linux-kernel - Re: [PATCH v3 2/2] cpufreq: CPPC: Change FIE default

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Message-ID: <20220830054455.pezg63zqxg6az77n@vireshk-i7>
Date:   Tue, 30 Aug 2022 11:14:55 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     linux-acpi@...r.kernel.org, rafael@...nel.or, lenb@...nel.org,
        robert.moore@...el.com, punit.agrawal@...edance.com,
        ionela.voinescu@....com, pierre.gondois@....com,
        linux-kernel@...r.kernel.org, devel@...ica.org,
        linux-pm@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Souvik Chakravarty <souvik.chakravarty@....com>,
        Jeremy Linton <jeremy.linton@....com>,
        vincent.guittot@...aro.org
Subject: Re: [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@....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