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Date:   Wed, 10 Aug 2022 13:29:08 +0100
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Subject: Re: [PATCH v2 1/1] ACPI: CPPC: Disable FIE if registers in PCC
 regions
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To:     Jeremy Linton <jeremy.linton@arm.com>
Cc:     rafael@kernel.org, lenb@kernel.org, viresh.kumar@linaro.org,
        robert.moore@intel.com, devel@acpica.org,
        linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org, vschneid@redhat.com,
        Ionela Voinescu <ionela.voinescu@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>
References: <20220728221043.4161903-1-jeremy.linton@arm.com>
 <20220728221043.4161903-2-jeremy.linton@arm.com>
From:   Lukasz Luba <lukasz.luba@arm.com>
In-Reply-To: <20220728221043.4161903-2-jeremy.linton@arm.com>
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Hi Jeremy,

+CC Valentin since he might be interested in this finding
+CC Ionela, Dietmar

I have a few comments for this patch.


On 7/28/22 23:10, Jeremy Linton wrote:
> PCC regions utilize a mailbox to set/retrieve register values used by
> the CPPC code. This is fine as long as the operations are
> infrequent. With the FIE code enabled though the overhead can range
> from 2-11% of system CPU overhead (ex: as measured by top) on Arm
> based machines.
> 
> So, before enabling FIE assure none of the registers used by
> cppc_get_perf_ctrs() are in the PCC region. Furthermore lets also
> enable a module parameter which can also disable it at boot or module
> reload.
> 
> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
> ---
>   drivers/acpi/cppc_acpi.c       | 41 ++++++++++++++++++++++++++++++++++
>   drivers/cpufreq/cppc_cpufreq.c | 19 ++++++++++++----
>   include/acpi/cppc_acpi.h       |  5 +++++
>   3 files changed, 61 insertions(+), 4 deletions(-)


1. You assume that all platforms would have this big overhead when
    they have the PCC regions for this purpose.
    Do we know which version of HW mailbox have been implemented
    and used that have this 2-11% overhead in a platform?
    Do also more recent MHU have such issues, so we could block
    them by default (like in your code)?

2. I would prefer to simply change the default Kconfig value to 'n' for
    the ACPI_CPPC_CPUFREQ_FIE, instead of creating a runtime
    check code which disables it.
    We have probably introduce this overhead for older platforms with
    this commit:

commit 4c38f2df71c8e33c0b64865992d693f5022eeaad
Author: Viresh Kumar <viresh.kumar@linaro.org>
Date:   Tue Jun 23 15:49:40 2020 +0530

     cpufreq: CPPC: Add support for frequency invariance


If the test server with this config enabled performs well
in the stress-tests, then on production server the config may be
set to 'y' (or 'm' and loaded).

I would vote to not add extra code, which then after a while might be
decided to bw extended because actually some HW is actually capable (so
we could check in runtime and enable it). IMO this create an additional
complexity in our diverse configuration/tunnable space in our code.

When we don't compile-in this, we should fallback to old-style
FIE, which has been used on these old platforms.

BTW (I have to leave it here) the first-class solution for those servers
is to implement AMU counters, so the overhead to retrieve this info is
really low.

Regards,
Lukasz