Re: RE: RE: [PATCH v3] drivers: hv: vmbus: Use kthread for vmbus interrupts on PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-03-19 03:43:12 [+0000], Michael Kelley wrote:
> Indeed, yes, that would remove the need for all the per-CPU interrupt hackery
> on x86/x64. I don't have any objection to someone pursuing that path, but it's
> not something I can do. Full disclosure:  You'll see my name on Hyper-V and
> VMBus stuff in the Linux kernel, with Microsoft as my employer. But I retired
> from Microsoft 2.5 years ago, and my current involvement in Linux kernel work
> is purely as a very part-time volunteer. I also lack access to hardware and the
> test machinery needed to make more significant changes, particularly if multiple
> versions of Hyper-V must be tested.

right. Then I would only ask for better annotation instead this current
thingy.

> > I would be worried if the host would storming interrupts to the guest
> > because it makes no progress.
> 
> No, that kind of storming won't happen. The Hyper-V host<->guest
> interface is based on message queues. The host interrupts the guest
> if it puts a message in the queue that transitions the queue from
> "empty" to "not empty". Eventually the tasklet enabled in vmbus_isr()
> and its subsidiaries gets around to emptying the queue, which effectively
> re-arms the interrupt. The host may add more messages to the queue,
> but it doesn't interrupt again for that queue until the queue is empty.
> If the guest is delayed in doing that emptying, nothing bad happens.

Okay.

> > > > Moving on. This (trying very hard here) even schedules tasklets. Why?
> > > > You need to disable BH before doing so. Otherwise it ends in ksoftirqd.
> > > > You don't want that.
> > >
> > > Again, Jan can comment on the impact of delays due to ending up
> > > in ksoftirqd.
> > 
> > My point is that having this with threaded interrupt support would
> > eliminate the usage of tasklets.
> 
> Agreed, probably. For the non-RT case, the latency in getting to the
> tasklet code *does* matter. I'm not familiar with how tasklets compare
> to threaded interrupts on latency.

There shouldn't be much difference on level where it actually matters.

Sebastian