On Tue, 16 Jun 2026 14:07:34 +0800 (CST), Lange Tang <[email protected]> wrote:
> At 2026-06-16 11:27:12, "Xuan Zhuo" <[email protected]> wrote:
> >On Tue, 16 Jun 2026 11:00:29 +0800 (CST), Lange Tang <[email protected]>
> >wrote:
> >> At 2026-06-15 18:01:40, "Xuan Zhuo" <[email protected]> wrote:
> >> >On Mon, 15 Jun 2026 17:45:50 +0800, Longjun Tang <[email protected]>
> >> >wrote:
> >> >> From: Longjun Tang <[email protected]>
> >> >>
> >> >> 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 <[email protected]>
> >> >> ---
> >> >> 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.
>
> >
> >
> >> >
> >> >
> >> >> }
> >> >> }
> >> >>
> >> >> --
> >> >> 2.25.1
> >> >>
> >>
>
