Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 24 Jun 2020 11:00:06 +0200 | Subject | Re: [PATCH v2 0/2] sched: Optionally skip uclamp logic in fast path |
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On Tue, 23 Jun 2020 at 19:40, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote: > > On 19/06/2020 19:20, Qais Yousef wrote> This series attempts to address the report that uclamp logic could be expensive > > sometimes and shows a regression in netperf UDP_STREAM under certain > > conditions. > > > > The first patch is a fix for how struct uclamp_rq is initialized which is > > required by the 2nd patch which contains the real 'fix'. > > > > Worth noting that the root cause of the overhead is believed to be system > > specific or related to potential certain code/data layout issues, leading to > > worse I/D $ performance. > > > > Different systems exhibited different behaviors and the regression did > > disappear in certain kernel version while attempting to reporoduce. > > > > More info can be found here: > > > > https://lore.kernel.org/lkml/20200616110824.dgkkbyapn3io6wik@e107158-lin/ > > > > Having the static key seemed the best thing to do to ensure the effect of > > uclamp is minimized for kernels that compile it in but don't have a userspace > > that uses it, which will allow distros to distribute uclamp capable kernels by > > default without having to compromise on performance for some systems that could > > be affected. > > My test data indicates that the static key w/o any uclamp users (3) > brings the performance number for the 'perf bench sched pipe' > workload back (i.e. from w/ !CONFIG_UCLAMP_TASK) (1). > > platform: > > Arm64 Hikey960 (only little CPUs [0-3]), no CPUidle, > performance CPUfreq governor > > workload: > > perf stat -n -r 20 -- perf bench sched pipe -T -l 100000 > > > (A) *** Performance results *** > > (1) tip/sched/core > # CONFIG_UCLAMP_TASK is not set > > *1.39285* +- 0.00191 seconds time elapsed ( +- 0.14% ) > > (2) tip/sched/core > CONFIG_UCLAMP_TASK=y > > *1.42877* +- 0.00181 seconds time elapsed ( +- 0.13% ) > > (3) tip/sched/core + opt_skip_uclamp_v2 > CONFIG_UCLAMP_TASK=y > > *1.38833* +- 0.00291 seconds time elapsed ( +- 0.21% ) > > (4) tip/sched/core + opt_skip_uclamp_v2 > CONFIG_UCLAMP_TASK=y > echo 512 > /proc/sys/kernel/sched_util_clamp_min (enable uclamp) > > *1.42062* +- 0.00238 seconds time elapsed ( +- 0.17% ) > > > (B) *** Profiling on CPU0 and CPU1 *** > > (further hp'ing out CPU2 and CPU3 to get consistent hit numbers) > > (1) > > CPU0: Function Hit Time Avg s^2 > -------- --- ---- --- --- > deactivate_task 1997346 2207642 us *1.105* us 0.033 us > activate_task 1997391 1840057 us *0.921* us 0.054 us > > CPU1: Function Hit Time Avg s^2 > -------- --- ---- --- --- > deactivate_task 1997455 2225960 us 1.114 us 0.034 us > activate_task 1997410 1842603 us 0.922 us 0.052 us > > (2) > > CPU0: Function Hit Time Avg s^2 > -------- --- ---- --- --- > deactivate_task 1998538 2419719 us *1.210* us 0.061 us > activate_task 1997119 1960401 us *0.981* us 0.034 us > > CPU1: Function Hit Time Avg s^2 > -------- --- ---- --- --- > deactivate_task 1996597 2400760 us 1.202 us 0.059 us > activate_task 1998016 1985013 us 0.993 us 0.028 us > > (3) > > CPU0: Function Hit Time Avg s^2 > -------- --- ---- --- --- > deactivate_task 1997525 2155416 us *1.079* us 0.020 us > activate_task 1997874 1899002 us *0.950* us 0.044 us > > CPU1: Function Hit Time Avg s^2 > -------- --- ---- --- --- > deactivate_task 1997935 2118648 us 1.060 us 0.017 us > activate_task 1997586 1895162 us 0.948 us 0.044 us > > (4) > > CPU0: Function Hit Time Avg s^2 > -------- --- ---- --- --- > deactivate_task 1998246 2428121 us *1.215* us 0.062 us > activate_task 1998252 2132141 us *1.067* us 0.020 us > > CPU1: Function Hit Time Avg s^2 > -------- --- ---- --- --- > deactivate_task 1996154 2414194 us 1.209 us 0.060 us > activate_task 1996148 2140667 us 1.072 us 0.021 us
I have rerun the tests that I ran previously on my octo core arm64 (hikey): 20 iteration of perf bench sched pipe -T -l 50000 tip stands for tip/sched/core uclamp enabled means both uclamp task and uclamp cgroup the stdev is around 0.25% for all tests
root level 1 level 2 level 3
tip uclamp disabled 50653 47188 44568 41925 tip uclamp enabled 48800(-3.66%) 45600(-3.37%) 42822(-3.92%) 40257(-3.98%) /w patch uclamp disabled 50615(-0.08%) 47198(+0.02%) 44609(+0.09%) 41735(-0.45%) /w patch uclamp enabled 49661(-1.96%) 46611(-1.22%) 43803(-1.72%) 41243(-1.63%)
Results are better with your patch
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