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    Subject[PATCH -tip 02/32] 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.

    Tested-by: Julien Desfossez <jdesfossez@digitalocean.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: Vineeth Remanan Pillai <viremana@linux.microsoft.com>
    Signed-off-by: Julien Desfossez <jdesfossez@digitalocean.com>
    Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
    ---
    kernel/sched/deadline.c | 16 ++++++++++++++--
    kernel/sched/fair.c | 32 +++++++++++++++++++++++++++++++-
    kernel/sched/idle.c | 8 ++++++++
    kernel/sched/rt.c | 15 +++++++++++++--
    kernel/sched/sched.h | 3 +++
    kernel/sched/stop_task.c | 14 ++++++++++++--
    6 files changed, 81 insertions(+), 7 deletions(-)

    diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
    index 0f2ea0a3664c..abfc8b505d0d 100644
    --- a/kernel/sched/deadline.c
    +++ b/kernel/sched/deadline.c
    @@ -1867,7 +1867,7 @@ 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;
    @@ -1879,7 +1879,18 @@ 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;
    }

    @@ -2551,6 +2562,7 @@ DEFINE_SCHED_CLASS(dl) = {

    #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 52ddfec7cea6..12cf068eeec8 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -4459,7 +4459,7 @@ 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) {
    @@ -7017,6 +7017,35 @@ 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)
    {
    @@ -11219,6 +11248,7 @@ DEFINE_SCHED_CLASS(fair) = {

    #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 50e128b899c4..33864193a2f9 100644
    --- a/kernel/sched/idle.c
    +++ b/kernel/sched/idle.c
    @@ -406,6 +406,13 @@ 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;
    @@ -473,6 +480,7 @@ DEFINE_SCHED_CLASS(idle) = {

    #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 a6f9d132c24f..a0e245b0c4bd 100644
    --- a/kernel/sched/rt.c
    +++ b/kernel/sched/rt.c
    @@ -1626,7 +1626,7 @@ 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;

    @@ -1634,7 +1634,17 @@ 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;
    }

    @@ -2483,6 +2493,7 @@ DEFINE_SCHED_CLASS(rt) = {

    #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 f794c9337047..5a0dd2b312aa 100644
    --- a/kernel/sched/sched.h
    +++ b/kernel/sched/sched.h
    @@ -1839,6 +1839,9 @@ struct sched_class {
    #ifdef CONFIG_SMP
    int (*balance)(struct rq *rq, struct task_struct *prev, struct rq_flags *rf);
    int (*select_task_rq)(struct task_struct *p, int task_cpu, int flags);
    +
    + struct task_struct * (*pick_task)(struct rq *rq);
    +
    void (*migrate_task_rq)(struct task_struct *p, int new_cpu);

    void (*task_woken)(struct rq *this_rq, struct task_struct *task);
    diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
    index 55f39125c0e1..f988ebe3febb 100644
    --- a/kernel/sched/stop_task.c
    +++ b/kernel/sched/stop_task.c
    @@ -34,15 +34,24 @@ 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)
    {
    @@ -123,6 +132,7 @@ DEFINE_SCHED_CLASS(stop) = {

    #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
    --
    2.29.2.299.gdc1121823c-goog
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