Re: [RFC PATCH 3/3] sched/fair: Traverse cpufreq policies to detect capacity inversion

[Vincent Guittot]

On Fri, 23 Dec 2022 at 12:58, Qais Yousef <qyousef@xxxxxxxxxxx> wrote:
>
> On 12/20/22 14:50, Vincent Guittot wrote:
>
> Thanks for the patch!
>
> > Hereafter is what I came with in order to decouple misfit task with cpu
> > overutilized. We keep using util_fits_cpu but with 3 values so we can keep
> > using it with cpu_overutilized but exclude the case of misfit task
> > because uclmap_min. Also select_idle_capacity() and feec() keep selecting the
> > big cpu even if it doesn't fit only because of uclamp_min
> >
> >
> > Subject: [PATCH] sched/fair: unlink misfit task from cpu overutilized
> >
> > By taking into account uclamp_min, the 1:1 relation between task misfit and
> > cpu overutilized is no more true as a task with a util_avg of 20as an
> > example may not fit a 1024 capacity cpu because of a uclamp_min constraint.
> >
> > Add a new state in util_fits_cpu() to reflect the case that task would fit
> > a CPU except for the uclamp_min hint which is a bandwidth requriement.
>
> nit: mixing uclamp with bandwidth has been a source of a lot of confusion when
> discussing uclamp. Can we use performance requirement instead please?

ok

>
> >
> > Use -1 to reflect that a CPU doesn't fit only because of uclamp_min so we
> > can use this new value to take additional action to select the best cpu
> > that doesn't match uclamp_min.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > ---
> >  kernel/sched/fair.c | 73 ++++++++++++++++++++++++++++++---------------
> >  1 file changed, 49 insertions(+), 24 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 4423681baf15..705335d6af65 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4578,8 +4578,7 @@ static inline int util_fits_cpu(unsigned long util,
> >        *     2. The system is being saturated when we're operating near
> >        *        max capacity, it doesn't make sense to block overutilized.
> >        */
> > -     uclamp_max_fits = (capacity_orig == SCHED_CAPACITY_SCALE) && (uclamp_max == SCHED_CAPACITY_SCALE);
> > -     uclamp_max_fits = !uclamp_max_fits && (uclamp_max <= capacity_orig);
> > +     uclamp_max_fits = (uclamp_max <= capacity_orig) || (capacity_orig == SCHED_CAPACITY_SCALE);
> >       fits = fits || uclamp_max_fits;
> >
> >       /*
> > @@ -4614,8 +4613,8 @@ static inline int util_fits_cpu(unsigned long util,
> >        * handle the case uclamp_min > uclamp_max.
> >        */
> >       uclamp_min = min(uclamp_min, uclamp_max);
> > -     if (util < uclamp_min && capacity_orig != SCHED_CAPACITY_SCALE)
> > -             fits = fits && (uclamp_min <= capacity_orig_thermal);
> > +     if (fits && (util < uclamp_min) && (uclamp_min > capacity_orig_thermal))
> > +             return -1;
> >
> >       return fits;
>
> nit: return !!fits?
>
> We check explicitly == 1 below and I'm not sure all the boolean check above
> will guarantee we will end up return 1 for true on all combination of
> compilerls/archs.
>
> >  }
> > @@ -4625,7 +4624,7 @@ static inline int task_fits_cpu(struct task_struct *p, int cpu)
> >       unsigned long uclamp_min = uclamp_eff_value(p, UCLAMP_MIN);
> >       unsigned long uclamp_max = uclamp_eff_value(p, UCLAMP_MAX);
> >       unsigned long util = task_util_est(p);
> > -     return util_fits_cpu(util, uclamp_min, uclamp_max, cpu);
> > +     return (util_fits_cpu(util, uclamp_min, uclamp_max, cpu) == 1);
>
> Or make this >  0?

yes, will use > 0

>
> >  }
> >
> >  static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
> > @@ -6064,7 +6063,10 @@ static inline void hrtick_update(struct rq *rq)
> >  #ifdef CONFIG_SMP
> >  static inline bool cpu_overutilized(int cpu)
> >  {
> > -     return !fits_capacity(cpu_util_cfs(cpu), capacity_of(cpu));
> > +     unsigned long rq_util_min = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MIN);
> > +     unsigned long rq_util_max = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MAX);
> > +
> > +     return !util_fits_cpu(cpu_util_cfs(cpu), rq_util_min, rq_util_max, cpu);
> >  }
> >
> >  static inline void update_overutilized_status(struct rq *rq)
> > @@ -6857,6 +6859,7 @@ static int
> >  select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> >  {
> >       unsigned long task_util, util_min, util_max, best_cap = 0;
> > +     int fits, best_fits = -1;
> >       int cpu, best_cpu = -1;
> >       struct cpumask *cpus;
> >
> > @@ -6872,12 +6875,24 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> >
> >               if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu))
> >                       continue;
> > -             if (util_fits_cpu(task_util, util_min, util_max, cpu))
> > +
> > +             fits = util_fits_cpu(task_util, util_min, util_max, cpu);
> > +
> > +             /* This cpu fits with all capacity requirements */
>
> nit: s#capacity#capacity & performance#?
>
> > +             if (fits > 0)
> >                       return cpu;
> > +             /*
> > +              * Only the min bandwidth (i.e. uclamp_min) doesn't fit. Look
> > +              * for the cpu with highest bandwidth capacity.
> > +              */
>
> s/bandwidth/performance/?
>
> > +             else if (fits < 0)
> > +                     cpu_cap = capacity_of(cpu) - thermal_load_avg(cpu_rq(cpu));
>
> Hmm. Isn't capacity_of() already takes into account thermal_load_avg()?
>
> Did you mean capacity_orig_of()?

Yes

>
> >
> > -             if (cpu_cap > best_cap) {
> > +             if ((fits > best_fits) ||
> > +                 ((fits == best_fits) && (cpu_cap > best_cap))) {
> >                       best_cap = cpu_cap;
> >                       best_cpu = cpu;
> > +                     best_fits = fits;
>
> I'm not sure if this logic is correct. It's a bit of a mind  bender.
>
>         @iter#0
>
>                 fits <= 0
>                 best_fits <= -1
>
>                 if (fits > best_fits) // 0 > -1 => True
>                         ...     // update best_cap if larger
>                         best_fits <= 0
>
>         @iter#1
>
>                 fits <= -1
>                 best_fits <= 0
>
>                 if (fits > best_fits) // -1 > 0 => False
>
>                 if (fits == best_fits) // -1 == 0 => False
>
>                 // We will never update best_cap for all fits = -1 after
>                 // encountering the first fits = 0
>
> I think we should reverse the initial values and split the conditions

The copy/paste from feec() was too quick. It should be :

+             if ((fits < best_fits) ||
+                 ((fits == best_fits) && (cpu_cap > best_cap))) {

I don't think that the split gives any benefit but makes it more
difficult to read. I will add a comment
/*
* Select the CPU which fits better first (-1 being better than 0).
* Then, select the one with the largest capacity at the same level.
*/

>
>         int fits, best_fits = 0;
>
>                 if ((fits < best_fits)) {
>                         /* Reset best_cap for first "fits_but" */
>                         best_cap = cpu_cap;
>                         best_cpu = cpu;
>                         best_fits = fits;
>                 } else if ((fits == best_fits) && (cpu_cap > best_cap))) {
>                         best_cap = cpu_cap;
>                         best_cpu = cpu;
>                 }
>
> Which give us
>
>         @iter#0
>
>                 fits <= 0
>                 best_fits <= 0
>
>                 if (fits < best_fits) // 0 < 0 => False
>
>                 if (fits == best_fits) // 0 == 0 => True
>                         ...     // update best_cap if larger
>
>         @iter#1
>
>                 fits <= -1
>                 best_fits <= 0
>
>                 if (fits < best_fits) // -1 < 0 => True
>                         ...     // reset best_cap to first "fits_but" hit
>                         best_fits <= -1
>
>         @iter#2
>
>                 fits <= 0
>                 best_fits <= -1
>
>                 if (fits < best_fits) // 0 < -1 => False
>
>                 if (fits == best_fits) // 0 == -1 => False
>
>                 // We should never update best_cap for all fits == 0 now
>
>         @iter#3
>
>                 fits <= -1
>                 best_fits <= -1
>
>                 if (fits < best_fits) // -1 < -1 => False
>
>                 if (fits == best_fits) // -1 == -1 => True
>                         ...     // update best_cap if larger
>
>                 // Only fits = -1 will update best_cap if larger now
>
> Of course any hit with fits = 1 will return the cpu immediately.
>
>
> >               }
> >       }
> >
> > @@ -6890,7 +6905,7 @@ static inline bool asym_fits_cpu(unsigned long util,
> >                                int cpu)
> >  {
> >       if (sched_asym_cpucap_active())
> > -             return util_fits_cpu(util, util_min, util_max, cpu);
> > +             return (util_fits_cpu(util, util_min, util_max, cpu) > 0);
> >
> >       return true;
> >  }
> > @@ -7257,6 +7272,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >       unsigned long p_util_max = uclamp_is_used() ? uclamp_eff_value(p, UCLAMP_MAX) : 1024;
> >       struct root_domain *rd = this_rq()->rd;
> >       int cpu, best_energy_cpu, target = -1;
> > +     int prev_fits = -1, best_fits = -1;
> >       struct sched_domain *sd;
> >       struct perf_domain *pd;
> >       struct energy_env eenv;
> > @@ -7288,10 +7304,11 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >               unsigned long cpu_cap, cpu_thermal_cap, util;
> >               unsigned long cur_delta, max_spare_cap = 0;
> >               unsigned long rq_util_min, rq_util_max;
> > -             unsigned long util_min, util_max;
> > +             unsigned long util_min = 0, util_max = 1024;
>
> Why this change? Are you hitting the same warning reported by Dan?

While debugging, I got random util_min|max values passed to
util_fits_cpu(). I agree that this is not a real problem because it
means that !uclamp_is_used() and the values will not be used in
util_fits_cpu() in this case but this is a hidden dependency which
seems a bit weak.

I can probably remove it from this patch as it's out of the scope

>
> >               unsigned long prev_spare_cap = 0;
> >               int max_spare_cap_cpu = -1;
> >               unsigned long base_energy;
> > +             int fits, max_fits = -1;
> >
> >               cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);
> >
> > @@ -7344,7 +7361,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >                                       util_max = max(rq_util_max, p_util_max);
> >                               }
> >                       }
> > -                     if (!util_fits_cpu(util, util_min, util_max, cpu))
> > +
> > +                     fits = util_fits_cpu(util, util_min, util_max, cpu);
> > +                     if (!fits)
> >                               continue;
> >
> >                       lsub_positive(&cpu_cap, util);
> > @@ -7352,7 +7371,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >                       if (cpu == prev_cpu) {
> >                               /* Always use prev_cpu as a candidate. */
> >                               prev_spare_cap = cpu_cap;
> > -                     } else if (cpu_cap > max_spare_cap) {
> > +                             prev_fits = fits;
> > +                     } else if ((fits > max_fits) ||
> > +                                ((fits == max_fits) && (cpu_cap > max_spare_cap))) {
> >                               /*
> >                                * Find the CPU with the maximum spare capacity
> >                                * among the remaining CPUs in the performance
> > @@ -7360,6 +7381,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >                                */
> >                               max_spare_cap = cpu_cap;
> >                               max_spare_cap_cpu = cpu;
> > +                             max_fits = fits;
>
> Should we reset best_delta here?
>
> Because we update max_fits here..
>
> >                       }
> >               }
> >
> > @@ -7389,15 +7411,18 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >                       if (cur_delta < base_energy)
> >                               goto unlock;
> >                       cur_delta -= base_energy;
> > -                     if (cur_delta < best_delta) {
> > +                     if ((fits > max_fits) ||
> > +                         ((fits == max_fits) && (cur_delta < best_delta))) {
>
> .. on first first transitions from -1 to 1; this condition will be
> skipped if cur_delta is lower than best delta. best_delta here could be the
> previous -1 fitting cpu.

But we want a cpu that fits in priority then the one with the smallest delta.

>
> We should reset best_delta on first transition then look if we encounter
> something with a better delta?

my mistake... This should be

+                     if ((max_fits > best_fits) ||
+                         ((max_fits == best_fits) && (cur_delta <
best_delta))) {

I'm going to prepare a new version

>
>
> Thanks!
>
> --
> Qais Yousef
>
> >                               best_delta = cur_delta;
> >                               best_energy_cpu = max_spare_cap_cpu;
> > +                             best_fits = max_fits;
> >                       }
> >               }
> >       }
> >       rcu_read_unlock();
> >
> > -     if (best_delta < prev_delta)
> > +     if ((best_fits > prev_fits) ||
> > +         ((best_fits == prev_fits) && (best_delta < prev_delta)))
> >               target = best_energy_cpu;
> >
> >       return target;
> > @@ -10164,24 +10189,23 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
> >        */
> >       update_sd_lb_stats(env, &sds);
> >
> > -     if (sched_energy_enabled()) {
> > -             struct root_domain *rd = env->dst_rq->rd;
> > -
> > -             if (rcu_dereference(rd->pd) && !READ_ONCE(rd->overutilized))
> > -                     goto out_balanced;
> > -     }
> > -
> > -     local = &sds.local_stat;
> > -     busiest = &sds.busiest_stat;
> > -
> >       /* There is no busy sibling group to pull tasks from */
> >       if (!sds.busiest)
> >               goto out_balanced;
> >
> > +     busiest = &sds.busiest_stat;
> > +
> >       /* Misfit tasks should be dealt with regardless of the avg load */
> >       if (busiest->group_type == group_misfit_task)
> >               goto force_balance;
> >
> > +     if (sched_energy_enabled()) {
> > +             struct root_domain *rd = env->dst_rq->rd;
> > +
> > +             if (rcu_dereference(rd->pd) && !READ_ONCE(rd->overutilized))
> > +                     goto out_balanced;
> > +     }
> > +
> >       /* ASYM feature bypasses nice load balance check */
> >       if (busiest->group_type == group_asym_packing)
> >               goto force_balance;
> > @@ -10194,6 +10218,7 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
> >       if (busiest->group_type == group_imbalanced)
> >               goto force_balance;
> >
> > +     local = &sds.local_stat;
> >       /*
> >        * If the local group is busier than the selected busiest group
> >        * don't try and pull any tasks.
> > --
> > 2.17.1
> >
> >
> >
> > >
> > >
> > > Thanks!!
> > >
> > > --
> > > Qais Yousef