Messages in this thread Patch in this message | | | Date | Thu, 12 Jan 2023 14:26:03 +0000 | From | Qais Yousef <> | Subject | Re: [PATCH v2] sched/fair: unlink misfit task from cpu overutilized |
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Hi Vincent
On 12/28/22 17:54, Vincent Guittot wrote: > 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 small util_avg of > may not may not fit a high capacity cpu because of uclamp_min constraint.
Wouldn't it be better to split this into two patches
* Unlink/Decouple misfit ... * Unlink/Decouple util_fits_cpu from HMP
?
> > 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 performance requirement. > > 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 hint.
This part has nothing to do with the commit subject. I think it's better to split the patches if it's not too much work for you.
> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > > Change since v1: > - fix some wrong conditions > - take into account more cases > > kernel/sched/fair.c | 99 +++++++++++++++++++++++++++++++++------------ > 1 file changed, 74 insertions(+), 25 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 1649e7d71d24..57077f0a897e 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4582,8 +4582,7 @@ 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; > > /* > @@ -4618,8 +4617,8 @@ 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; > } > @@ -4629,7 +4628,7 @@ 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) > 0);
So the big difference between your approach and my approach is that task_fits_cpu() and asym_fits_cpu() now are very strict in regards to thermal pressure since with your approach we delegate the smartness to the caller.
Should we add a comment for these 2 users to make it obvious we intentionally ignore the '-1' value and why it is okay?
I'm not sure I can write a reasonable rationale myself. I'm actually worried this might subtly break decisions made by select_idle_capacity() or feec() when doing the LB.
Have you considered this?
> } > > static inline void update_misfit_status(struct task_struct *p, struct rq *rq) > @@ -6864,6 +6863,7 @@ 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 = 0; > int cpu, best_cpu = -1; > struct cpumask *cpus; > > @@ -6879,12 +6879,28 @@ 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 and performance requirements */ > + if (fits > 0) > return cpu; > + /* > + * Only the min performance (i.e. uclamp_min) doesn't fit. Look > + * for the CPU with highest performance capacity. > + */ > + else if (fits < 0) > + cpu_cap = capacity_orig_of(cpu) - thermal_load_avg(cpu_rq(cpu)); > > - if (cpu_cap > best_cap) { > + /* > + * First, select cpu which fits better (-1 being better than 0). > + * Then, select the one with largest capacity at same level. > + */ > + if ((fits < best_fits) || > + ((fits == best_fits) && (cpu_cap > best_cap))) { > best_cap = cpu_cap; > best_cpu = cpu; > + best_fits = fits; > } > } > > @@ -6897,7 +6913,7 @@ 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; > } > @@ -7264,6 +7280,9 @@ 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; > + unsigned long best_thermal_cap = 0; > + unsigned long prev_thermal_cap = 0; > struct sched_domain *sd; > struct perf_domain *pd; > struct energy_env eenv; > @@ -7299,6 +7318,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) > 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); > > @@ -7351,7 +7371,9 @@ 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); > @@ -7359,7 +7381,9 @@ 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 > @@ -7367,6 +7391,7 @@ 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; > } > } > > @@ -7385,26 +7410,50 @@ 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; > best_delta = min(best_delta, prev_delta); > } > > /* Evaluate the energy impact of using max_spare_cap_cpu. */ > if (max_spare_cap_cpu >= 0 && max_spare_cap > prev_spare_cap) { > + /* Current best energy cpu fits better */ > + if (max_fits < best_fits) > + continue; > + > + /* > + * Both don't fit performance (i.e. uclamp_min) but > + * best energy cpu has better performance. > + */ > + if ((max_fits < 0) && > + (cpu_thermal_cap <= best_thermal_cap)) > + continue; > + > cur_delta = compute_energy(&eenv, pd, cpus, p, > max_spare_cap_cpu); > /* CPU utilization has changed */ > if (cur_delta < base_energy) > goto unlock; > cur_delta -= base_energy; > - if (cur_delta < best_delta) { > - best_delta = cur_delta; > - best_energy_cpu = max_spare_cap_cpu; > - } > + > + /* > + * Both fit for the task but best energy cpu has lower > + * energy impact. > + */ > + if ((max_fits > 0) &&
Shouldn't this be
if ((max_fits > 0) && (max_fits == best_fits) && ?
We should update best_delta unconditionally first time we hit max_fits = 1, no?
I think it's worth extending the comment with something along the lines of
* ... except for the first time max_fits becomes 1 * then we must update best_delta unconditionally
> + (cur_delta >= best_delta)) > + continue; > + > + best_delta = cur_delta; > + best_energy_cpu = max_spare_cap_cpu; > + best_fits = max_fits; > + best_thermal_cap = cpu_thermal_cap; > } > } > rcu_read_unlock(); > > - if (best_delta < prev_delta) > + if ((best_fits > prev_fits) || > + ((best_fits > 0) && (best_delta < prev_delta)) || > + ((best_fits < 0) && (best_thermal_cap > prev_thermal_cap))) > target = best_energy_cpu;
Overall I think the approach is sound. I tested it on my pinebook pro and couldn't catch obvious breakage at least.
I am still worried though about spilling the knowledge outside of util_fits_cpu() is creating extra complexity in the callers and potentially more fragility when these callers evolve overtime e.g: task_fits_cpu()/asym_fits_cpu() gain a new user that must actually care about the -1 return value.
I think we can still optimize the capacity inversion logic to use no loops without having to spill the knowledge to the users/callers of util_fits_cpu(), no?
That said except for the few comments I had this LGTM anyway. Thanks for your effort!
Cheers
-- Qais Yousef
> > return target; > @@ -10228,24 +10277,23 @@ 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; > @@ -10258,6 +10306,7 @@ 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 >
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