Re: [PATCH net-next v2] vhost/net: Defer TX queue re-enable until after sendmsg

Jon Kohler
2025-04-26T19:21:53Z

> On Apr 26, 2025, at 3:06 PM, Jon Kohler <jon@nutanix.com> wrote:
>
>
>
>> On Apr 24, 2025, at 8:11 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
>>
>> !-------------------------------------------------------------------|
>> CAUTION: External Email
>>
>> |-------------------------------------------------------------------!
>>
>> On Thu, Apr 24, 2025 at 01:48:53PM +0200, Paolo Abeni wrote:
>>> On 4/20/25 3:05 AM, Jon Kohler wrote:
>>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>>>> index b9b9e9d40951..9b04025eea66 100644
>>>> --- a/drivers/vhost/net.c
>>>> +++ b/drivers/vhost/net.c
>>>> @@ -769,13 +769,17 @@ 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) {
>>>> + /* If interrupted while doing busy polling, requeue
>>>> + * the handler to be fair handle_rx as well as other
>>>> + * tasks waiting on cpu
>>>> + */
>>>> 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;
>>>> }
>>>> + /* Kicks are disabled at this point, break loop and
>>>> + * process any remaining batched packets. Queue will
>>>> + * be re-enabled afterwards.
>>>> + */
>>>> break;
>>>> }
>>>
>>> It's not clear to me why the zerocopy path does not need a similar change.
>>
>> It can have one, it's just that Jon has a separate patch to drop
>> it completely. A commit log comment mentioning this would be a good
>> idea, yes.
>
> Yea, the utility of the ZC side is a head scratcher for me, I can’t get it to work
> well to save my life. I’ve got a separate thread I need to respond to Eugenio
> on, will try to circle back on that next week.
>
> The reason this one works so well is that the last batch in the copy path can
> take a non-trivial amount of time, so it opens up the guest to a real saw tooth
> pattern. Getting rid of that, and all that comes with it (exits, stalls, etc), just
> pays off.
>
>>
>>>> @@ -825,7 +829,14 @@ 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);
>>>> +
>>>> + /* 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);
>>>
>>> This will call vhost_poll_queue() regardless of the 'busyloop_intr' flag
>>> value, while AFAICS prior to this patch vhost_poll_queue() is only
>>> performed with busyloop_intr == true. Why don't we need to take care of
>>> such flag here?
>>
>> Hmm I agree this is worth trying, a free if possibly small performance
>> gain, why not. Jon want to try?
>
> I mentioned in the commit msg that the reason we’re doing this is to be
> fair to handle_rx. If my read of vhost_net_busy_poll_try_queue is correct,
> we would only call vhost_poll_queue iff:
> 1. The TX ring is not empty, in which case we want to run handle_tx again
> 2. When we go to reenable kicks, it returns non-zero, which means we
> should run handle_tx again anyhow
>
> In the ring is truly empty, and we can re-enable kicks with no drama, we
> would not run vhost_poll_queue.
>
> That said, I think what you’re saying here is, we should check the busy
> flag and *not* try vhost_net_busy_poll_try_queue, right? If so, great, I did
> that in an internal version of this patch; however, it adds another conditional
> which for the vast majority of users is not going to add any value (I think)
>
> Happy to dig deeper, either on this change series, or a follow up?

Sorry, I do not know why this email sent itself again when I opened my
laptop, my mistake somehow.

>
>>
>>
>>> @Michael: I assume you prefer that this patch will go through the
>>> net-next tree, right?
>>>
>>> Thanks,
>>>
>>> Paolo
>>
>> I don't mind and this seems to be what Jon wants.
>> I could queue it too, but extra review it gets in the net tree is good.
>
> My apologies, I thought all non-bug fixes had to go thru net-next,
> which is why I sent the v2 to net-next; however if you want to queue
> right away, I’m good with either. Its a fairly well contained patch with
> a huge upside :)
>
>>
>> --
>> MST
>>
>