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    Subject[RFC PATCH v7 02/23] sched: Introduce sched_class::pick_task()
    Date
    From: Peter Zijlstra <peterz@infradead.org>

    Because sched_class::pick_next_task() also implies
    sched_class::set_next_task() (and possibly put_prev_task() and
    newidle_balance) it is not state invariant. This makes it unsuitable
    for remote task selection.

    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: Vineeth Remanan Pillai <vpillai@digitalocean.com>
    Signed-off-by: Julien Desfossez <jdesfossez@digitalocean.com>
    ---
    kernel/sched/deadline.c | 16 ++++++++++++++--
    kernel/sched/fair.c | 36 +++++++++++++++++++++++++++++++++---
    kernel/sched/idle.c | 8 ++++++++
    kernel/sched/rt.c | 14 ++++++++++++--
    kernel/sched/sched.h | 3 +++
    kernel/sched/stop_task.c | 13 +++++++++++--
    6 files changed, 81 insertions(+), 9 deletions(-)

    diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
    index c588a06057dc..6389aff32558 100644
    --- a/kernel/sched/deadline.c
    +++ b/kernel/sched/deadline.c
    @@ -1824,7 +1824,7 @@ static struct sched_dl_entity *pick_next_dl_entity(struct rq *rq,
    return rb_entry(left, struct sched_dl_entity, rb_node);
    }

    -static struct task_struct *pick_next_task_dl(struct rq *rq)
    +static struct task_struct *pick_task_dl(struct rq *rq)
    {
    struct sched_dl_entity *dl_se;
    struct dl_rq *dl_rq = &rq->dl;
    @@ -1836,7 +1836,18 @@ static struct task_struct *pick_next_task_dl(struct rq *rq)
    dl_se = pick_next_dl_entity(rq, dl_rq);
    BUG_ON(!dl_se);
    p = dl_task_of(dl_se);
    - set_next_task_dl(rq, p, true);
    +
    + return p;
    +}
    +
    +static struct task_struct *pick_next_task_dl(struct rq *rq)
    +{
    + struct task_struct *p;
    +
    + p = pick_task_dl(rq);
    + if (p)
    + set_next_task_dl(rq, p, true);
    +
    return p;
    }

    @@ -2493,6 +2504,7 @@ const struct sched_class dl_sched_class

    #ifdef CONFIG_SMP
    .balance = balance_dl,
    + .pick_task = pick_task_dl,
    .select_task_rq = select_task_rq_dl,
    .migrate_task_rq = migrate_task_rq_dl,
    .set_cpus_allowed = set_cpus_allowed_dl,
    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 9be6cf92cc3d..1a1bf726264a 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -4446,7 +4446,7 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
    * Avoid running the skip buddy, if running something else can
    * be done without getting too unfair.
    */
    - if (cfs_rq->skip == se) {
    + if (cfs_rq->skip && cfs_rq->skip == se) {
    struct sched_entity *second;

    if (se == curr) {
    @@ -4464,13 +4464,13 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
    /*
    * Prefer last buddy, try to return the CPU to a preempted task.
    */
    - if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
    + if (left && cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
    se = cfs_rq->last;

    /*
    * Someone really wants this to run. If it's not unfair, run it.
    */
    - if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
    + if (left && cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
    se = cfs_rq->next;

    clear_buddies(cfs_rq, se);
    @@ -6970,6 +6970,35 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
    set_last_buddy(se);
    }

    +#ifdef CONFIG_SMP
    +static struct task_struct *pick_task_fair(struct rq *rq)
    +{
    + struct cfs_rq *cfs_rq = &rq->cfs;
    + struct sched_entity *se;
    +
    + if (!cfs_rq->nr_running)
    + return NULL;
    +
    + do {
    + struct sched_entity *curr = cfs_rq->curr;
    +
    + se = pick_next_entity(cfs_rq, NULL);
    +
    + if (curr) {
    + if (se && curr->on_rq)
    + update_curr(cfs_rq);
    +
    + if (!se || entity_before(curr, se))
    + se = curr;
    + }
    +
    + cfs_rq = group_cfs_rq(se);
    + } while (cfs_rq);
    +
    + return task_of(se);
    +}
    +#endif
    +
    struct task_struct *
    pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    {
    @@ -11152,6 +11181,7 @@ const struct sched_class fair_sched_class

    #ifdef CONFIG_SMP
    .balance = balance_fair,
    + .pick_task = pick_task_fair,
    .select_task_rq = select_task_rq_fair,
    .migrate_task_rq = migrate_task_rq_fair,

    diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
    index 5c33beb1a525..8c41b023dd14 100644
    --- a/kernel/sched/idle.c
    +++ b/kernel/sched/idle.c
    @@ -408,6 +408,13 @@ static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool fir
    schedstat_inc(rq->sched_goidle);
    }

    +#ifdef CONFIG_SMP
    +static struct task_struct *pick_task_idle(struct rq *rq)
    +{
    + return rq->idle;
    +}
    +#endif
    +
    struct task_struct *pick_next_task_idle(struct rq *rq)
    {
    struct task_struct *next = rq->idle;
    @@ -475,6 +482,7 @@ const struct sched_class idle_sched_class

    #ifdef CONFIG_SMP
    .balance = balance_idle,
    + .pick_task = pick_task_idle,
    .select_task_rq = select_task_rq_idle,
    .set_cpus_allowed = set_cpus_allowed_common,
    #endif
    diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
    index e57fca05b660..a5851c775270 100644
    --- a/kernel/sched/rt.c
    +++ b/kernel/sched/rt.c
    @@ -1624,7 +1624,7 @@ static struct task_struct *_pick_next_task_rt(struct rq *rq)
    return rt_task_of(rt_se);
    }

    -static struct task_struct *pick_next_task_rt(struct rq *rq)
    +static struct task_struct *pick_task_rt(struct rq *rq)
    {
    struct task_struct *p;

    @@ -1632,7 +1632,16 @@ static struct task_struct *pick_next_task_rt(struct rq *rq)
    return NULL;

    p = _pick_next_task_rt(rq);
    - set_next_task_rt(rq, p, true);
    +
    + return p;
    +}
    +
    +static struct task_struct *pick_next_task_rt(struct rq *rq)
    +{
    + struct task_struct *p = pick_task_rt(rq);
    + if (p)
    + set_next_task_rt(rq, p, true);
    +
    return p;
    }

    @@ -2443,6 +2452,7 @@ const struct sched_class rt_sched_class

    #ifdef CONFIG_SMP
    .balance = balance_rt,
    + .pick_task = pick_task_rt,
    .select_task_rq = select_task_rq_rt,
    .set_cpus_allowed = set_cpus_allowed_common,
    .rq_online = rq_online_rt,
    diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
    index 9145be83d80b..293d031480d8 100644
    --- a/kernel/sched/sched.h
    +++ b/kernel/sched/sched.h
    @@ -1793,6 +1793,9 @@ struct sched_class {

    #ifdef CONFIG_SMP
    int (*balance)(struct rq *rq, struct task_struct *prev, struct rq_flags *rf);
    +
    + struct task_struct * (*pick_task)(struct rq *rq);
    +
    int (*select_task_rq)(struct task_struct *p, int task_cpu, int sd_flag, int flags);
    void (*migrate_task_rq)(struct task_struct *p, int new_cpu);

    diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
    index 394bc8126a1e..8f92915dd95e 100644
    --- a/kernel/sched/stop_task.c
    +++ b/kernel/sched/stop_task.c
    @@ -34,15 +34,23 @@ static void set_next_task_stop(struct rq *rq, struct task_struct *stop, bool fir
    stop->se.exec_start = rq_clock_task(rq);
    }

    -static struct task_struct *pick_next_task_stop(struct rq *rq)
    +static struct task_struct *pick_task_stop(struct rq *rq)
    {
    if (!sched_stop_runnable(rq))
    return NULL;

    - set_next_task_stop(rq, rq->stop, true);
    return rq->stop;
    }

    +static struct task_struct *pick_next_task_stop(struct rq *rq)
    +{
    + struct task_struct *p = pick_task_stop(rq);
    + if (p)
    + set_next_task_stop(rq, p, true);
    +
    + return p;
    +}
    +
    static void
    enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
    {
    @@ -124,6 +132,7 @@ const struct sched_class stop_sched_class

    #ifdef CONFIG_SMP
    .balance = balance_stop,
    + .pick_task = pick_task_stop,
    .select_task_rq = select_task_rq_stop,
    .set_cpus_allowed = set_cpus_allowed_common,
    #endif
    --
    2.17.1
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