Messages in this thread | | | Date | Thu, 11 Aug 2022 08:29:38 +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 19:04, Jeremy Linton wrote: > Hi, > > On 8/10/22 09:32, Lukasz Luba wrote: >> >> >> On 8/10/22 15:08, 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)? >>> >>> 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? >> >> How we define the 'benefit' here - it's a better task utilization. >> How much better it would be vs. previous approach with old-style FIE? >> >> TBH, I haven't found any test results from the development of the patch >> set. Maybe someone could point me to the test results which bring >> this benefit of better utilization. >> >> In the RFC I could find that statement [1]: >> >> "This is tested with some hacks, as I didn't have access to the right >> hardware, on the ARM64 hikey board to check the overall functionality >> and that works fine." >> >> There should be a rule that such code is tested on a real server with >> many CPUs under some stress-test. >> >> Ionela do you have some test results where this new FIE feature >> introduces some better & meaningful accuracy improvement to the >> tasks utilization? >> >> With this overhead measured on a real server platform I think >> it's not worth to keep it 'y' in default. >> >> The design is heavy, as stated in the commit message: >> " On an invocation of cppc_scale_freq_tick(), we schedule an irq work >> (since we reach here from hard-irq context), which then schedules a >> normal work item and cppc_scale_freq_workfn() updates the per_cpu >> arch_freq_scale variable based on the counter updates since the last >> tick. >> " >> >> As you said Jeremy, this mailbox would always be with overhead. IMO >> untill we cannot be sure we have some powerful new HW mailbox, this >> feature should be disabled. > > > Right, the design of the feature would be completely different if it > were a simple register read to get the delivered perf avoiding all the > jumping around you quoted. > > Which sorta implies that its not really fixable as is, which IMHO means > that 'n' isn't really strong enough, it should probably be under > CONFIG_EXPERT as well if such a change were made to discourage its use. >
That's something that I also started to consider, since we are aware of the impact.
You have my vote when you decide to go forward with that config change.
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