Re: [PATCH v4 2/5] sched: Take cpufreq feedback into account

Qais Yousef
2024-01-30T00:26:56Z

On 01/09/24 17:46, Vincent Guittot wrote:
> Aggregate the different pressures applied on the capacity of CPUs and
> create a new function that returns the actual capacity of the CPU:
> get_actual_cpu_capacity()
>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
> ---
> kernel/sched/fair.c | 45 +++++++++++++++++++++++++--------------------
> 1 file changed, 25 insertions(+), 20 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9cc20855dc2b..e54bbf8b4936 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4910,13 +4910,22 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
> trace_sched_util_est_se_tp(&p->se);
> }
>
> +static inline unsigned long get_actual_cpu_capacity(int cpu)
> +{
> + unsigned long capacity = arch_scale_cpu_capacity(cpu);
> +
> + capacity -= max(thermal_load_avg(cpu_rq(cpu)), cpufreq_get_pressure(cpu));

Does cpufreq_get_pressure() reflect thermally throttled frequency, or just the
policy->max being capped by user etc? I didn't see an update to cpufreq when we
topology_update_hw_pressure(). Not sure if it'll go through another path.

maxing with thermal_load_avg() will change the behavior below where we used to
compare against instantaneous pressure. The concern was that it not just can
appear quickly, but disappear quickly too. thermal_load_avg() will decay
slowly, no? This means we'll lose a lot of opportunities for better task
placement until this decays which can take relatively long time.

So maxing handles the direction where a pressure suddenly appears. But it
doesn't handle where it disappears.

I suspect your thoughts are that if it was transient then thermal_load_avg()
should be small anyway - which I think makes sense.

I think we need a comment to explain these nuance differences.

> +
> + return capacity;
> +}
> +
> static inline int util_fits_cpu(unsigned long util,
> unsigned long uclamp_min,
> unsigned long uclamp_max,
> int cpu)
> {
> - unsigned long capacity_orig, capacity_orig_thermal;
> unsigned long capacity = capacity_of(cpu);
> + unsigned long capacity_orig;
> bool fits, uclamp_max_fits;
>
> /*
> @@ -4948,7 +4957,6 @@ static inline int util_fits_cpu(unsigned long util,
> * goal is to cap the task. So it's okay if it's getting less.
> */
> capacity_orig = arch_scale_cpu_capacity(cpu);
> - capacity_orig_thermal = capacity_orig - arch_scale_thermal_pressure(cpu);
>
> /*
> * We want to force a task to fit a cpu as implied by uclamp_max.
> @@ -5023,7 +5031,8 @@ static inline int util_fits_cpu(unsigned long util,
> * handle the case uclamp_min > uclamp_max.
> */
> uclamp_min = min(uclamp_min, uclamp_max);
> - if (fits && (util < uclamp_min) && (uclamp_min > capacity_orig_thermal))
> + if (fits && (util < uclamp_min) &&
> + (uclamp_min > get_actual_cpu_capacity(cpu)))
> return -1;
>
> return fits;
> @@ -7404,7 +7413,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> * Look for the CPU with best capacity.
> */
> else if (fits < 0)
> - cpu_cap = arch_scale_cpu_capacity(cpu) - thermal_load_avg(cpu_rq(cpu));
> + cpu_cap = get_actual_cpu_capacity(cpu);
>
> /*
> * First, select CPU which fits better (-1 being better than 0).
> @@ -7897,8 +7906,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> struct root_domain *rd = this_rq()->rd;
> int cpu, best_energy_cpu, target = -1;
> int prev_fits = -1, best_fits = -1;
> - unsigned long best_thermal_cap = 0;
> - unsigned long prev_thermal_cap = 0;
> + unsigned long best_actual_cap = 0;
> + unsigned long prev_actual_cap = 0;
> struct sched_domain *sd;
> struct perf_domain *pd;
> struct energy_env eenv;
> @@ -7928,7 +7937,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>
> for (; pd; pd = pd->next) {
> unsigned long util_min = p_util_min, util_max = p_util_max;
> - unsigned long cpu_cap, cpu_thermal_cap, util;
> + unsigned long cpu_cap, cpu_actual_cap, util;
> long prev_spare_cap = -1, max_spare_cap = -1;
> unsigned long rq_util_min, rq_util_max;
> unsigned long cur_delta, base_energy;
> @@ -7940,18 +7949,17 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> if (cpumask_empty(cpus))
> continue;
>
> - /* Account thermal pressure for the energy estimation */
> + /* Account external pressure for the energy estimation */
> cpu = cpumask_first(cpus);
> - cpu_thermal_cap = arch_scale_cpu_capacity(cpu);
> - cpu_thermal_cap -= arch_scale_thermal_pressure(cpu);
> + cpu_actual_cap = get_actual_cpu_capacity(cpu);
>
> - eenv.cpu_cap = cpu_thermal_cap;
> + eenv.cpu_cap = cpu_actual_cap;
> eenv.pd_cap = 0;
>
> for_each_cpu(cpu, cpus) {
> struct rq *rq = cpu_rq(cpu);
>
> - eenv.pd_cap += cpu_thermal_cap;
> + eenv.pd_cap += cpu_actual_cap;
>
> if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
> continue;
> @@ -8022,7 +8030,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> if (prev_delta < base_energy)
> goto unlock;
> prev_delta -= base_energy;
> - prev_thermal_cap = cpu_thermal_cap;
> + prev_actual_cap = cpu_actual_cap;
> best_delta = min(best_delta, prev_delta);
> }
>
> @@ -8037,7 +8045,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> * but best energy cpu has better capacity.
> */
> if ((max_fits < 0) &&
> - (cpu_thermal_cap <= best_thermal_cap))
> + (cpu_actual_cap <= best_actual_cap))
> continue;
>
> cur_delta = compute_energy(&eenv, pd, cpus, p,
> @@ -8058,14 +8066,14 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> best_delta = cur_delta;
> best_energy_cpu = max_spare_cap_cpu;
> best_fits = max_fits;
> - best_thermal_cap = cpu_thermal_cap;
> + best_actual_cap = cpu_actual_cap;
> }
> }
> rcu_read_unlock();
>
> if ((best_fits > prev_fits) ||
> ((best_fits > 0) && (best_delta < prev_delta)) ||
> - ((best_fits < 0) && (best_thermal_cap > prev_thermal_cap)))
> + ((best_fits < 0) && (best_actual_cap > prev_actual_cap)))
> target = best_energy_cpu;
>
> return target;
> @@ -9441,8 +9449,8 @@ static inline void init_sd_lb_stats(struct sd_lb_stats *sds)
>
> static unsigned long scale_rt_capacity(int cpu)
> {
> + unsigned long max = get_actual_cpu_capacity(cpu);
> struct rq *rq = cpu_rq(cpu);
> - unsigned long max = arch_scale_cpu_capacity(cpu);
> unsigned long used, free;
> unsigned long irq;
>
> @@ -9454,12 +9462,9 @@ static unsigned long scale_rt_capacity(int cpu)
> /*
> * avg_rt.util_avg and avg_dl.util_avg track binary signals
> * (running and not running) with weights 0 and 1024 respectively.
> - * avg_thermal.load_avg tracks thermal pressure and the weighted
> - * average uses the actual delta max capacity(load).
> */
> used = READ_ONCE(rq->avg_rt.util_avg);
> used += READ_ONCE(rq->avg_dl.util_avg);
> - used += thermal_load_avg(rq);
>
> if (unlikely(used >= max))
> return 1;
> --
> 2.34.1
>