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    Subject[PATCH] x86,sched: Fix sched_smt_power_savings totally broken
    Date
    sched_smt_power_savings is totally broken at lastest linux and -tip tree.

    sched_smt_power_savings is set to 1, the scheduler tries to schedule processes
    on the least number of hyper-threads on a core as possible. In other words,
    the process load is distributed such that all the hyper-threads in a core and
    all the cores within the same processor are busy before the load is distributed
    to other hyper-threads and cores in another processor.

    Test On Intel Xeon machine with 2 physical CPUs and each CPU has 8 cores / 16
    threads. physical CPU 0 includes cpu[0~7] and cpu[16~23]; while physical CPU 1
    includes cpu[8~15] and cpu[24~31].

    At latest -tip tree:
    echo 1 > /sys/devices/system/cpu/sched_smt_power_savings
    ./ebizzy -t 16 -S 100 & sleep 10 ; cat /proc/sched_debug | grep -A 1 cpu# > tmp.log

    cpu#0, 2693.564 MHz
    .nr_running : 0
    --
    cpu#1, 2693.564 MHz
    .nr_running : 1
    --
    cpu#2, 2693.564 MHz
    .nr_running : 0
    --
    cpu#3, 2693.564 MHz
    .nr_running : 1
    --
    cpu#4, 2693.564 MHz
    .nr_running : 1
    --
    cpu#5, 2693.564 MHz
    .nr_running : 1
    --
    cpu#6, 2693.564 MHz
    .nr_running : 1
    --
    cpu#7, 2693.564 MHz
    .nr_running : 1
    --
    cpu#8, 2693.564 MHz
    .nr_running : 1
    --
    cpu#9, 2693.564 MHz
    .nr_running : 1
    --
    cpu#10, 2693.564 MHz
    .nr_running : 1
    --
    cpu#11, 2693.564 MHz
    .nr_running : 0
    --
    cpu#12, 2693.564 MHz
    .nr_running : 1
    --
    cpu#13, 2693.564 MHz
    .nr_running : 1
    --
    cpu#14, 2693.564 MHz
    .nr_running : 0
    --
    cpu#15, 2693.564 MHz
    .nr_running : 0
    --
    cpu#16, 2693.564 MHz
    .nr_running : 0
    --
    cpu#17, 2693.564 MHz
    .nr_running : 0
    --
    cpu#18, 2693.564 MHz
    .nr_running : 1
    --
    cpu#19, 2693.564 MHz
    .nr_running : 0
    --
    cpu#20, 2693.564 MHz
    .nr_running : 0
    --
    cpu#21, 2693.564 MHz
    .nr_running : 0
    --
    cpu#22, 2693.564 MHz
    .nr_running : 0
    --
    cpu#23, 2693.564 MHz
    .nr_running : 0
    --
    cpu#24, 2693.564 MHz
    .nr_running : 0
    --
    cpu#25, 2693.564 MHz
    .nr_running : 1
    --
    cpu#26, 2693.564 MHz
    .nr_running : 1
    --
    cpu#27, 2693.564 MHz
    .nr_running : 1
    --
    cpu#28, 2693.564 MHz
    .nr_running : 0
    --
    cpu#29, 2693.564 MHz
    .nr_running : 0
    --
    cpu#30, 2693.564 MHz
    .nr_running : 1
    --
    cpu#31, 2693.564 MHz
    .nr_running : 1

    From above, we notice 16 threads are distributed among 2 physical CPUs.
    After apply the patch, 16 threads are only distributed at one physical CPU.
    In this case, we can notice 30% power saving.
    Following are the result after apply the patch:

    cpu#0, 2693.384 MHz
    .nr_running : 1
    --
    cpu#1, 2693.384 MHz
    .nr_running : 1
    --
    cpu#2, 2693.384 MHz
    .nr_running : 1
    --
    cpu#3, 2693.384 MHz
    .nr_running : 1
    --
    cpu#4, 2693.384 MHz
    .nr_running : 1
    --
    cpu#5, 2693.384 MHz
    .nr_running : 1
    --
    cpu#6, 2693.384 MHz
    .nr_running : 1
    --
    cpu#7, 2693.384 MHz
    .nr_running : 1
    --
    cpu#8, 2693.384 MHz
    .nr_running : 0
    --
    cpu#9, 2693.384 MHz
    .nr_running : 0
    --
    cpu#10, 2693.384 MHz
    .nr_running : 0
    --
    cpu#11, 2693.384 MHz
    .nr_running : 0
    --
    cpu#12, 2693.384 MHz
    .nr_running : 0
    --
    cpu#13, 2693.384 MHz
    .nr_running : 0
    --
    cpu#14, 2693.384 MHz
    .nr_running : 0
    --
    cpu#15, 2693.384 MHz
    .nr_running : 0
    --
    cpu#16, 2693.384 MHz
    .nr_running : 1
    --
    cpu#17, 2693.384 MHz
    .nr_running : 1
    --
    cpu#18, 2693.384 MHz
    .nr_running : 1
    --
    cpu#19, 2693.384 MHz
    .nr_running : 1
    --
    cpu#20, 2693.384 MHz
    .nr_running : 1
    --
    cpu#21, 2693.384 MHz
    .nr_running : 1
    --
    cpu#22, 2693.384 MHz
    .nr_running : 1
    --
    cpu#23, 2693.384 MHz
    .nr_running : 1
    --
    cpu#24, 2693.384 MHz
    .nr_running : 0
    --
    cpu#25, 2693.384 MHz
    .nr_running : 0
    --
    cpu#26, 2693.384 MHz
    .nr_running : 0
    --
    cpu#27, 2693.384 MHz
    .nr_running : 0
    --
    cpu#28, 2693.384 MHz
    .nr_running : 0
    --
    cpu#29, 2693.384 MHz
    .nr_running : 0
    --
    cpu#30, 2693.384 MHz
    .nr_running : 0
    --
    cpu#31, 2693.384 MHz
    .nr_running : 1


    This patch will set SMT sibling power capability to SCHED_POWER_SCALE
    (1024) when sched_smt_power_savings set. So when there is possible do power
    saving during scheduling, scheduler will truly schedule processes as
    sched_smt_power_savings should do.


    Signed-off-by: Youquan Song <youquan.song@intel.com>
    Tested-by: Anhua Xu <anhua.xu@intel.com>
    ---
    kernel/sched/fair.c | 3 +++
    1 files changed, 3 insertions(+), 0 deletions(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index a4d2b7a..5be1d43 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -3715,6 +3715,9 @@ unsigned long default_scale_smt_power(struct sched_domain *sd, int cpu)
    unsigned long weight = sd->span_weight;
    unsigned long smt_gain = sd->smt_gain;

    + if (sched_smt_power_savings)
    + return SCHED_POWER_SCALE;
    +
    smt_gain /= weight;

    return smt_gain;
    --
    1.6.4.2


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