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

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Date:   Thu, 7 Nov 2019 11:49:01 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Thara Gopinath <thara.gopinath@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, ionela.voinescu@....com,
        rui.zhang@...el.com, edubezval@...il.com, qperret@...gle.com,
        linux-kernel@...r.kernel.org, amit.kachhap@...il.com,
        javi.merino@...nel.org, daniel.lezcano@...aro.org
Subject: Re: [Patch v5 6/6] sched/fair: Enable tuning of decay period

Le Tuesday 05 Nov 2019 à 13:49:46 (-0500), Thara Gopinath a écrit :
> 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@...aro.org>
> ---
> 
> 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");
> +
> +	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)));

Would be better to create

+static inline u64 rq_clock_thermal(struct rq *rq)
+{
+       lockdep_assert_held(&rq->lock);
+       assert_clock_updated(rq);
+
+       return rq_clock_task(rq) >> sched_thermal_decay_shift;
+}
+

and use it when calling update_thermal_load_avg(rq_clock_thermal(rq)...


>  #endif
>  }
>  
> -- 
> 2.1.4
>