Messages in this thread | | | Date | Wed, 10 Aug 2022 15:37:54 +0100 | Subject | Re: [PATCH v2 1/1] ACPI: CPPC: Disable FIE if registers in PCC regions | From | Lukasz Luba <> |
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On 8/10/22 15:30, Jeremy Linton wrote: > 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@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(-) >> >> >> 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.
Exactly, I'm afraid the same, that we would never get such fast mailbox-based platform. Newer platforms would just use AMU, so completely different code and no one would even bother to test if their HW mailbox is fast-enough for this FIE purpose ;)
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