Hi,
On 7/29/22 07:59, Punit Agrawal wrote:
Hi Jeremy,
One comment / query below.
Jeremy Linton <jeremy.linton@xxxxxxx> 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@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(-)
[...]
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!
