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    SubjectRe: [PATCH v4 5/7] sched/fair: skip SIS domain search if fully busy
    Hello Abel,


    On Sun, Jun 19, 2022 at 08:04:49PM +0800, Abel Wu wrote:
    > If a full scan on SIS domain failed, then no unoccupied cpus available
    > and the LLC is fully busy. In this case we'd better spend the time on
    > something more useful, rather than wasting it trying to find an idle
    > cpu that probably not exist.
    >
    > The fully busy status will be re-evaluated when any core of this LLC
    > domain enters load balancing, and cleared once idle cpus found.
    >
    > Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
    > ---

    [..snip..]

    > @@ -6197,24 +6201,44 @@ static inline int __select_idle_cpu(int cpu, struct task_struct *p)
    > DEFINE_STATIC_KEY_FALSE(sched_smt_present);
    > EXPORT_SYMBOL_GPL(sched_smt_present);
    >
    > -static inline void set_idle_cores(int cpu, int val)
    > +static inline void sd_set_state(int cpu, enum sd_state state)

    Nit: We are setting the state of only the LLC domain and not any other
    domain via this function. So should we name it as
    set_llc_state()/get_llc_state() for better readability ?


    > {
    > struct sched_domain_shared *sds;
    >
    > sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
    > if (sds)
    > - WRITE_ONCE(sds->has_idle_cores, val);
    > + WRITE_ONCE(sds->state, state);
    > }
    >
    > -static inline bool test_idle_cores(int cpu)
    > +static inline enum sd_state sd_get_state(int cpu)
    > {
    > struct sched_domain_shared *sds;
    >
    > sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
    > if (sds)
    > - return READ_ONCE(sds->has_idle_cores);
    > + return READ_ONCE(sds->state);
    >
    > - return false;
    > + return sd_has_icpus;
    > +}
    > +
    > +static inline void set_idle_cores(int cpu, int idle)
    ^^^^^
    I agree with Josh. We can use core_idle instead of idle here.

    > +{
    > + sd_set_state(cpu, idle ? sd_has_icores : sd_has_icpus);
    > +}
    > +
    > +static inline bool test_idle_cores(int cpu)
    > +{
    > + return sd_get_state(cpu) == sd_has_icores;
    > +}
    > +
    > +static inline void set_idle_cpus(int cpu, int idle)
    > +{
    > + sd_set_state(cpu, idle ? sd_has_icpus : sd_is_busy);
    > +}
    > +
    > +static inline bool test_idle_cpus(int cpu)
    > +{
    > + return sd_get_state(cpu) != sd_is_busy;
    > }
    >
    > /*

    [...]


    > @@ -8661,6 +8702,12 @@ 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 void sd_classify(struct sd_lb_stats *sds, struct rq *rq)
    > +{
    > + if (sds->sd_state != sd_has_icpus && unoccupied_rq(rq))

    Nit: sds->sd_state can either be sd_has_icpus or sd_is_busy. So for
    better readability, we can just use the positive check

    if (sds->sd_state == sd_is_busy && unoccupied_rq(rq))
    sds->sd_state = sd_has_icpus;


    > + sds->sd_state = sd_has_icpus;
    > +}
    > +
    > /**
    > * update_sg_lb_stats - Update sched_group's statistics for load balancing.
    > * @env: The load balancing environment.
    > @@ -8675,11 +8722,12 @@ static inline void update_sg_lb_stats(struct lb_env *env,
    > struct sg_lb_stats *sgs,
    > int *sg_status)
    > {
    > - int i, nr_running, local_group;
    > + int i, nr_running, local_group, update_core;
    >
    > memset(sgs, 0, sizeof(*sgs));
    >
    > local_group = group == sds->local;
    > + update_core = env->sd->flags & SD_SHARE_CPUCAPACITY;
    >
    > for_each_cpu_and(i, sched_group_span(group), env->cpus) {
    > struct rq *rq = cpu_rq(i);
    > @@ -8692,6 +8740,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,
    > nr_running = rq->nr_running;
    > sgs->sum_nr_running += nr_running;
    >
    > + if (update_core)
    > + sd_classify(sds, rq);
    > +
    > if (nr_running > 1)
    > *sg_status |= SG_OVERLOAD;
    >
    > @@ -9220,6 +9271,12 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
    > return idlest;
    > }
    >
    > +static void sd_update_state(struct lb_env *env, struct sd_lb_stats *sds)
    > +{
    > + if (sds->sd_state == sd_has_icpus && !test_idle_cpus(env->dst_cpu))
    > + set_idle_cpus(env->dst_cpu, true);

    We could enter this if condition when env->dst_cpu is the only idle
    CPU in the SMT domain (which is likely to be the case every time we do
    a NEW_IDLE balance). By the end of this load-balancing round, the
    env->dst_cpu can pull a task from some other CPU and thereby no longer
    remain idle but the LLC state would still be sd_has_icpus.

    That would mean that some CPU on this LLC would do a full scan during
    the wakeup only to find no idle CPU and reset the state to
    sd_is_busy. Have you seen instances where this false-positive pattern
    can result in wasteful scan thereby cause a performance degradation?
    Ideally it should not be worse that what we currently have.

    Apart from this, patch looks good to me.

    I would be worth the while to explore if the LLC state can be used
    early on in select_task_rq_fair() to determine if we need to do a
    wake-affine or allow the task to stick to its previous LLC depending
    on which among the previous LLC and the waker's LLC have an idle CPU.

    --
    Thanks and Regards
    gautham.

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