Hi,
On 8/10/22 07:29, 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)?
Well, the mailbox nature of PCC pretty much assures its "slow", relative the alternative of providing an actual register. If a platform provides direct access to say MHU registers, then of course they won't actually be in a PCC region and the FIE will remain on.
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:
The problem here is that these ACPI kernels are being shipped as single images in distro's which expect them to run on a wide range of platforms (including x86/amd in this case), and preform optimally on all of them.
So the 'n' option basically is saying that the latest FIE code doesn't provide a befit anywhere?