by Qais Yousef

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

On 01/30/24 10:35, Vincent Guittot wrote:
> On Tue, 30 Jan 2024 at 01:50, Qais Yousef <[email protected]> wrote:
> >
> > On 01/30/24 00:26, Qais Yousef wrote:
> > > 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 <[email protected]>
> > > > Reviewed-by: Lukasz Luba <[email protected]>
> > > > ---
> > > > 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.
> >
> > It is done via the cooling device. And assume any limitations on freq due to
> > power etc are assumed to always to cause the policy->max to change.
> >
> > (sorry if I missed earlier discussions about this)
>
> I assume that you have answered all your questions.
>
> We have now 2 distinct signals:
> - hw high freq update which is averaged with PELT and go through
> topology_update_hw_pressure
> - cpufreq pressure which is not averaged (including cpufreq cooling
> device with patch 3)

Yes. I think a comment like suggested below is useful to help keeping the code
understandable to new comers. But FWIW

Reviewed-by: Qais Yousef <[email protected]>

>
> >
> > >
> > > 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
> > > >