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

From: Jeremy Linton
Date: Wed Aug 10 2022 - 13:43:33 EST


Hi,

On 8/10/22 07:51, Ionela Voinescu wrote:
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.

I'm not sure what that would look like, I will have to mess with it a bit more, but as you can see its really just the two init entry points (one for the module, and one for the registered cpufreq), and their associated exit's which I'm not sure I see a way to simplify that short of maybe creating a second cpufreq_driver table, which replaces the .init calls with ones which include cppc_cpufreq_cpu_fie_init. The alternative is runtime setting the .init to switch between an init with FIE and one without. I'm not sure that clarifies what is happening in the code, and I thought in general dynamic runtime dispatch was to be avoided in the ACPI code when possible. Neither choice of course affects actual runtime because they are both firing during module load/unload.


- 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.

Tristating the module parameter with default=detect, ON, OFF is a reasonable idea, and one I considered, but ignored because in the hisi quirk case even with ON it will have to be OFF, so it really ends up with 4 states default=detect, request ON, ON, OFF.

I'm good with any of this if people feel strongly about it.


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


Thanks for looking at this!