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Date: Thu, 7 Jul 2022 18:42:48 +0200 From: Dietmar Eggemann <dietmar.eggemann@....com> To: Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com, linux-kernel@...r.kernel.org, david.chen@...anix.com, zhangqiao22@...wei.com Subject: Re: [PATCH] sched/fair: fix case with reduced capacity CPU On 02/07/2022 06:52, Vincent Guittot wrote: > The capacity of the CPU available for CFS tasks can be reduced because of > other activities running on the latter. In such case, it's worth trying to > move CFS tasks on a CPU with more available capacity. > > The rework of the load balance has filterd the case when the CPU is > classified to be fully busy but its capacity is reduced. > > Check if CPU's capacity is reduced while gathering load balance statistics > and classify it group_misfit_task instead of group_fully_busy so we can > try to move the load on another CPU. > > Reported-by: David Chen <david.chen@...anix.com> > Reported-by: Zhang Qiao <zhangqiao22@...wei.com> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org> > --- > > David, Zhang, > > I haven't put your tested-by because I have reworked and cleaned the patch to > cover more cases. > > Could you run some tests with this version ? > > Thanks > > kernel/sched/fair.c | 50 ++++++++++++++++++++++++++++++++++++--------- > 1 file changed, 40 insertions(+), 10 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index a78d2e3b9d49..126b82ef4279 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -8798,6 +8798,19 @@ sched_asym(struct lb_env *env, struct sd_lb_stats *sds, struct sg_lb_stats *sgs > return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu); > } > > +static inline bool > +sched_reduced_capacity(struct rq *rq, struct sched_domain *sd) > +{ > + /* > + * When there is more than 1 task, the group_overloaded case already > + * takes care of cpu with reduced capacity > + */ > + if (rq->cfs.h_nr_running != 1) > + return false; > + > + return check_cpu_capacity(rq, sd); > +} > + > /** > * update_sg_lb_stats - Update sched_group's statistics for load balancing. > * @env: The load balancing environment. > @@ -8820,8 +8833,9 @@ static inline void update_sg_lb_stats(struct lb_env *env, > > for_each_cpu_and(i, sched_group_span(group), env->cpus) { > struct rq *rq = cpu_rq(i); > + unsigned long load = cpu_load(rq); > > - sgs->group_load += cpu_load(rq); > + sgs->group_load += load; > sgs->group_util += cpu_util_cfs(i); > sgs->group_runnable += cpu_runnable(rq); > sgs->sum_h_nr_running += rq->cfs.h_nr_running; > @@ -8851,11 +8865,17 @@ static inline void update_sg_lb_stats(struct lb_env *env, > if (local_group) > continue; > > - /* Check for a misfit task on the cpu */ > - if (env->sd->flags & SD_ASYM_CPUCAPACITY && > - sgs->group_misfit_task_load < rq->misfit_task_load) { > - sgs->group_misfit_task_load = rq->misfit_task_load; > - *sg_status |= SG_OVERLOAD; > + if (env->sd->flags & SD_ASYM_CPUCAPACITY) { > + /* Check for a misfit task on the cpu */ > + if (sgs->group_misfit_task_load < rq->misfit_task_load) { > + sgs->group_misfit_task_load = rq->misfit_task_load; > + *sg_status |= SG_OVERLOAD; > + } > + } else if ((env->idle != CPU_NOT_IDLE) && > + sched_reduced_capacity(rq, env->sd) && > + (sgs->group_misfit_task_load < load)) { > + /* Check for a task running on a CPU with reduced capacity */ > + sgs->group_misfit_task_load = load; > } > } > > @@ -8908,7 +8928,8 @@ static bool update_sd_pick_busiest(struct lb_env *env, > * CPUs in the group should either be possible to resolve > * internally or be covered by avg_load imbalance (eventually). > */ > - if (sgs->group_type == group_misfit_task && > + if ((env->sd->flags & SD_ASYM_CPUCAPACITY) && > + (sgs->group_type == group_misfit_task) && > (!capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) || > sds->local_stat.group_type != group_has_spare)) > return false; > @@ -9517,9 +9538,18 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s > busiest = &sds->busiest_stat; > > if (busiest->group_type == group_misfit_task) { > - /* Set imbalance to allow misfit tasks to be balanced. */ > - env->migration_type = migrate_misfit; > - env->imbalance = 1; > + if (env->sd->flags & SD_ASYM_CPUCAPACITY) { > + /* Set imbalance to allow misfit tasks to be balanced. */ > + env->migration_type = migrate_misfit; > + env->imbalance = 1; > + } else { > + /* > + * Set load imbalance to allow moving task from cpu > + * with reduced capacity > + */ > + env->migration_type = migrate_load; > + env->imbalance = busiest->group_misfit_task_load; I'm wondering why you've chosen that hybrid approach `group_misfit_task -> migrate_load` and not `group_misfit_task -> migrate_misfit`. It looks like this `rq->cfs.h_nr_running = 1` case almost (since we check `busiest->nr_running > 1`) always ends up in the load_balance() `if (!ld_moved)` condition and need_active_balance() can return 1 in case `if ((env->idle != CPU_NOT_IDLE) && ...` condition. This leads to active load_balance and this IMHO, the same you can achieve when you would stay with `group_misfit_task -> migrate_misfit`. I think cpu_load(rq) can be used instead of `rq->misfit_task_load` in the migrate_misfit case of find_busiest_queue() too. [...]
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