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)?
I posted that other email before being awake and conflated MHU with AMU
(which could potentially expose the values directly). But the CPPC code
isn't aware of whether a MHU or some other mailbox is in use. Either
way, its hard to imagine a general mailbox with a doorbell/wait for
completion handshake will ever be fast enough to consider running at the
granularity this code is running at. If there were a case like that, the
kernel would have to benchmark it at runtime to differentiate it from
something that is talking over a slow link to a slowly responding mgmt
processor.
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.
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
