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SubjectRe: [PATCH v3 2/2] cpufreq: CPPC: Change FIE default
On 24-08-22, 15:04, Lukasz Luba wrote:
> On 8/24/22 07:14, Viresh Kumar wrote:
> > On 18-08-22, 16:16, Jeremy Linton wrote:
> > > FIE is mostly implemented as PCC mailboxes on arm machines. This was
> > > enabled by default without any data suggesting that it does anything
> > > but hurt system performance. Lets change the default to 'n' until
> > > hardware appears which clearly benefits.
> > >
> > > Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
> > > ---
> > > drivers/cpufreq/Kconfig.arm | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/cpufreq/Kconfig.arm b/drivers/cpufreq/Kconfig.arm
> > > index 954749afb5fe..ad66d8f15db0 100644
> > > --- a/drivers/cpufreq/Kconfig.arm
> > > +++ b/drivers/cpufreq/Kconfig.arm
> > > @@ -22,7 +22,7 @@ config ACPI_CPPC_CPUFREQ
> > > config ACPI_CPPC_CPUFREQ_FIE
> > > bool "Frequency Invariance support for CPPC cpufreq driver"
> > > depends on ACPI_CPPC_CPUFREQ && GENERIC_ARCH_TOPOLOGY
> > > - default y
> > > + default n
> > > help
> > > This extends frequency invariance support in the CPPC cpufreq driver,
> > > by using CPPC delivered and reference performance counters.
> >
> > Why is this required after we have the first patch in ?
> >

Well, my question was for the way the patches were added. If we are
disabling the feature based on platform, then there is no need to
disable the feature by default.

> There are a few issues with this ACPI_CPPC_CPUFREQ_FIE solution:
> 1. The design is very heavy and that kernel thread can be ~512 util
> (that's what we have been told by one of our partners from servers'
> world)
> 2. The HW & FW design is not suited for this task. Newer HW will just
> have AMU counters (on Arm64) for FIE
> 3. The patches haven't been tested in terms of performance overhead
> AFAIK. Although, it affects existing Arm64 servers with their
> workloads.
> 4. AFAIK non of our server partners wasn't complaining about issues with
> old FIE mechanism.
>
> In our team we are not allowed to send code that we cannot prove in many
> ways.
>
> I would just not compile this at all (or even revert this feature).
> If someone compiled in this by accident - make sure we disable it
> after checks like in the patch 1/2. I'll add also some comments
> to that patch.

If we don't really want the feature, which is open for discussion
(added Vincent to have a look as well), then we better mark it BROKEN
in Kconfig.

--
viresh

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