On Wed, Aug 30, 2017 at 03:41:18PM +0100, Dietmar Eggemann wrote: > The following 'capacity-dmips-mhz' dt property values are used: > > Cortex-A15: 1024, Cortex-A7: 539 > > They have been derived from the cpu_efficiency values: > > Cortex-A15: 3891, Cortex-A7: 2048 > > by scaling them so that the Cortex-A15s (big cores) use 1024. > > The cpu_efficiency values were originally derived from the "Big.LITTLE > Processing with ARM Cortex™-A15 & Cortex-A7" white paper > (http://www.cl.cam.ac.uk/~rdm34/big.LITTLE.pdf). Table 1 lists 1.9x > (3891/2048) as the Cortex-A15 vs Cortex-A7 performance ratio for the > Dhrystone benchmark. > > The following platforms are affected once cpu-invariant accounting > support is re-connected to the task scheduler: > > arndale-octa, peach-pi, peach-pit, smdk5420 > > The patch has been tested on Samsung Chromebook 2 13" (peach-pi, Exynos > 5800). > > $ cat /sys/devices/system/cpu/cpu*/cpu_capacity > 1024 > 1024 > 1024 > 1024 > 389 > 389 > 389 > 389 > > The Cortex-A15 vs Cortex-A7 performance ratio is 1024/389 = 2.63. > > The values derived with the 'cpu_efficiency/clock-frequency dt property' > solution are: > > $ cat /sys/devices/system/cpu/cpu*/cpu_capacity > 1535 > 1535 > 1535 > 1535 > 448 > 448 > 448 > 448 > > The Cortex-A15 vs Cortex-A7 performance ratio is 1535/448 = 3.43. > > The discrepancy between 2.63 and 3.43 is due to the false assumption > when using the 'cpu_efficiency/clock-frequency dt property' solution > that the max cpu frequency of the little cpus is 1 GHZ and not 1.3 GHz. > The Cortex-A7 cluster runs with a max cpu frequency of 1.3 GHZ whereas > the 'clock-frequency' property value is set to 1 GHz. > > 3.43/1.3 = 2.64 > > $ cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_max_freq > 1800000 > 1800000 > 1800000 > 1800000 > 1300000 <-- max cpu frequency of the Cortex-A7s (little cores) > 1300000 > 1300000 > 1300000 > > Running another benchmark (single-threaded sysbench affine to the > individual cpus) with performance cpufreq governor on the Samsung > Chromebook 2 13" showed the following numbers: > > $ for i in `seq 0 7`; do taskset -c $i sysbench --test=cpu > --num-threads=1 --max-time=10 run | grep "total number of events:"; > done > > total number of events: 1083 > total number of events: 1085 > total number of events: 1085 > total number of events: 1085 > total number of events: 454 > total number of events: 454 > total number of events: 454 > total number of events: 454 > > The Cortex-A15 vs Cortex-A7 performance ratio is 2.39, i.e. very close > to the one derived from the Dhrystone based one of the "Big.LITTLE > Processing with ARM Cortex™-A15 & Cortex-A7" white paper (2.63). > > We don't aim for exact values for the cpu capacity values. Besides the > CPI (Cycles Per Instruction), the instruction mix and whether the system > runs cpu-bound or memory-bound has an impact on the cpu capacity values > derived from these benchmark results. > > Cc: Rob Herring <robh+dt@xxxxxxxxxx> > Cc: Mark Rutland <mark.rutland@xxxxxxx> > Cc: Russell King <linux@xxxxxxxxxxxxxxx> > Cc: Kukjin Kim <kgene@xxxxxxxxxx> > Cc: Krzysztof Kozlowski <krzk@xxxxxxxxxx> > Signed-off-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx> > --- > arch/arm/boot/dts/exynos5420-cpus.dtsi | 8 ++++++++ > 1 file changed, 8 insertions(+) > Thanks, applied (with s/arm/ARM/ change in subject). Best regards, Krzysztof
