Re:Re: [PATCH 1/2] virtio_net: disable cb when in busy-polling

["Lange Tang" <lange_tang@163.com>]

At 2026-04-03 10:57:58, "Jason Wang" <jasowang@redhat.com> wrote:
>On Tue, Mar 31, 2026 at 6:27 PM Longjun Tang <lange_tang@163.com> wrote:
>>
>> From: Longjun Tang <tanglongjun@kylinos.cn>
>>
>> virtnet_poll() is shared between NAPI softirq context and the busy-poll
>> path (sk_busy_loop).  In the busy-poll path the caller drives the loop
>> directly and does not rely on interrupts to schedule NAPI; keeping
>> callbacks enabled is wasteful and risks a flood of IRQ_NONE returns that
>> can trigger the kernel's spurious-IRQ detector.
>>
>> Signed-off-by: Longjun Tang <tanglongjun@kylinos.cn>
>> ---
>>  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 72d6a9c6a5a2..df365fba2c40 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -3078,6 +3078,9 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
>>         unsigned int xdp_xmit = 0;
>>         bool napi_complete;
>>
>> +       if (READ_ONCE(napi->state) & NAPIF_STATE_IN_BUSY_POLL)
>> +               virtqueue_disable_cb(rq->vq);
>> +
>
>I guess the root cause is virtuqeue_disable_cb() doesn't disable cb in
>this case?
>
>static void virtqueue_napi_schedule(struct napi_struct *napi,
>                                    struct virtqueue *vq)
>{
>        if (napi_schedule_prep(napi)) {
>virtqueue_disable_cb(vq);
>                __napi_schedule(napi);
>        }
>}
>
>If there's an interrupt in the middle of NAPIF_STATE_IN_BUSY_POLL,
>napi_schedule_prep() will return false, so we lose the chance to
>disable cb.
>

Sorry for the late reply!

Regarding the root cause you mentioned, the chance to disable cb is indeed lost 
when the following three conditions are met:
1. In NAPIF_STATE_IN_BUSY_POLL
2. Interrupt occurs
3. more_used returns true

However, in another scenario:
1. In NAPIF_STATE_IN_BUSY_POLL
2. Interrupt occurs
3. more_used returns false
In this second scenario, there is no chance to disable cb during the execution 
of the virtnet_poll until virtqueue_napi_complete.

>Thanks
>
>>         virtnet_poll_cleantx(rq, budget);
>>
>>         received = virtnet_receive(rq, budget, &xdp_xmit);
>> --
>> 2.43.0
>>