Re: [PATCH] intel_pstate: Change busy calculation to use fixed point math.
From: Rafael J. Wysocki
Date: Tue Feb 25 2014 - 19:24:35 EST
On Tuesday, February 25, 2014 10:35:37 AM dirk.brandewie@xxxxxxxxx wrote:
> From: Dirk Brandewie <dirk.j.brandewie@xxxxxxxxx>
>
> Commit fcb6a15c2e Take core C0 time into account for core busy calculation.
>
> Introduced a regression on some processor SKUs supported by
> intel_pstate. This was caused by the truncation caused by using
> integer math to calculate core busy and C0 percentages.
>
> On a i7-4770K processor operating at 800Mhz going to 100% utilization
> the percent busy of the CPU using integer math is 22% it actually is
> 22.85%. This value scaled to the current frequency returned 97 which
> the PID interpreted as no error and did not adjust the P state.
>
> Tested on i7-4770K, i7-2600, i5-3230M
>
> References:
> https://lkml.org/lkml/2014/2/19/626
> https://bugzilla.kernel.org/show_bug.cgi?id=70941
>
> Signed-off-by: Dirk Brandewie <dirk.j.brandewie@xxxxxxxxx>
Queued up as a fix for 3.14.
Thanks!
> ---
> drivers/cpufreq/intel_pstate.c | 28 ++++++++++++++++++----------
> 1 file changed, 18 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index e908161..2cd36b9 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -39,9 +39,10 @@
> #define BYT_TURBO_RATIOS 0x66c
>
>
> -#define FRAC_BITS 8
> +#define FRAC_BITS 6
> #define int_tofp(X) ((int64_t)(X) << FRAC_BITS)
> #define fp_toint(X) ((X) >> FRAC_BITS)
> +#define FP_ROUNDUP(X) ((X) += 1 << FRAC_BITS)
>
> static inline int32_t mul_fp(int32_t x, int32_t y)
> {
> @@ -556,18 +557,20 @@ static void intel_pstate_get_cpu_pstates(struct cpudata *cpu)
> static inline void intel_pstate_calc_busy(struct cpudata *cpu,
> struct sample *sample)
> {
> - u64 core_pct;
> - u64 c0_pct;
> + int32_t core_pct;
> + int32_t c0_pct;
>
> - core_pct = div64_u64(sample->aperf * 100, sample->mperf);
> + core_pct = div_fp(int_tofp((sample->aperf)),
> + int_tofp((sample->mperf)));
> + core_pct = mul_fp(core_pct, int_tofp(100));
> + FP_ROUNDUP(core_pct);
> +
> + c0_pct = div_fp(int_tofp(sample->mperf), int_tofp(sample->tsc));
>
> - c0_pct = div64_u64(sample->mperf * 100, sample->tsc);
> sample->freq = fp_toint(
> - mul_fp(int_tofp(cpu->pstate.max_pstate),
> - int_tofp(core_pct * 1000)));
> + mul_fp(int_tofp(cpu->pstate.max_pstate * 1000), core_pct));
>
> - sample->core_pct_busy = mul_fp(int_tofp(core_pct),
> - div_fp(int_tofp(c0_pct + 1), int_tofp(100)));
> + sample->core_pct_busy = mul_fp(core_pct, c0_pct);
> }
>
> static inline void intel_pstate_sample(struct cpudata *cpu)
> @@ -579,6 +582,10 @@ static inline void intel_pstate_sample(struct cpudata *cpu)
> rdmsrl(MSR_IA32_MPERF, mperf);
> tsc = native_read_tsc();
>
> + aperf = aperf >> FRAC_BITS;
> + mperf = mperf >> FRAC_BITS;
> + tsc = tsc >> FRAC_BITS;
> +
> cpu->sample_ptr = (cpu->sample_ptr + 1) % SAMPLE_COUNT;
> cpu->samples[cpu->sample_ptr].aperf = aperf;
> cpu->samples[cpu->sample_ptr].mperf = mperf;
> @@ -610,7 +617,8 @@ static inline int32_t intel_pstate_get_scaled_busy(struct cpudata *cpu)
> core_busy = cpu->samples[cpu->sample_ptr].core_pct_busy;
> max_pstate = int_tofp(cpu->pstate.max_pstate);
> current_pstate = int_tofp(cpu->pstate.current_pstate);
> - return mul_fp(core_busy, div_fp(max_pstate, current_pstate));
> + core_busy = mul_fp(core_busy, div_fp(max_pstate, current_pstate));
> + return FP_ROUNDUP(core_busy);
> }
>
> static inline void intel_pstate_adjust_busy_pstate(struct cpudata *cpu)
>
--
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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