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Date: Tue, 04 Apr 2023 21:49:10 +0100
From: Marc Zyngier <maz@...nel.org>
To: David Dai <davidai@...gle.com>,
Oliver Upton <oliver.upton@...ux.dev>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
Viresh Kumar <viresh.kumar@...aro.org>,
Rob Herring <robh+dt@...nel.org>,
Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Jonathan Corbet <corbet@....net>,
James Morse <james.morse@....com>,
Suzuki K Poulose <suzuki.poulose@....com>,
Zenghui Yu <yuzenghui@...wei.com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Lorenzo Pieralisi <lpieralisi@...nel.org>,
Sudeep Holla <sudeep.holla@....com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
kernel-team@...roid.com, linux-pm@...r.kernel.org,
devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org, linux-doc@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev
Subject: Re: [RFC PATCH 0/6] Improve VM DVFS and task placement behavior
On Tue, 04 Apr 2023 20:43:40 +0100,
Oliver Upton <oliver.upton@...ux.dev> 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.
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.
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
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