linux-kernel - Re: [PATCH 1/2] sched: introduce helper function to calculate distribution over sched class

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Date: Wed, 21 Feb 2024 18:51:23 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: "zhaoyang.huang" <zhaoyang.huang@...soc.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Juri Lelli <juri.lelli@...hat.com>, Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Zhaoyang Huang <huangzhaoyang@...il.com>, 
	steve.kang@...soc.com
Subject: Re: [PATCH 1/2] sched: introduce helper function to calculate
 distribution over sched class

On Tue, 20 Feb 2024 at 07:16, zhaoyang.huang <zhaoyang.huang@...soc.com> wrote:
>
> From: Zhaoyang Huang <zhaoyang.huang@...soc.com>
>
> As RT, DL, IRQ time could be deemed as lost time of CFS's task, some

It's lost only if cfs has been actually preempted

> timing value want to know the distribution of how these spread
> approximately by using utilization account value (nivcsw is not enough
> sometimes). This commit would like to introduce a helper function to
> achieve this goal.
>
> eg.
> Effective part of A = Total_time * cpu_util_cfs / cpu_util
>
> Timing value A
> (should be a process last for several TICKs or statistics of a repeadted
> process)
>
> Timing start
> |
> |
> preempted by RT, DL or IRQ
> |\
> | This period time is nonvoluntary CPU give up, need to know how long
> |/

preempted means that a cfs task stops running on the cpu and lets
another rt/dl task or an irq run on the cpu instead. We can't know
that. We know an average ratio of time spent in rt/dl and irq contexts
but not if the cpu was idle or running cfs task

> sched in again
> |
> |
> |
> Timing end
>
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> ---
>  include/linux/sched.h |  1 +
>  kernel/sched/core.c   | 20 ++++++++++++++++++++
>  2 files changed, 21 insertions(+)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 77f01ac385f7..99cf09c47f72 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2318,6 +2318,7 @@ static inline bool owner_on_cpu(struct task_struct *owner)
>
>  /* Returns effective CPU energy utilization, as seen by the scheduler */
>  unsigned long sched_cpu_util(int cpu);
> +unsigned long cfs_prop_by_util(struct task_struct *tsk, unsigned long val);
>  #endif /* CONFIG_SMP */
>
>  #ifdef CONFIG_RSEQ
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 802551e0009b..217e2220fdc1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7494,6 +7494,26 @@ unsigned long sched_cpu_util(int cpu)
>  {
>         return effective_cpu_util(cpu, cpu_util_cfs(cpu), ENERGY_UTIL, NULL);
>  }
> +
> +/*
> + * Calculate the approximate proportion of timing value consumed in cfs.
> + * The user must be aware of this is done by avg_util which is tracked by
> + * the geometric series as decaying the load by y^32 = 0.5 (unit is 1ms).
> + * That is, only the period last for at least several TICKs or the statistics
> + * of repeated timing value are suitable for this helper function.
> + */
> +unsigned long cfs_prop_by_util(struct task_struct *tsk, unsigned long val)
> +{
> +       unsigned int cpu = task_cpu(tsk);
> +       struct rq *rq = cpu_rq(cpu);
> +       unsigned long util;
> +
> +       if (tsk->sched_class != &fair_sched_class)
> +               return val;
> +       util = cpu_util_rt(rq) + cpu_util_cfs(cpu) + cpu_util_irq(rq) + cpu_util_dl(rq);

This is not correct as irq is not on the same clock domain: look at
effective_cpu_util()

You don't care about idle time ?

> +       return min(val, cpu_util_cfs(cpu) * val / util);
> +}
> +
>  #endif /* CONFIG_SMP */
>
>  /**
> --
> 2.25.1
>