet_poll_cleantx(rq, budget);
>
>         received = virtnet_receive(rq, budget, &xdp_xmit);
> --
> 2.43.0
>

Re: [PATCH v2] virtio_net: disable cb when NAPI is busy-polled

[Michael S. Tsirkin]

On Tue, Jun 23, 2026 at 02:55:30AM -0700, Eric Dumazet wrote:
> On Tue, Jun 23, 2026 at 2:19 AM Longjun Tang <lange_tang@163.com> wrote:
> >
> > From: Longjun Tang <tanglongjun@kylinos.cn>
> >
> > When busy-poll is active, napi_schedule_prep() returns false in
> > virtqueue_napi_schedule(), so virtqueue_disable_cb() is skipped.
> > The device may keep firing irqs until reaches virtqueue_napi_complete().
> > Under load (received == budget), it will lead to a large number
> > of spurious interrupts.
> >
> > Fix it by disabling the callback at the virtnet_poll() entry. This keeps
> > the callback off while we poll and re-enable by virtqueue_napi_complete()
> > when going idle.
> >
> > Signed-off-by: Longjun Tang <tanglongjun@kylinos.cn>
> >
> 
> I added netdev@ to get more attention from networking napi polling experts,
> 
> Please add a Fixes: tag as this will ease code review.
> 
> My rough guess is:
> 
> Fixes: ceef438d613f ("virtio_net: remove custom busy_poll")
> 
> Thanks.

Exactly. The old custom virtnet_busy_poll did napi_schedule_prep + virtqueue_disable_cb itself.

I'd even say CC stable interrupt storms are devastating to performance.


> > ---
> > V1 -> V2: Remain agnostic to busy polling
> > ---
> >  drivers/net/virtio_net.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index f4adcfee7a80..0a11f2b32500 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -3008,6 +3008,11 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
> >         unsigned int xdp_xmit = 0;
> >         bool napi_complete;
> >
> > +       /* Keep callbacks suppressed for the duration of this poll,
> > +        * busy-poll need.
> > +        */
> > +       virtqueue_disable_cb(rq->vq);
> > +
> >         virtnet_poll_cleantx(rq, budget);
> >
> >         received = virtnet_receive(rq, budget, &xdp_xmit);
> > --
> > 2.43.0
> >

Re: [PATCH v2] virtio_net: disable cb when NAPI is busy-polled

[Michael S. Tsirkin]

On Tue, Jun 23, 2026 at 05:19:01PM +0800, Longjun Tang wrote:
> From: Longjun Tang <tanglongjun@kylinos.cn>
> 
> When busy-poll is active, napi_schedule_prep() returns false in
> virtqueue_napi_schedule(), so virtqueue_disable_cb() is skipped.
> The device may keep firing irqs until reaches virtqueue_napi_complete().
> Under load (received == budget), it will lead to a large number
> of spurious interrupts.
> 
> Fix it by disabling the callback at the virtnet_poll() entry. This keeps
> the callback off while we poll and re-enable by virtqueue_napi_complete()
> when going idle.
> 
> Signed-off-by: Longjun Tang <tanglongjun@kylinos.cn>


Patch is correct


Acked-by: Michael S. Tsirkin <mst@redhat.com>

Ideally, I like to see performance numbers in such cases, to be included with commit log.

> ---
> V1 -> V2: Remain agnostic to busy polling
> ---
>  drivers/net/virtio_net.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index f4adcfee7a80..0a11f2b32500 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3008,6 +3008,11 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
>  	unsigned int xdp_xmit = 0;
>  	bool napi_complete;
>  
> +	/* Keep callbacks suppressed for the duration of this poll,
> +	 * busy-poll need.
> +	 */
> +	virtqueue_disable_cb(rq->vq);
> +
>  	virtnet_poll_cleantx(rq, budget);
>  
>  	received = virtnet_receive(rq, budget, &xdp_xmit);
> -- 
> 2.43.0