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Date: Tue, 28 Jan 2014 10:46:18 -0800
From: bsegall@...gle.com
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, mingo@...nel.org,
daniel.lezcano@...aro.org, pjt@...gle.com,
Steven Rostedt <rostedt@...dmis.org>,
Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 5/9] sched: Push put_prev_task() into pick_next_task()
Peter Zijlstra <peterz@...radead.org> writes:
> In order to avoid having to do put/set on a whole cgroup hierarchy
> when we context switch, push the put into pick_next_task() so that
> both operations are in the same function. Further changes then allow
> us to possibly optimize away redundant work.
>
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> ---
> kernel/sched/core.c | 21 ++++++++-------------
> kernel/sched/deadline.c | 4 +++-
> kernel/sched/fair.c | 5 ++++-
> kernel/sched/idle_task.c | 5 ++++-
> kernel/sched/rt.c | 26 +++++++++++++++-----------
> kernel/sched/sched.h | 8 +++++++-
> kernel/sched/stop_task.c | 14 ++++++++------
> 7 files changed, 49 insertions(+), 34 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2556,18 +2556,11 @@ static inline void schedule_debug(struct
> schedstat_inc(this_rq(), sched_count);
> }
>
> -static void put_prev_task(struct rq *rq, struct task_struct *prev)
> -{
> - if (prev->on_rq || rq->skip_clock_update < 0)
> - update_rq_clock(rq);
> - prev->sched_class->put_prev_task(rq, prev);
> -}
> -
> /*
> * Pick up the highest-prio task:
> */
> static inline struct task_struct *
> -pick_next_task(struct rq *rq)
> +pick_next_task(struct rq *rq, struct task_struct *prev)
> {
> const struct sched_class *class;
> struct task_struct *p;
> @@ -2577,13 +2570,13 @@ pick_next_task(struct rq *rq)
> * the fair class we can call that function directly:
> */
> if (likely(rq->nr_running == rq->cfs.h_nr_running)) {
> - p = fair_sched_class.pick_next_task(rq);
> + p = fair_sched_class.pick_next_task(rq, prev);
> if (likely(p))
> return p;
> }
>
> for_each_class(class) {
> - p = class->pick_next_task(rq);
> + p = class->pick_next_task(rq, prev);
> if (p)
> return p;
> }
> @@ -2691,8 +2684,10 @@ static void __sched __schedule(void)
> rq->idle_stamp = 0;
> }
>
> - put_prev_task(rq, prev);
> - next = pick_next_task(rq);
> + if (prev->on_rq || rq->skip_clock_update < 0)
> + update_rq_clock(rq);
> +
> + next = pick_next_task(rq, prev);
> clear_tsk_need_resched(prev);
> clear_preempt_need_resched();
> rq->skip_clock_update = 0;
> @@ -4734,7 +4729,7 @@ static void migrate_tasks(unsigned int d
> if (rq->nr_running == 1)
> break;
>
> - next = pick_next_task(rq);
> + next = pick_next_task(rq, NULL);
This seems to be incorrect without the if (prev) lines in
pick_next_task_foo, since foo_nr_running isn't enough to save us.
> BUG_ON(!next);
> next->sched_class->put_prev_task(rq, next);
>
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -989,7 +989,7 @@ static struct sched_dl_entity *pick_next
> return rb_entry(left, struct sched_dl_entity, rb_node);
> }
>
> -struct task_struct *pick_next_task_dl(struct rq *rq)
> +struct task_struct *pick_next_task_dl(struct rq *rq, struct task_struct *prev)
> {
> struct sched_dl_entity *dl_se;
> struct task_struct *p;
> @@ -1000,6 +1000,8 @@ struct task_struct *pick_next_task_dl(st
> if (unlikely(!dl_rq->dl_nr_running))
> return NULL;
>
> + prev->sched_class->put_prev_task(rq, prev);
> +
> dl_se = pick_next_dl_entity(rq, dl_rq);
> BUG_ON(!dl_se);
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4659,7 +4659,8 @@ static void check_preempt_wakeup(struct
> set_last_buddy(se);
> }
>
> -static struct task_struct *pick_next_task_fair(struct rq *rq)
> +static struct task_struct *
> +pick_next_task_fair(struct rq *rq, struct task_struct *prev)
> {
> struct task_struct *p;
> struct cfs_rq *cfs_rq = &rq->cfs;
> @@ -4668,6 +4669,8 @@ static struct task_struct *pick_next_tas
> if (!cfs_rq->nr_running)
> return NULL;
>
> + prev->sched_class->put_prev_task(rq, prev);
> +
> do {
> se = pick_next_entity(cfs_rq);
> set_next_entity(cfs_rq, se);
> --- a/kernel/sched/idle_task.c
> +++ b/kernel/sched/idle_task.c
> @@ -33,8 +33,11 @@ static void check_preempt_curr_idle(stru
> resched_task(rq->idle);
> }
>
> -static struct task_struct *pick_next_task_idle(struct rq *rq)
> +static struct task_struct *
> +pick_next_task_idle(struct rq *rq, struct task_struct *prev)
> {
> + prev->sched_class->put_prev_task(rq, prev);
> +
> schedstat_inc(rq, sched_goidle);
> #ifdef CONFIG_SMP
> /* Trigger the post schedule to do an idle_enter for CFS */
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1310,15 +1310,7 @@ static struct task_struct *_pick_next_ta
> {
> struct sched_rt_entity *rt_se;
> struct task_struct *p;
> - struct rt_rq *rt_rq;
> -
> - rt_rq = &rq->rt;
> -
> - if (!rt_rq->rt_nr_running)
> - return NULL;
> -
> - if (rt_rq_throttled(rt_rq))
> - return NULL;
> + struct rt_rq *rt_rq = &rq->rt;
>
> do {
> rt_se = pick_next_rt_entity(rq, rt_rq);
> @@ -1332,9 +1324,21 @@ static struct task_struct *_pick_next_ta
> return p;
> }
>
> -static struct task_struct *pick_next_task_rt(struct rq *rq)
> +static struct task_struct *
> +pick_next_task_rt(struct rq *rq, struct task_struct *prev)
> {
> - struct task_struct *p = _pick_next_task_rt(rq);
> + struct task_struct *p;
> + struct rt_rq *rt_rq = &rq->rt;
> +
> + if (!rt_rq->rt_nr_running)
> + return NULL;
> +
> + if (rt_rq_throttled(rt_rq))
> + return NULL;
> +
> + prev->sched_class->put_prev_task(rq, prev);
> +
> + p = _pick_next_task_rt(rq);
>
> /* The running task is never eligible for pushing */
> if (p)
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1123,7 +1123,13 @@ struct sched_class {
>
> void (*check_preempt_curr) (struct rq *rq, struct task_struct *p, int flags);
>
> - struct task_struct * (*pick_next_task) (struct rq *rq);
> + /*
> + * It is the responsibility of the pick_next_task() method that will
> + * return the next task to call put_prev_task() on the @prev task or
> + * something equivalent.
> + */
> + struct task_struct * (*pick_next_task) (struct rq *rq,
> + struct task_struct *prev);
> void (*put_prev_task) (struct rq *rq, struct task_struct *p);
>
> #ifdef CONFIG_SMP
> --- a/kernel/sched/stop_task.c
> +++ b/kernel/sched/stop_task.c
> @@ -23,16 +23,18 @@ check_preempt_curr_stop(struct rq *rq, s
> /* we're never preempted */
> }
>
> -static struct task_struct *pick_next_task_stop(struct rq *rq)
> +static struct task_struct *
> +pick_next_task_stop(struct rq *rq, struct task_struct *prev)
> {
> struct task_struct *stop = rq->stop;
>
> - if (stop && stop->on_rq) {
> - stop->se.exec_start = rq_clock_task(rq);
> - return stop;
> - }
> + if (!stop || !stop->on_rq)
> + return NULL;
>
> - return NULL;
> + prev->sched_class->put_prev_task(rq, prev);
> + stop->se.exec_start = rq_clock_task(rq);
> +
> + return stop;
> }
>
> static void
--
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