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

Jon Kohler
2025-09-12T15:40:56Z

> On Sep 12, 2025, at 11:38 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
>
> !-------------------------------------------------------------------|
> CAUTION: External Email
>
> |-------------------------------------------------------------------!
>
> On Fri, Sep 12, 2025 at 03:33:32PM +0000, Jon Kohler wrote:
>>
>>
>>> 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
>>>>>
>>>>> |-------------------------------------------------------------------!
>>>>>
>>>>> 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.
>
>
> Oh I forgot we had the tx batch in there.
> OK then, I don't have a problem with this.
>
>
> However, what I like about Jason's patch is that
> it is actually simply revert of your patch +
> a single call to
> vhost_tx_batch(net, nvq, sock, &msg);
>
> So it is a more obviosly correct approach.
>
>
> I'll be fine with doing what you propose on top,
> with testing that they are benefitial for performance.

Ok fair enough, agreed, let’s fix the bug business first,
then reoptimize on top.

>
>
>
>
>
>
>>>
>>>
>>>> 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.
>>>
>>
>