Re: [PATCH v2 1/1] ACPI: CPPC: Disable FIE if registers in PCC regions

[Lukasz Luba]

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@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?
> >
> > 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.