RE: [PATCH] [v4] x86, suspend: Save/restore extra MSR registers for suspend

[Chen, Yu C]

Hi, Doug,
Sorry for my late response, and thanks for your information:

> -----Original Message-----
> From: Doug Smythies [mailto:dsmythies@xxxxxxxxx]
> Sent: Sunday, October 11, 2015 11:47 PM
> To: Chen, Yu C
> Cc: Wysocki, Rafael J; tglx@xxxxxxxxxxxxx; hpa@xxxxxxxxx; bp@xxxxxxxxx;
> Zhang, Rui; linux-pm@xxxxxxxxxxxxxxx; x86@xxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; Brown, Len; 'Ingo Molnar'; 'Pavel Machek'; 'Kristen
> Carlson Accardi'; Doug Smythies
> Subject: RE: [PATCH] [v4] x86, suspend: Save/restore extra MSR registers for
> suspend
> 
> Hi Yu, thanks for your reply.
> 
> On 2015.10.10 19:27 Chen, Yu C wrote:
> > On 2105.10.10 02:56 Doug Smythies wrote:
> >
> >>> The current version of the intel_pstate driver is incompatible with
> >>> any use of Clock Modulation, always resulting in driving the target
> >>> pstate to the minimum, regardless of load. The result is the
> >>> apparent CPU frequency stuck at minimum * modulation percent.
> >>
> >>> The acpi-cpufreq driver works fine with Clock Modulation, resulting
> >>> in desired frequency * modulation percent.
> >>
> 
> > [Yu] Why intel_pstate driver is incompatible with Clock Modulation?
> 
> It is simply how the current control algorithm responds to the scenario.
> 
> The problem is in intel_pstate_get_scaled_busy, here:
> 
>         /*
>          * core_busy is the ratio of actual performance to max
>          * max_pstate is the max non turbo pstate available
>          * current_pstate was the pstate that was requested during
>          *      the last sample period.
>          *
>          * We normalize core_busy, which was our actual percent
>          * performance to what we requested during the last sample
>          * period. The result will be a percentage of busy at a
>          * specified pstate.
>          */
>         core_busy = cpu->sample.core_pct_busy;
>         max_pstate = int_tofp(cpu->pstate.max_pstate);
>         current_pstate = int_tofp(cpu->pstate.current_pstate);
>         core_busy = mul_fp(core_busy, div_fp(max_pstate, current_pstate));
> 
> With Clock Modulation enabled, the actual performance percent will always
> be less than what was asked for, basically meaning current_pstate is much
> less than what was asked for. Thus the algorithm will drive down the target
> pstate regardless of load.
> 
[Yu] Do you mean, there is some problem with the normalization,and we should use 
the actual pstate rather than the theoretical current_pstate, for example, 
the pseudocode might looked like:

-  current_pstate = int_tofp(cpu->pstate.current_pstate);
+ current_pstate = int_tofp(cpu->pstate.current_pstat)*0.85;

Best Regards,
Yu
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