Messages in this thread | | | Date | Fri, 29 Jul 2022 10:20:46 -0500 | Subject | Re: [PATCH v2 1/1] ACPI: CPPC: Disable FIE if registers in PCC regions | From | Jeremy Linton <> |
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Hi,
On 7/29/22 07:59, Punit Agrawal wrote: > Hi Jeremy, > > One comment / query below. > > Jeremy Linton <jeremy.linton@arm.com> writes: > >> 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(-) >> > > [...] > >> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c >> index 24eaf0ec344d..ed607e27d6bb 100644 >> --- a/drivers/cpufreq/cppc_cpufreq.c >> +++ b/drivers/cpufreq/cppc_cpufreq.c > > [...] > >> @@ -229,7 +233,12 @@ static void __init cppc_freq_invariance_init(void) >> }; >> int ret; >> >> - if (cppc_cpufreq_driver.get == hisi_cppc_cpufreq_get_rate) >> + if (cppc_perf_ctrs_in_pcc()) { >> + pr_debug("FIE not enabled on systems with registers in PCC\n"); > > The message should probably be promoted to a pr_info() and exposed as > part of the kernel logs. It is a change in the default behaviour we've > had until now. The message will provide some hint about why it was > disabled. > > Thoughts?
I personally flip flopped between making it pr_info or pr_debug and settled on debug because no one else was complaining about the cppc_fie consumption. Which to me, meant that the users of platforms utilizing PCC regions weren't sensitive to the problem, or weren't yet running a distro/kernel with it enabled, or any number of other reasons why the problem wasn't getting more attention. Mostly I concluded the FIE code hadn't shown up in "enterprise" distros yet..
But, yah, if no one is going to complain about the extra messages pr_info() is a better plan.
Thanks for looking at this!
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