[PATCH 0/2 v2] cpufreq, intel_pstate, Fix rounding errors

Prarit Bhargava
2015-11-20T12:32:45Z

I have a Intel (6,63) processor with a "marketing" frequency (from
/proc/cpuinfo) of 2100MHz, and a max turbo frequency of 2600MHz. I
can execute

cpupower frequency-set -g powersave --min 1200MHz --max 2100MHz

and the max_freq_pct is set to 80. When adding load to the system I noticed
that the cpu frequency only reached 2000MHZ and not 2100MHz as expected.

I wrote a little test program to set the frequencies in decrements of
100MHz and compared the targeted frequency (the frequency set through
the cpupower command) and the actual frequency (from /proc/cpuinfo), as
well as dumping out the value of the MSR_IA32_PERF_CTL.

Target Achieved Difference MSR(0x199)
3300 2900 -400 0x1e00
3200 2900 -300 0x1e00
3100 2900 -200 0x1e00
3000 2900 -100 0x1d00
2900 2800 -100 0x1c00
2800 2700 -100 0x1b00
2700 2600 -100 0x1a00
2600 2500 -100 0x1900
2500 2400 -100 0x1800
2400 2300 -100 0x1700
2300 2200 -100 0x1600
2200 2100 -100 0x1500
2100 2000 -100 0x1400
2000 1900 -100 0x1300
1900 1800 -100 0x1200
1800 1700 -100 0x1100
1700 1600 -100 0x1000
1600 1500 -100 0xf00
1500 1400 -100 0xe00
1400 1300 -100 0xd00
1300 1200 -100 0xc00
1200 1200 0 0xc00

As can be seen the frequencies are consistently off by 100MHz. After
some examination I found a rounding error in intel_pstate_set_policy() for the
calculation of limits->max_policy_pct which needs to be rounded up to the
nearest percentage point. For example, setting a frequency of 2100MHz on this
system results in limits->max_policy_pct = ((2100 * 100) / 2600) = 80.
However, ((2100 * 100) / 2600) is actually 80.7, or 81. This is fixed
by expanding the calculation an extra decimal point and rounding to the
nearest percentage point.

A second rounding error was found in the calculation of limits->max_perf
in intel_pstate_set_policy(), which is used to calculate the max and min
pstate values in intel_pstate_get_min_max(). In this case, limits->max_perf
is truncated to 2 hex digits such that, for example, 0x169 was incorrectly
truncated to 0x16 instead of 0x17. This resulted in the pstate being set
one level too low.

After applying these two fixes we consistently reach the targeted
frequency.

Target Achieved Difference MSR(0x199)
3300 2900 -400 0x1e00
3200 2900 -300 0x1e00
3100 2900 -200 0x1e00
3000 2900 -100 0x1d00
2900 2900 0 0x1d00
2800 2800 0 0x1c00
2700 2700 0 0x1b00
2600 2600 0 0x1a00
2500 2500 0 0x1900
2400 2400 0 0x1800
2300 2300 0 0x1700
2200 2200 0 0x1600
2100 2100 0 0x1500
2000 2000 0 0x1400
1900 1900 0 0x1300
1800 1800 0 0x1200
1700 1700 0 0x1100
1600 1600 0 0x1000
1500 1500 0 0xf00
1400 1400 0 0xe00
1300 1300 0 0xd00
1200 1200 0 0xc00

Additional tests were run on a (6,78) with HWP enabled and a (6,79)
system. Testing on both systems showed that the problem was resolved.

Cc: Srinivas Pandruvada <srinivas.pandruvada@intel.com>
Cc: Len Brown <len.brown@intel.com>
Cc: Alexandra Yates <alexandra.yates@intel.com>
Cc: Kristen Carlson Accardi <kristen@linux.intel.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: linux-pm@vger.kernel.org
Signed-off-by: Prarit Bhargava <prarit@redhat.com>

[v2]: Separate into two patches, and rebase onto git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git linux-next
Prarit Bhargava (2):
cpufreq, intel_pstate, Fix limits->max_policy_pct rounding error
cpufreq, intel_pstate, fix limits->max_perf rounding error

drivers/cpufreq/intel_pstate.c | 3 +++
1 file changed, 3 insertions(+)

--
1.8.3.1