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SubjectRe: [PATCH] sched: Fast idling of CPU when system is partially loaded
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On Thu, 2014-06-12 at 23:01 -0700, Jason Low wrote:
> On Thu, 2014-06-12 at 14:25 -0700, Tim Chen wrote:
>
> > Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> > ---
> > kernel/sched/core.c | 12 ++++++++----
> > kernel/sched/fair.c | 23 +++++++++++++++++++++--
> > kernel/sched/sched.h | 10 ++++++++--
> > 3 files changed, 37 insertions(+), 8 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index c6b9879..4f57221 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -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) {
>
> Hi Tim,
>
> If it is skipping if no cpu has more than 1 task, should the
> above have the additional check for (rq->nr_running > 1) instead
> of (rq->nr_running > 0)?

If you have a job on your local cpu, you do want to have the scheduler
pick the task to run.

> > @@ -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);
>
> Would it make the code cleaner if we always call:
>
> + update_sg_lb_stats(env, sg, load_idx, local_group, sgs,
> &overload);
>
> and in update_sg_lb_stats():
>
> + bool is_root_domain = (env->sd->parent == NULL)
>
>
> + /* gather overload info if we are at root domain */
> + if (is_root_domain && rq->nr_running > 1)
> + *overload = true;
>

I want to have the caller to update_sg_lb_stats make the decision
on whether there's a need to calculate the indicator, and not
make the decision in update_sg_lb_stats.

This allows the flexibility later on if we want such indicator
in a lower sched domain hierarchy.

> > 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);
>
> Perhaps we could just use:
>
> struct rq *rq = env->dst_rq;
>

Yes, your suggested change is more concise. I'll update the code with
this change.

Tim



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