Re: [Patch v5 6/6] sched/fair: Enable tuning of decay period

[Amit Kucheria]

On Wed, Nov 6, 2019 at 12:20 AM Thara Gopinath
<thara.gopinath@xxxxxxxxxx> wrote:
>
> Thermal pressure follows pelt signas which means the
> decay period for thermal pressure is the default pelt
> decay period. Depending on soc charecteristics and thermal
> activity, it might be beneficial to decay thermal pressure
> slower, but still in-tune with the pelt signals.
> One way to achieve this is to provide a command line parameter
> to set a decay shift parameter to an integer between 0 and 10.
>
> Signed-off-by: Thara Gopinath <thara.gopinath@xxxxxxxxxx>
> ---
>
> v4->v5:
>         - Changed _coeff to _shift as per review comments on the list.
>
>  Documentation/admin-guide/kernel-parameters.txt |  5 +++++
>  kernel/sched/fair.c                             | 25 +++++++++++++++++++++++--
>  2 files changed, 28 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index c82f87c..0b8f55e 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4281,6 +4281,11 @@
>                         incurs a small amount of overhead in the scheduler
>                         but is useful for debugging and performance tuning.
>
> +       sched_thermal_decay_shift=
> +                       [KNL, SMP] Set decay shift for thermal pressure signal.
> +                       Format: integer betweer 0 and 10
> +                       Default is 0.
> +
>         skew_tick=      [KNL] Offset the periodic timer tick per cpu to mitigate
>                         xtime_lock contention on larger systems, and/or RCU lock
>                         contention on all systems with CONFIG_MAXSMP set.
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5f6c371..61a020b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -91,6 +91,18 @@ const_debug unsigned int sysctl_sched_migration_cost = 500000UL;
>   * and maximum available capacity due to thermal events.
>   */
>  static DEFINE_PER_CPU(unsigned long, thermal_pressure);
> +/**
> + * By default the decay is the default pelt decay period.
> + * The decay shift can change the decay period in
> + * multiples of 32.
> + *  Decay shift                Decay period(ms)
> + *     0                       32
> + *     1                       64
> + *     2                       128
> + *     3                       256
> + *     4                       512
> + */
> +static int sched_thermal_decay_shift;
>
>  static void trigger_thermal_pressure_average(struct rq *rq);
>
> @@ -10435,6 +10447,15 @@ void update_thermal_pressure(int cpu, unsigned long capped_capacity)
>         delta = arch_scale_cpu_capacity(cpu) - capped_capacity;
>         per_cpu(thermal_pressure, cpu) = delta;
>  }
> +
> +static int __init setup_sched_thermal_decay_shift(char *str)
> +{
> +       if (kstrtoint(str, 0, &sched_thermal_decay_shift))
> +               pr_warn("Unable to set scheduler thermal pressure decay shift parameter\n");

You're reading straight from the cmdline into a kernel variable w/o
any bounds checking. Perhaps use clamp or clamp_val to make sure it is
between 0 and 10?


> +
> +       return 1;
> +}
> +__setup("sched_thermal_decay_shift=", setup_sched_thermal_decay_shift);
>  #endif
>
>  /**
> @@ -10444,8 +10465,8 @@ void update_thermal_pressure(int cpu, unsigned long capped_capacity)
>  static void trigger_thermal_pressure_average(struct rq *rq)
>  {
>  #ifdef CONFIG_SMP
> -       update_thermal_load_avg(rq_clock_task(rq), rq,
> -                               per_cpu(thermal_pressure, cpu_of(rq)));
> +       update_thermal_load_avg(rq_clock_task(rq) >> sched_thermal_decay_shift,
> +                               rq, per_cpu(thermal_pressure, cpu_of(rq)));
>  #endif
>  }
>
> --
> 2.1.4
>