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    Subject[RFC PATCH v2 07/17] sched: Allow put_prev_task() to drop rq->lock
    Date
    From: Peter Zijlstra (Intel) <peterz@infradead.org>

    Currently the pick_next_task() loop is convoluted and ugly because of
    how it can drop the rq->lock and needs to restart the picking.

    For the RT/Deadline classes, it is put_prev_task() where we do
    balancing, and we could do this before the picking loop. Make this
    possible.

    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    ---
    kernel/sched/core.c | 2 +-
    kernel/sched/deadline.c | 14 +++++++++++++-
    kernel/sched/fair.c | 2 +-
    kernel/sched/idle.c | 2 +-
    kernel/sched/rt.c | 14 +++++++++++++-
    kernel/sched/sched.h | 4 ++--
    kernel/sched/stop_task.c | 2 +-
    7 files changed, 32 insertions(+), 8 deletions(-)

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 32ea79fb8d29..9dfa0c53deb3 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -5595,7 +5595,7 @@ static void calc_load_migrate(struct rq *rq)
    atomic_long_add(delta, &calc_load_tasks);
    }

    -static void put_prev_task_fake(struct rq *rq, struct task_struct *prev)
    +static void put_prev_task_fake(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    {
    }

    diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
    index fadfbfe7d573..56791c0318a2 100644
    --- a/kernel/sched/deadline.c
    +++ b/kernel/sched/deadline.c
    @@ -1773,13 +1773,25 @@ pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    return p;
    }

    -static void put_prev_task_dl(struct rq *rq, struct task_struct *p)
    +static void put_prev_task_dl(struct rq *rq, struct task_struct *p, struct rq_flags *rf)
    {
    update_curr_dl(rq);

    update_dl_rq_load_avg(rq_clock_pelt(rq), rq, 1);
    if (on_dl_rq(&p->dl) && p->nr_cpus_allowed > 1)
    enqueue_pushable_dl_task(rq, p);
    +
    + if (rf && !on_dl_rq(&p->dl) && need_pull_dl_task(rq, p)) {
    + /*
    + * This is OK, because current is on_cpu, which avoids it being
    + * picked for load-balance and preemption/IRQs are still
    + * disabled avoiding further scheduler activity on it and we've
    + * not yet started the picking loop.
    + */
    + rq_unpin_lock(rq, rf);
    + pull_dl_task(rq);
    + rq_repin_lock(rq, rf);
    + }
    }

    /*
    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index f7e631e692a3..41ec5e68e1c5 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -7081,7 +7081,7 @@ done: __maybe_unused;
    /*
    * Account for a descheduled task:
    */
    -static void put_prev_task_fair(struct rq *rq, struct task_struct *prev)
    +static void put_prev_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    {
    struct sched_entity *se = &prev->se;
    struct cfs_rq *cfs_rq;
    diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
    index dd64be34881d..1b65a4c3683e 100644
    --- a/kernel/sched/idle.c
    +++ b/kernel/sched/idle.c
    @@ -373,7 +373,7 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl
    resched_curr(rq);
    }

    -static void put_prev_task_idle(struct rq *rq, struct task_struct *prev)
    +static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    {
    }

    diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
    index adec98a94f2b..51ee87c5a28a 100644
    --- a/kernel/sched/rt.c
    +++ b/kernel/sched/rt.c
    @@ -1593,7 +1593,7 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    return p;
    }

    -static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
    +static void put_prev_task_rt(struct rq *rq, struct task_struct *p, struct rq_flags *rf)
    {
    update_curr_rt(rq);

    @@ -1605,6 +1605,18 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
    */
    if (on_rt_rq(&p->rt) && p->nr_cpus_allowed > 1)
    enqueue_pushable_task(rq, p);
    +
    + if (rf && !on_rt_rq(&p->rt) && need_pull_rt_task(rq, p)) {
    + /*
    + * This is OK, because current is on_cpu, which avoids it being
    + * picked for load-balance and preemption/IRQs are still
    + * disabled avoiding further scheduler activity on it and we've
    + * not yet started the picking loop.
    + */
    + rq_unpin_lock(rq, rf);
    + pull_rt_task(rq);
    + rq_repin_lock(rq, rf);
    + }
    }

    #ifdef CONFIG_SMP
    diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
    index bfcbcbb25646..4cbe2bef92e4 100644
    --- a/kernel/sched/sched.h
    +++ b/kernel/sched/sched.h
    @@ -1675,7 +1675,7 @@ struct sched_class {
    struct task_struct * (*pick_next_task)(struct rq *rq,
    struct task_struct *prev,
    struct rq_flags *rf);
    - void (*put_prev_task)(struct rq *rq, struct task_struct *p);
    + void (*put_prev_task)(struct rq *rq, struct task_struct *p, struct rq_flags *rf);
    void (*set_next_task)(struct rq *rq, struct task_struct *p);

    #ifdef CONFIG_SMP
    @@ -1721,7 +1721,7 @@ struct sched_class {
    static inline void put_prev_task(struct rq *rq, struct task_struct *prev)
    {
    WARN_ON_ONCE(rq->curr != prev);
    - prev->sched_class->put_prev_task(rq, prev);
    + prev->sched_class->put_prev_task(rq, prev, NULL);
    }

    static inline void set_next_task(struct rq *rq, struct task_struct *next)
    diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
    index 47a3d2a18a9a..8f414018d5e0 100644
    --- a/kernel/sched/stop_task.c
    +++ b/kernel/sched/stop_task.c
    @@ -59,7 +59,7 @@ static void yield_task_stop(struct rq *rq)
    BUG(); /* the stop task should never yield, its pointless. */
    }

    -static void put_prev_task_stop(struct rq *rq, struct task_struct *prev)
    +static void put_prev_task_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
    {
    struct task_struct *curr = rq->curr;
    u64 delta_exec;
    --
    2.17.1
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