Re: [RFC PATCH 10/11] Drivers: hv: vmbus: Introduce the CHANNELMSG_MODIFYCHANNEL message type

[Andrea Parri <parri.andrea@gmail.com>]

On Thu, Mar 26, 2020 at 03:46:23PM +0100, Vitaly Kuznetsov wrote:
> "Andrea Parri (Microsoft)" <parri.andrea@gmail.com> writes:
> 
> > VMBus version 4.1 and later support the CHANNELMSG_MODIFYCHANNEL(22)
> > message type which can be used to request Hyper-V to change the vCPU
> > that a channel will interrupt.
> >
> > Introduce the CHANNELMSG_MODIFYCHANNEL message type, and define the
> > vmbus_send_modifychannel() function to send CHANNELMSG_MODIFYCHANNEL
> > requests to the host via a hypercall.  The function is then used to
> > define a sysfs "store" operation, which allows to change the (v)CPU
> > the channel will interrupt by using the sysfs interface.  The feature
> > can be used for load balancing or other purposes.
> >
> > One interesting catch here is that Hyper-V can *not* currently ACK
> > CHANNELMSG_MODIFYCHANNEL messages with the promise that (after the ACK
> > is sent) the channel won't send any more interrupts to the "old" CPU.
> >
> > The peculiarity of the CHANNELMSG_MODIFYCHANNEL messages is problematic
> > if the user want to take a CPU offline, since we don't want to take a
> > CPU offline (and, potentially, "lose" channel interrupts on such CPU)
> > if the host is still processing a CHANNELMSG_MODIFYCHANNEL message
> > associated to that CPU.
> >
> > It is worth mentioning, however, that we have been unable to observe
> > the above mentioned "race": in all our tests, CHANNELMSG_MODIFYCHANNEL
> > requests appeared *as if* they were processed synchronously by the
> > host.
> 
> Hyper-V engineers never want to make our lifes easier :-)

Haha.  I'd say more exciting!  ;-) ;-)


> I can only think of a 'lazy' approach to setting channel CPU affinity:
> we actually re-assign it to the target CPU when we receive first
> interrupt there - but it's not very different from re-assigning it there
> but still handling interrupts on the old CPU like you do.

Interesting!  I'm wondering whether it'd make sense to use a similar
approach to (lazily) "unblock" the "old" CPUs; let me think more...


> One more thing: it was already discussed several times but we never get
> to it. I think this question was even raised on Michael's latest
> 'Hyper-V on ARM' submission. What about implmenting a Hyper-V specific
> IRQ chip which would now support setting CPU affinity? The greatest
> benefit is that we don't need new tools to do e.g. load balancing,
> irqbalance will just work.

Thank you for the suggestions; TBH, I haven't considered such approach
so far (and I'd need more time to come up with an informed comment...)

OTOH, I had some initial investigations about the current (in-kernel)
balancing scheme/init_vp_index() and possible improvements/extensions
there...  Hopefully another, follow-up series to come soon!

Thanks,
  Andrea