Re: [PATCH v2 0/2] sched: Optionally skip uclamp logic in fast path

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

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