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

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Date:   Fri, 16 Jun 2017 21:35:01 -0400
From:   Len Brown <lenb@...nel.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Len Brown <len.brown@...el.com>
Subject: Re: [PATCH 4/5] intel_pstate: skip scheduler hook when in
 "performance" mode.

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