Re: [PATCH 4/5] intel_pstate: skip scheduler hook when in "performance" mode.

From: Len Brown
Date: Fri Jun 16 2017 - 21:35:13 EST

sorry, that was a premature send...

>>> > What about update_turbo_pstate()?
>>> >
>>> > In theory MSR_IA32_MISC_ENABLE_TURBO_DISABLE can be set at any time, so
>>> > wouldn't that become problematic after this change?
>>> yes, the sysfs "no_turbo" attribute can be modified at any time, invoking
>>> update_turbo_state(), which will update MSR_IA32_MISC_ENABLE_TURBO_DISABLE
>> If that was the only way it could change, I wouldn't worry about it, but what
>> about changes by BMCs and similar? Are they not a concern?
>>> But how is the presence or change in turbo related to the lack of a
>>> need to hook the scheduler callback in "performance" mode? The hook
>>> literally does nothing in this case, except consume cycles, no?
>> No.
>> It actually sets the P-state to the current maximum (which admittedly is
>> excessive) exactly because the maximum may change on the fly in theory.
> There are 2 cases.
> If turbo was enabled and were we requesting max turbo
> and "somebody" disabled turbo in an MSR, then the HW would
> simply clip our excessive
request to what the hardware supports.

if turbo was disabled and we were requesting max non-turbo,
and "somebody" enabled turbo in the MSR,
then it is highly likely that current request is already sufficiently
high enough
to enable turbo. "highly likely" = would work on 100% of the
machines I have ever seen, including those with mis-configured TAR.

ie. it should not matter.

>> If it can't change on the fly (or we don't care), we can do some more
>> simplifications there. :-)
> I do not think it is Linux's responsibility to monitor changes to MSRs
> such as Turbo enable/disable done behind its back by a BMC at run-time.
> (if this is even possible)
> --
> Len Brown, Intel Open Source Technology Center