Re:Re:Re:Re: [PATCH] virtio_net: disable cb when napi_schedule_prep fails during busy-poll

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

At 2026-06-16 14:49:25, "Xuan Zhuo" <xuanzhuo@linux.alibaba.com> wrote:
>On Tue, 16 Jun 2026 14:07:34 +0800 (CST), Lange Tang <lange_tang@163.com> wrote:
>> At 2026-06-16 11:27:12, "Xuan Zhuo" <xuanzhuo@linux.alibaba.com> wrote:
>> >On Tue, 16 Jun 2026 11:00:29 +0800 (CST), Lange Tang <lange_tang@163.com> wrote:
>> >> At 2026-06-15 18:01:40, "Xuan Zhuo" <xuanzhuo@linux.alibaba.com> wrote:
>> >> >On Mon, 15 Jun 2026 17:45:50 +0800, Longjun Tang <lange_tang@163.com> wrote:
>> >> >> From: Longjun Tang <tanglongjun@kylinos.cn>
>> >> >>
>> >> >> When busy-poll is active, napi_schedule_prep() returns false in
>> >> >> skb_recv_done(), so virtqueue_disable_cb() is skipped. The device
>> >> >> may keep firing irqs until the next poll round reaches
>> >> >> virtqueue_napi_complete(). If cb is enabled under busy-poll case,
>> >> >> it will lead to a large number of spurious interrupts. Explicitly
>> >> >> disable callbacks in this case to prevent spurious interrupts.
>> >> >>
>> >> >> Signed-off-by: Longjun Tang <tanglongjun@kylinos.cn>
>> >> >> ---
>> >> >>  drivers/net/virtio_net.c | 2 ++
>> >> >>  1 file changed, 2 insertions(+)
>> >> >>
>> >> >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> >> >> index f4adcfee7a80..6d675fddc59b 100644
>> >> >> --- a/drivers/net/virtio_net.c
>> >> >> +++ b/drivers/net/virtio_net.c
>> >> >> @@ -728,6 +728,8 @@ static void virtqueue_napi_schedule(struct napi_struct *napi,
>> >> >>  	if (napi_schedule_prep(napi)) {
>> >> >>  		virtqueue_disable_cb(vq);
>> >> >>  		__napi_schedule(napi);
>> >> >> +	} else if (test_bit(NAPI_STATE_IN_BUSY_POLL, &napi->state)) {
>> >> >> +		virtqueue_disable_cb(vq);
>> >> >
>> >> >I see, but we should avoid checking NAPI_STATE_IN_BUSY_POLL directly in the
>> >> >drivers. The NIC driver should remain agnostic to busy polling. I think we need
>> >> >a better way, maybe we should rewrite virtqueue_napi_schedule instead.
>> >>
>> >> How about rewrite it like this?
>> >> static void virtqueue_napi_schedule(struct napi_struct *napi,
>> >>                                     struct virtqueue *vq)
>> >> {
>> >>         virtqueue_disable_cb(vq);
>> >>         if (napi_schedule_prep(napi))
>> >>                 __napi_schedule(napi);
>> >> }
>> >> Any comments are welcome.
>> >
>> >
>> >Another CPU could be running NAPI and has just enabled the callbacks (cb).
>> >Meanwhile, this side unconditionally disables the cb. Since NAPI on the other
>> >CPU hasn't exited yet, the subsequent prep on this side fails, leaving no one to
>> >re-enable the cb.
>> >
>> >Thanks.
>>
>> Regarding the case you described, when NAPI on another CPU exits, the virtqueue_napi_complete func
>> will be executed to re-enable cb.  and if there is still unconsumed data in the virtqueue, virtqueue_napi_schedule
>> will be called again to schedule NAPI.
>>
>> In summary, I think that the disable_cb and __napi_schedule within the virtqueue_napi_schedule func do not need to be bound together.
>>
>> Any comments are welcome. Thinks.
>
>
><Your code>
>static void virtqueue_napi_schedule(struct napi_struct *napi,
>                                    struct virtqueue *vq)
>{
>
>							       |static bool virtqueue_napi_complete(struct napi_struct *napi,
>							       |				    struct virtqueue *vq, int processed)
>							       |{
>							       |	int opaque;
>							       |
>							       |	opaque = virtqueue_enable_cb_prepare(vq);
>                                                               |
>        virtqueue_disable_cb(vq);                              |
>        if (napi_schedule_prep(napi))                          |
>                __napi_schedule(napi);                         |
>							       |	if (napi_complete_done(napi, processed)) {
>							       |		if (unlikely(virtqueue_poll(vq, opaque)))
>							       |			virtqueue_napi_schedule(napi, vq);
>							       |		else
>							       |			 return true; // return directly
>							       |	} else {
>							       |		virtqueue_disable_cb(vq);
>							       |	}
>							       |
>							       |	return false;
>							       |}
>}
>
>1. new packets (notified by irq) are consumed by napi before virtqueue_napi_complete
>2. poll is not called by irq, maybe xsk wake up. So irq is not disabled.
>
>
>Thanks.

Based on your code analysis above, I got .  thanks.
disable_cb and __napi_schedule in the virtqueue_napi_schedule func indeed cannot be separated.

Regarding the issue of not being able to disable cb in a busy-poll context, do you have any suggestions?

Any comments are welcome. Thanks.
>
>
>>
>> >
>> >
>> >> >
>> >> >
>> >> >>  	}
>> >> >>  }
>> >> >>
>> >> >> --
>> >> >> 2.25.1
>> >> >>
>> >>
>>