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    Subject[RFC PATCH v4 09/19] 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 | 21 ++++++++++++++++-----
    kernel/sched/fair.c | 36 +++++++++++++++++++++++++++++++++---
    kernel/sched/idle.c | 10 +++++++++-
    kernel/sched/rt.c | 21 ++++++++++++++++-----
    kernel/sched/sched.h | 2 ++
    kernel/sched/stop_task.c | 21 ++++++++++++++++-----
    6 files changed, 92 insertions(+), 19 deletions(-)

    diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
    index 0c7a51745813..461f1c1b791d 100644
    --- a/kernel/sched/deadline.c
    +++ b/kernel/sched/deadline.c
    @@ -1722,15 +1722,12 @@ 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, struct task_struct *prev, struct rq_flags *rf)
    +static struct task_struct *pick_task_dl(struct rq *rq)
    {
    struct sched_dl_entity *dl_se;
    struct task_struct *p;
    struct dl_rq *dl_rq;

    - WARN_ON_ONCE(prev || rf);
    -
    dl_rq = &rq->dl;

    if (unlikely(!dl_rq->dl_nr_running))
    @@ -1741,7 +1738,19 @@ pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)

    p = dl_task_of(dl_se);

    - set_next_task_dl(rq, p);
    + return p;
    +}
    +
    +static struct task_struct *
    +pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    +{
    + struct task_struct *p;
    +
    + WARN_ON_ONCE(prev || rf);
    +
    + p = pick_task_dl(rq);
    + if (p)
    + set_next_task_dl(rq, p);

    return p;
    }
    @@ -2380,6 +2389,8 @@ const struct sched_class dl_sched_class = {
    .set_next_task = set_next_task_dl,

    #ifdef CONFIG_SMP
    + .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 850e235c9209..cc78b334e1c3 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -4211,7 +4211,7 @@ 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) {
    @@ -4229,13 +4229,13 @@ 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);
    @@ -6786,6 +6786,34 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
    set_last_buddy(se);
    }

    +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);
    +}
    +
    static struct task_struct *
    pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    {
    @@ -10469,6 +10497,8 @@ const struct sched_class fair_sched_class = {
    .set_next_task = set_next_task_fair,

    #ifdef CONFIG_SMP
    + .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 5bb8d44fbff9..87c55e3b988d 100644
    --- a/kernel/sched/idle.c
    +++ b/kernel/sched/idle.c
    @@ -374,6 +374,12 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl
    resched_curr(rq);
    }

    +static struct task_struct *
    +pick_task_idle(struct rq *rq)
    +{
    + return rq->idle;
    +}
    +
    static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    {
    }
    @@ -387,11 +393,12 @@ static void set_next_task_idle(struct rq *rq, struct task_struct *next)
    static struct task_struct *
    pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    {
    - struct task_struct *next = rq->idle;
    + struct task_struct *next;

    if (prev)
    put_prev_task(rq, prev);

    + next = pick_task_idle(rq);
    set_next_task_idle(rq, next);

    return next;
    @@ -459,6 +466,7 @@ const struct sched_class idle_sched_class = {
    .set_next_task = set_next_task_idle,

    #ifdef CONFIG_SMP
    + .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 09c317a916aa..4714630a90b9 100644
    --- a/kernel/sched/rt.c
    +++ b/kernel/sched/rt.c
    @@ -1548,20 +1548,29 @@ 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, struct task_struct *prev, struct rq_flags *rf)
    +static struct task_struct *pick_task_rt(struct rq *rq)
    {
    struct task_struct *p;
    struct rt_rq *rt_rq = &rq->rt;

    - WARN_ON_ONCE(prev || rf);
    -
    if (!rt_rq->rt_queued)
    return NULL;

    p = _pick_next_task_rt(rq);

    - set_next_task_rt(rq, p);
    + return p;
    +}
    +
    +static struct task_struct *
    +pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    +{
    + struct task_struct *p;
    +
    + WARN_ON_ONCE(prev || rf);
    +
    + p = pick_task_rt(rq);
    + if (p)
    + set_next_task_rt(rq, p);

    return p;
    }
    @@ -2364,6 +2373,8 @@ const struct sched_class rt_sched_class = {
    .set_next_task = set_next_task_rt,

    #ifdef CONFIG_SMP
    + .pick_task = pick_task_rt,
    +
    .select_task_rq = select_task_rq_rt,

    .set_cpus_allowed = set_cpus_allowed_common,
    diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
    index 16c9654ac750..44f45aad3f97 100644
    --- a/kernel/sched/sched.h
    +++ b/kernel/sched/sched.h
    @@ -1725,6 +1725,8 @@ struct sched_class {
    void (*set_next_task)(struct rq *rq, struct task_struct *p);

    #ifdef CONFIG_SMP
    + 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 7e1cee4e65b2..fb6c436cba6c 100644
    --- a/kernel/sched/stop_task.c
    +++ b/kernel/sched/stop_task.c
    @@ -29,20 +29,30 @@ static void set_next_task_stop(struct rq *rq, struct task_struct *stop)
    }

    static struct task_struct *
    -pick_next_task_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    +pick_task_stop(struct rq *rq)
    {
    struct task_struct *stop = rq->stop;

    - WARN_ON_ONCE(prev || rf);
    -
    if (!stop || !task_on_rq_queued(stop))
    return NULL;

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

    +static struct task_struct *
    +pick_next_task_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    +{
    + struct task_struct *p;
    +
    + WARN_ON_ONCE(prev || rf);
    +
    + p = pick_task_stop(rq);
    + if (p)
    + set_next_task_stop(rq, p);
    +
    + return p;
    +}
    +
    static void
    enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
    {
    @@ -129,6 +139,7 @@ const struct sched_class stop_sched_class = {
    .set_next_task = set_next_task_stop,

    #ifdef CONFIG_SMP
    + .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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