Re: [PATCH net 2/2] vhost-net: correctly flush batched packet before enabling notification

Jon Kohler
2025-09-12T15:33:45Z

> On Sep 12, 2025, at 11:30 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
>
> !-------------------------------------------------------------------|
> CAUTION: External Email
>
> |-------------------------------------------------------------------!
>
> On Fri, Sep 12, 2025 at 03:24:42PM +0000, Jon Kohler wrote:
>>
>>
>>> On Sep 12, 2025, at 4:50 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
>>>
>>> !-------------------------------------------------------------------|
>>> CAUTION: External Email
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>>>
>>> On Fri, Sep 12, 2025 at 04:26:58PM +0800, Jason Wang wrote:
>>>> Commit 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after
>>>> sendmsg") tries to defer the notification enabling by moving the logic
>>>> out of the loop after the vhost_tx_batch() when nothing new is
>>>> spotted. This will bring side effects as the new logic would be reused
>>>> for several other error conditions.
>>>>
>>>> One example is the IOTLB: when there's an IOTLB miss, get_tx_bufs()
>>>> might return -EAGAIN and exit the loop and see there's still available
>>>> buffers, so it will queue the tx work again until userspace feed the
>>>> IOTLB entry correctly. This will slowdown the tx processing and may
>>>> trigger the TX watchdog in the guest.
>>>
>>> It's not that it might.
>>> Pls clarify that it *has been reported* to do exactly that,
>>> and add a link to the report.
>>>
>>>
>>>> Fixing this by stick the notificaiton enabling logic inside the loop
>>>> when nothing new is spotted and flush the batched before.
>>>>
>>>> Reported-by: Jon Kohler <jon@nutanix.com>
>>>> Cc: stable@vger.kernel.org
>>>> Fixes: 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after sendmsg")
>>>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>>>
>>> So this is mostly a revert, but with
>>> vhost_tx_batch(net, nvq, sock, &msg);
>>> added in to avoid regressing performance.
>>>
>>> If you do not want to structure it like this (revert+optimization),
>>> then pls make that clear in the message.
>>>
>>>
>>>> ---
>>>> drivers/vhost/net.c | 33 +++++++++++++--------------------
>>>> 1 file changed, 13 insertions(+), 20 deletions(-)
>>>>
>>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>>>> index 16e39f3ab956..3611b7537932 100644
>>>> --- a/drivers/vhost/net.c
>>>> +++ b/drivers/vhost/net.c
>>>> @@ -765,11 +765,11 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
>>>> int err;
>>>> int sent_pkts = 0;
>>>> bool sock_can_batch = (sock->sk->sk_sndbuf == INT_MAX);
>>>> - bool busyloop_intr;
>>>> bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
>>>>
>>>> do {
>>>> - busyloop_intr = false;
>>>> + bool busyloop_intr = false;
>>>> +
>>>> if (nvq->done_idx == VHOST_NET_BATCH)
>>>> vhost_tx_batch(net, nvq, sock, &msg);
>>>>
>>>> @@ -780,10 +780,18 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
>>>> break;
>>>> /* Nothing new? Wait for eventfd to tell us they refilled. */
>>>> if (head == vq->num) {
>>>> - /* Kicks are disabled at this point, break loop and
>>>> - * process any remaining batched packets. Queue will
>>>> - * be re-enabled afterwards.
>>>> + /* Flush batched packets before enabling
>>>> + * virqtueue notification to reduce
>>>> + * unnecssary virtqueue kicks.
>>>
>>> typos: virtqueue, unnecessary
>>>
>>>> */
>>>> + vhost_tx_batch(net, nvq, sock, &msg);
>>>> + if (unlikely(busyloop_intr)) {
>>>> + vhost_poll_queue(&vq->poll);
>>>> + } else if (unlikely(vhost_enable_notify(&net->dev,
>>>> + vq))) {
>>>> + vhost_disable_notify(&net->dev, vq);
>>>> + continue;
>>>> + }
>>>> break;
>>>> }
>>
>> See my comment below, but how about something like this?
>> if (head == vq->num) {
>> /* Flush batched packets before enabling
>> * virtqueue notification to reduce
>> * unnecessary virtqueue kicks.
>> */
>> vhost_tx_batch(net, nvq, sock, &msg);
>> if (unlikely(busyloop_intr))
>> /* If interrupted while doing busy polling,
>> * requeue the handler to be fair handle_rx
>> * as well as other tasks waiting on cpu.
>> */
>> vhost_poll_queue(&vq->poll);
>> else
>> /* All of our work has been completed;
>> * however, before leaving the TX handler,
>> * do one last check for work, and requeue
>> * handler if necessary. If there is no work,
>> * queue will be reenabled.
>> */
>> vhost_net_busy_poll_try_queue(net, vq);
>
>
> I mean it's functionally equivalent, but vhost_net_busy_poll_try_queue
> checks the avail ring again and we just checked it.
> Why is this a good idea?
> This happens on good path so I dislike unnecessary work like this.

For the sake of discussion, let’s say vhost_tx_batch and the
sendmsg within took 1 full second to complete. A lot could potentially
happen in that amount of time. So sure, control path wise it looks like
we just checked it, but time wise, that could have been ages ago.

>
>
>> break;
>> }
>>
>>
>>>>
>>>> @@ -839,22 +847,7 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
>>>> ++nvq->done_idx;
>>>> } while (likely(!vhost_exceeds_weight(vq, ++sent_pkts, total_len)));
>>>>
>>>> - /* Kicks are still disabled, dispatch any remaining batched msgs. */
>>>> vhost_tx_batch(net, nvq, sock, &msg);
>>>> -
>>>> - if (unlikely(busyloop_intr))
>>>> - /* If interrupted while doing busy polling, requeue the
>>>> - * handler to be fair handle_rx as well as other tasks
>>>> - * waiting on cpu.
>>>> - */
>>>> - vhost_poll_queue(&vq->poll);
>>>> - else
>>>> - /* All of our work has been completed; however, before
>>>> - * leaving the TX handler, do one last check for work,
>>>> - * and requeue handler if necessary. If there is no work,
>>>> - * queue will be reenabled.
>>>> - */
>>>> - vhost_net_busy_poll_try_queue(net, vq);
>>
>> Note: the use of vhost_net_busy_poll_try_queue was intentional in my
>> patch as it was checking to see both conditionals.
>>
>> Can we simply hoist my logic up instead?
>>
>>>> }
>>>>
>>>> static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
>>>> --
>>>> 2.34.1
>>>
>>
>> Tested-by: Jon Kohler <jon@nutanix.com <mailto:jon@nutanix.com>>
>>
>> Tried this out on a 6.16 host / guest that locked up with iotlb miss loop,
>> applied this patch and all was well.
>