Messages in this thread | | | From | Vincent Guittot <> | Date | Tue, 29 Aug 2023 16:10:46 +0200 | Subject | Re: [PATCH] sched/fair: Check a task has a fitting cpu when updating misfit |
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On Sun, 20 Aug 2023 at 22:34, Qais Yousef <qyousef@layalina.io> wrote: > > From: Qais Yousef <qais.yousef@arm.com> > > If a misfit task is affined to a subset of the possible cpus, we need to > verify that one of these cpus can fit it. Otherwise the load balancer > code will continuously trigger needlessly leading the balance_interval > to increase in return and eventually end up with a situation where real > imbalances take a long time to address because of this impossible > imbalance situation. > > This can happen in Android world where it's common for background tasks > to be restricted to little cores. > > Similarly if we can't fit the biggest core, triggering misfit is > pointless as it is the best we can ever get on this system. > > To speed the search up, don't call task_fits_cpu() which will repeatedly > call uclamp_eff_value() for the same task. Call util_fits_cpu() instead. > And only do so when we see a cpu with higher capacity level than > passed cpu_of(rq). > > Signed-off-by: Qais Yousef <qais.yousef@arm.com> > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io> > --- > kernel/sched/fair.c | 50 ++++++++++++++++++++++++++++++++++++++------- > 1 file changed, 43 insertions(+), 7 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 0b7445cd5af9..f08c5f3bf895 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4853,17 +4853,50 @@ static inline int task_fits_cpu(struct task_struct *p, int cpu) > > static inline void update_misfit_status(struct task_struct *p, struct rq *rq) > { > + unsigned long uclamp_min, uclamp_max; > + unsigned long util, cap_level; > + bool has_fitting_cpu = false; > + int cpu = cpu_of(rq); > + > if (!sched_asym_cpucap_active()) > return; > > - if (!p || p->nr_cpus_allowed == 1) { > - rq->misfit_task_load = 0; > - return; > - } > + if (!p || p->nr_cpus_allowed == 1) > + goto out; > > - if (task_fits_cpu(p, cpu_of(rq))) { > - rq->misfit_task_load = 0; > - return; > + uclamp_min = uclamp_eff_value(p, UCLAMP_MIN); > + uclamp_max = uclamp_eff_value(p, UCLAMP_MAX); > + util = task_util_est(p); > + > + if (util_fits_cpu(util, uclamp_min, uclamp_max, cpu) > 0) > + goto out; > + > + cap_level = capacity_orig_of(cpu); > + > + /* If we can't fit the biggest CPU, that's the best we can ever get. */ > + if (cap_level == SCHED_CAPACITY_SCALE) > + goto out; > + > + /* > + * If the task affinity is not set to default, make sure it is not > + * restricted to a subset where no CPU can ever fit it. Triggering > + * misfit in this case is pointless as it has no where better to move > + * to. And it can lead to balance_interval to grow too high as we'll > + * continuously fail to move it anywhere. > + */ > + if (!cpumask_equal(p->cpus_ptr, cpu_possible_mask)) { > + for_each_cpu(cpu, p->cpus_ptr) {
I haven't looked at the problem in detail and at other possibilities so far but for_each_cpu doesn't scale and update_misfit_status() being called in pick_next_task_fair() so you must find another way to detect this
> + if (cap_level < capacity_orig_of(cpu)) { > + cap_level = capacity_orig_of(cpu); > + if (util_fits_cpu(util, uclamp_min, uclamp_max, cpu) > 0) { > + has_fitting_cpu = true; > + break; > + } > + } > + } > + > + if (!has_fitting_cpu) > + goto out; > } > > /* > @@ -4871,6 +4904,9 @@ static inline void update_misfit_status(struct task_struct *p, struct rq *rq) > * task_h_load() returns 0. > */ > rq->misfit_task_load = max_t(unsigned long, task_h_load(p), 1); > + return; > +out: > + rq->misfit_task_load = 0; > } > > #else /* CONFIG_SMP */ > -- > 2.34.1 >
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