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

[Lukasz Luba]

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@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.

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 ;)