Re: [RFC PATCH 0/6] Improve VM DVFS and task placement behavior

[Saravana Kannan]

On Wed, Apr 5, 2023 at 12:48 AM 'Quentin Perret' via kernel-team
<kernel-team@xxxxxxxxxxx> wrote:
>
> On Tuesday 04 Apr 2023 at 21:49:10 (+0100), Marc Zyngier wrote:
> > On Tue, 04 Apr 2023 20:43:40 +0100,
> > Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
> > >
> > > Folks,
> > >
> > > On Thu, Mar 30, 2023 at 03:43:35PM -0700, David Dai wrote:
> > >
> > > <snip>
> > >
> > > > PCMark
> > > > Higher is better
> > > > +-------------------+----------+------------+--------+-------+--------+
> > > > | Test Case (score) | Baseline |  Hypercall | %delta |  MMIO | %delta |
> > > > +-------------------+----------+------------+--------+-------+--------+
> > > > | Weighted Total    |     6136 |       7274 |   +19% |  6867 |   +12% |
> > > > +-------------------+----------+------------+--------+-------+--------+
> > > > | Web Browsing      |     5558 |       6273 |   +13% |  6035 |    +9% |
> > > > +-------------------+----------+------------+--------+-------+--------+
> > > > | Video Editing     |     4921 |       5221 |    +6% |  5167 |    +5% |
> > > > +-------------------+----------+------------+--------+-------+--------+
> > > > | Writing           |     6864 |       8825 |   +29% |  8529 |   +24% |
> > > > +-------------------+----------+------------+--------+-------+--------+
> > > > | Photo Editing     |     7983 |      11593 |   +45% | 10812 |   +35% |
> > > > +-------------------+----------+------------+--------+-------+--------+
> > > > | Data Manipulation |     5814 |       6081 |    +5% |  5327 |    -8% |
> > > > +-------------------+----------+------------+--------+-------+--------+
> > > >
> > > > PCMark Performance/mAh
> > > > Higher is better
> > > > +-----------+----------+-----------+--------+------+--------+
> > > > |           | Baseline | Hypercall | %delta | MMIO | %delta |
> > > > +-----------+----------+-----------+--------+------+--------+
> > > > | Score/mAh |       79 |        88 |   +11% |   83 |    +7% |
> > > > +-----------+----------+-----------+--------+------+--------+
> > > >
> > > > Roblox
> > > > Higher is better
> > > > +-----+----------+------------+--------+-------+--------+
> > > > |     | Baseline |  Hypercall | %delta |  MMIO | %delta |
> > > > +-----+----------+------------+--------+-------+--------+
> > > > | FPS |    18.25 |      28.66 |   +57% | 24.06 |   +32% |
> > > > +-----+----------+------------+--------+-------+--------+
> > > >
> > > > Roblox Frames/mAh
> > > > Higher is better
> > > > +------------+----------+------------+--------+--------+--------+
> > > > |            | Baseline |  Hypercall | %delta |   MMIO | %delta |
> > > > +------------+----------+------------+--------+--------+--------+
> > > > | Frames/mAh |    91.25 |     114.64 |   +26% | 103.11 |   +13% |
> > > > +------------+----------+------------+--------+--------+--------+
> > >
> > > </snip>
> > >
> > > > Next steps:
> > > > ===========
> > > > We are continuing to look into communication mechanisms other than
> > > > hypercalls that are just as/more efficient and avoid switching into the VMM
> > > > userspace. Any inputs in this regard are greatly appreciated.
> > >
> > > We're highly unlikely to entertain such an interface in KVM.
> > >
> > > The entire feature is dependent on pinning vCPUs to physical cores, for which
> > > userspace is in the driver's seat. That is a well established and documented
> > > policy which can be seen in the way we handle heterogeneous systems and
> > > vPMU.
> > >
> > > Additionally, this bloats the KVM PV ABI with highly VMM-dependent interfaces
> > > that I would not expect to benefit the typical user of KVM.
> > >
> > > Based on the data above, it would appear that the userspace implementation is
> > > in the same neighborhood as a KVM-based implementation, which only further
> > > weakens the case for moving this into the kernel.
> > >
> > > I certainly can appreciate the motivation for the series, but this feature
> > > should be in userspace as some form of a virtual device.
> >
> > +1 on all of the above.
>
> And I concur with all the above as well. Putting this in the kernel is
> not an obvious fit at all as that requires a number of assumptions about
> the VMM.
>
> As Oliver pointed out, the guest topology, and how it maps to the host
> topology (vcpu pinning etc) is very much a VMM policy decision and will
> be particularly important to handle guest frequency requests correctly.
>
> In addition to that, the VMM's software architecture may have an impact.
> Crosvm for example does device emulation in separate processes for
> security reasons, so it is likely that adjusting the scheduling
> parameters ('util_guest', uclamp, or else) only for the vCPU thread that
> issues frequency requests will be sub-optimal for performance, we may
> want to adjust those parameters for all the tasks that are on the
> critical path.
>
> And at an even higher level, assuming in the kernel a certain mapping of
> vCPU threads to host threads feels kinda wrong, this too is a host
> userspace policy decision I believe. Not that anybody in their right
> mind would want to do this, but I _think_ it would technically be
> feasible to serialize the execution of multiple vCPUs on the same host
> thread, at which point the util_guest thingy becomes entirely bogus. (I
> obviously don't want to conflate this use-case, it's just an example
> that shows the proposed abstraction in the series is not a perfect fit
> for the KVM userspace delegation model.)

See my reply to Oliver and Marc. To me it looks like we are converging
towards having shared memory between guest, host kernel and VMM and
that should address all our concerns.

The guest will see a MMIO device, writing to it will trigger the host
kernel to do the basic "set util_guest/uclamp for the vCPU thread that
corresponds to the vCPU" and then the VMM can do more on top as/if
needed (because it has access to the shared memory too). Does that
make sense?

Even in the extreme example, the stuff the kernel would do would still
be helpful, but not sufficient. You can aggregate the
util_guest/uclamp and do whatever from the VMM.
Technically in the extreme example, you don't need any of this. The
normal util tracking of the vCPU thread on the host side would be
sufficient.

Actually any time we have only 1 vCPU host thread per VM, we shouldn't
be using anything in this patch series and not instantiate the guest
device at all.

> So +1 from me to move this as a virtual device of some kind. And if the
> extra cost of exiting all the way back to userspace is prohibitive (is
> it btw?),

I think the "13% increase in battery consumption for games" makes it
pretty clear that going to userspace is prohibitive. And that's just
one example.

> then we can try to work on that. Maybe something a la vhost
> can be done to optimize, I'll have a think.
>
> > The one thing I'd like to understand that the comment seems to imply
> > that there is a significant difference in overhead between a hypercall
> > and an MMIO. In my experience, both are pretty similar in cost for a
> > handling location (both in userspace or both in the kernel). MMIO
> > handling is a tiny bit more expensive due to a guaranteed TLB miss
> > followed by a walk of the in-kernel device ranges, but that's all. It
> > should hardly register.
> >
> > And if you really want some super-low latency, low overhead
> > signalling, maybe an exception is the wrong tool for the job. Shared
> > memory communication could be more appropriate.
>
> I presume some kind of signalling mechanism will be necessary to
> synchronously update host scheduling parameters in response to guest
> frequency requests, but if the volume of data requires it then a shared
> buffer + doorbell type of approach should do.

Part of the communication doesn't need synchronous handling by the
host. So, what I said above.

> Thinking about it, using SCMI over virtio would implement exactly that.
> Linux-as-a-guest already supports it IIRC, so possibly the problem
> being addressed in this series could be 'simply' solved using an SCMI
> backend in the VMM...

This will be worse than all the options we've tried so far because it
has the userspace overhead AND uclamp overhead.

-Saravana