Re: [PATCH v2 1/1] ACPI: CPPC: Disable FIE if registers in PCC regions

[Ionela Voinescu]

Hi folks,

On Wednesday 10 Aug 2022 at 13:29:08 (+0100), Lukasz Luba wrote:
> 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@xxxxxxx>
> > ---
> >   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@xxxxxxxxxx>
> 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.
> 

I agree that having CONFIG_ACPI_CPPC_CPUFREQ_FIE default to no is the
simpler solution but it puts the decision in the hands of platform
providers which might result in this functionality not being used most
of the times, if at all. This being said, the use of CPPC counters is
meant as a last resort for FIE, if the platform does not have AMUs. This
is why I recommended this to default to no in the review of the original
patches.

But I don't see these runtime options as adding a lot of complexity
and therefore agree with the idea of this patch, versus the config
change above, with two design comments:
 - Rather than having a check for fie_disabled in multiple init and exit
   functions I think the code should be slightly redesigned to elegantly
   bail out of most functions if cppc_freq_invariance_init() failed.
 - Given the multiple options to disable this functionality (config,
   PCC check), I don't see a need for a module parameter or runtime user
   input, unless we make that overwrite all previous decisions, as in: if
   CONFIG_ACPI_CPPC_CPUFREQ_FIE=y, even if cppc_perf_ctrs_in_pcc(), if
   the fie_disabled module parameter is no, then counters should be used
   for FIE.

Thanks,
Ionela.


> 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