Re: [PATCH v7 00/11] track CPU utilization

[Viresh Kumar]

On 05-07-18, 14:36, Peter Zijlstra wrote:
> Subject: sched/cpufreq: Clarify sugov_get_util()
> 
> Add a few comments (hopefully) clarifying some of the magic in
> sugov_get_util().
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  cpufreq_schedutil.c |   69 ++++++++++++++++++++++++++++++++++++++--------------
>  1 file changed, 51 insertions(+), 18 deletions(-)
> 
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -177,6 +177,26 @@ static unsigned int get_next_freq(struct
>  	return cpufreq_driver_resolve_freq(policy, freq);
>  }
>  
> +/*
> + * This function computes an effective utilization for the given CPU, to be
> + * used for frequency selection given the linear relation: f = u * f_max.
> + *
> + * The scheduler tracks the following metrics:
> + *
> + *   cpu_util_{cfs,rt,dl,irq}()
> + *   cpu_bw_dl()
> + *
> + * Where the cfs,rt and dl util numbers are tracked with the same metric and
> + * synchronized windows and are thus directly comparable.
> + *
> + * The cfs,rt,dl utilization are the running times measured with rq->clock_task
> + * which excludes things like IRQ and steal-time. These latter are then accrued in
> + * the irq utilization.
> + *
> + * The DL bandwidth number otoh is not a measured meric but a value computed

                                                     metric

> + * based on the task model parameters and gives the minimal u required to meet

                                                               u ?

> + * deadlines.
> + */
>  static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu)
>  {
>  	struct rq *rq = cpu_rq(sg_cpu->cpu);
> @@ -188,26 +208,50 @@ static unsigned long sugov_get_util(stru
>  	if (rt_rq_is_runnable(&rq->rt))
>  		return max;
>  
> +	/*
> +	 * Early check to see if IRQ/steal time saturates the CPU, can be
> +	 * because of inaccuracies in how we track these -- see
> +	 * update_irq_load_avg().
> +	 */
>  	irq = cpu_util_irq(rq);
> -
>  	if (unlikely(irq >= max))
>  		return max;
>  
> -	/* Sum rq utilization */
> +	/*
> +	 * Because the time spend on RT/DL tasks is visible as 'lost' time to
> +	 * CFS tasks and we use the same metric to track the effective
> +	 * utilization (PELT windows are synchronized) we can directly add them
> +	 * to obtain the CPU's actual utilization.
> +	 */
>  	util = cpu_util_cfs(rq);
>  	util += cpu_util_rt(rq);
>  
>  	/*
> -	 * Interrupt time is not seen by rqs utilization nso we can compare
> -	 * them with the CPU capacity
> +	 * We do not make cpu_util_dl() a permanent part of this sum because we
> +	 * want to use cpu_bw_dl() later on, but we need to check if the
> +	 * CFS+RT+DL sum is saturated (ie. no idle time) such that we select
> +	 * f_max when there is no idle time.
> +	 *
> +	 * NOTE: numerical errors or stop class might cause us to not quite hit
> +	 * saturation when we should -- something for later.
>  	 */
>  	if ((util + cpu_util_dl(rq)) >= max)
>  		return max;
>  
>  	/*
> -	 * As there is still idle time on the CPU, we need to compute the
> -	 * utilization level of the CPU.
> +	 * There is still idle time; further improve the number by using the
> +	 * irq metric. Because IRQ/steal time is hidden from the task clock we
> +	 * need to scale the task numbers:
>  	 *
> +	 *              1 - irq
> +	 *   U' = irq + ------- * U
> +	 *                max
> +	 */
> +	util *= (max - irq);
> +	util /= max;
> +	util += irq;
> +
> +	/*
>  	 * Bandwidth required by DEADLINE must always be granted while, for
>  	 * FAIR and RT, we use blocked utilization of IDLE CPUs as a mechanism
>  	 * to gracefully reduce the frequency when no tasks show up for longer
> @@ -217,18 +261,7 @@ static unsigned long sugov_get_util(stru
>  	 * util_cfs + util_dl as requested freq. However, cpufreq is not yet
>  	 * ready for such an interface. So, we only do the latter for now.
>  	 */
> -
> -	/* Weight rqs utilization to normal context window */
> -	util *= (max - irq);
> -	util /= max;
> -
> -	/* Add interrupt utilization */
> -	util += irq;
> -
> -	/* Add DL bandwidth requirement */
> -	util += sg_cpu->bw_dl;
> -
> -	return min(max, util);
> +	return min(max, util + sg_cpu->bw_dl);
>  }
>  

Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>

-- 
viresh