On Sun, 2014-06-15 at 18:19 +0200, Peter Zijlstra wrote:
> On Thu, Jun 12, 2014 at 02:25:59PM -0700, Tim Chen wrote:
> > @@ -2630,7 +2630,7 @@ static inline struct task_struct *
> > pick_next_task(struct rq *rq, struct task_struct *prev)
> > {
> > const struct sched_class *class = &fair_sched_class;
> > - struct task_struct *p;
> > + struct task_struct *p = NULL;
> >
> > /*
> > * Optimization: we know that if all tasks are in
> > @@ -2638,9 +2638,13 @@ pick_next_task(struct rq *rq, struct task_struct
> > *prev)
> > */
> > if (likely(prev->sched_class == class &&
> > rq->nr_running == rq->cfs.h_nr_running)) {
> > - p = fair_sched_class.pick_next_task(rq, prev);
> > - if (unlikely(p == RETRY_TASK))
> > - goto again;
> > +
> > + /* If no cpu has more than 1 task, skip */
> > + if (rq->nr_running > 0 || rq->rd->overload) {
> > + p = fair_sched_class.pick_next_task(rq, prev);
> > + if (unlikely(p == RETRY_TASK))
> > + goto again;
> > + }
> >
> > /* assumes fair_sched_class->next == idle_sched_class */
> > if (unlikely(!p))
>
>
> Please move this into pick_next_task_fair(). You're slowing down the
> important fast path of picking a task when there actually is something
> to do.
Will do.
>
> Also, its a layering violation -- the idle balance things you're trying
> to avoid is a fair_sched_class affair.
>
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 9855e87..00ab38c 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5863,7 +5863,8 @@ static inline int sg_capacity(struct lb_env *env,
> > struct sched_group *group)
> > */
> > static inline void update_sg_lb_stats(struct lb_env *env,
> > struct sched_group *group, int load_idx,
> > - int local_group, struct sg_lb_stats *sgs)
> > + int local_group, struct sg_lb_stats *sgs,
> > + bool *overload)
> > {
> > unsigned long load;
> > int i;
> > @@ -5881,6 +5882,8 @@ static inline void update_sg_lb_stats(struct lb_env
> > *env,
> >
> > sgs->group_load += load;
> > sgs->sum_nr_running += rq->nr_running;
> > + if (overload && rq->nr_running > 1)
> > + *overload = true;
> > #ifdef CONFIG_NUMA_BALANCING
> > sgs->nr_numa_running += rq->nr_numa_running;
> > sgs->nr_preferred_running += rq->nr_preferred_running;
> > @@ -5991,6 +5994,7 @@ static inline void update_sd_lb_stats(struct lb_env
> > *env, struct sd_lb_stats *sd
> > struct sched_group *sg = env->sd->groups;
> > struct sg_lb_stats tmp_sgs;
> > int load_idx, prefer_sibling = 0;
> > + bool overload = false;
> >
> > if (child && child->flags & SD_PREFER_SIBLING)
> > prefer_sibling = 1;
> > @@ -6011,7 +6015,13 @@ static inline void update_sd_lb_stats(struct lb_env
> > *env, struct sd_lb_stats *sd
> > update_group_power(env->sd, env->dst_cpu);
> > }
> >
> > - update_sg_lb_stats(env, sg, load_idx, local_group, sgs);
> > + if (env->sd->parent)
> > + update_sg_lb_stats(env, sg, load_idx, local_group, sgs,
> > + NULL);
> > + else
> > + /* gather overload info if we are at root domain */
> > + update_sg_lb_stats(env, sg, load_idx, local_group, sgs,
> > + &overload);
> >
> > if (local_group)
> > goto next_group;
> > @@ -6045,6 +6055,15 @@ next_group:
> >
> > if (env->sd->flags & SD_NUMA)
> > env->fbq_type = fbq_classify_group(&sds->busiest_stat);
> > +
> > + if (!env->sd->parent) {
> > + /* update overload indicator if we are at root domain */
> > + int i = cpumask_first(sched_domain_span(env->sd));
> > + struct rq *rq = cpu_rq(i);
> > + if (rq->rd->overload != overload)
> > + rq->rd->overload = overload;
> > + }
> > +
> > }
> >
> > /**
>
> The worry I have is that this update is 'slow'. We could have grown many
> tasks since the last update.
The update to turn on the indicator is immediate and triggered in
add_nr_running. So if there are more than one tasks on any cpu,
we start load balancing again right away. It is only the
clearing of the indicator in update_sd_lb_stats that takes time.
That does no harm as the cleared indicator is for the skipping of load
balance, which can be delayed.
Thanks.
Tim
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