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Subject[PATCH 5.4 047/102] sched/core: Further clarify sched_class::set_next_task()
Date
From: Peter Zijlstra <peterz@infradead.org>

commit a0e813f26ebcb25c0b5e504498fbd796cca1a4ba upstream.

It turns out there really is something special to the first
set_next_task() invocation. In specific the 'change' pattern really
should not cause balance callbacks.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bsegall@google.com
Cc: dietmar.eggemann@arm.com
Cc: juri.lelli@redhat.com
Cc: ktkhai@virtuozzo.com
Cc: mgorman@suse.de
Cc: qais.yousef@arm.com
Cc: qperret@google.com
Cc: rostedt@goodmis.org
Cc: valentin.schneider@arm.com
Cc: vincent.guittot@linaro.org
Fixes: f95d4eaee6d0 ("sched/{rt,deadline}: Fix set_next_task vs pick_next_task")
Link: https://lkml.kernel.org/r/20191108131909.775434698@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/sched/deadline.c | 7 +++++--
kernel/sched/fair.c | 2 +-
kernel/sched/idle.c | 4 ++--
kernel/sched/rt.c | 7 +++++--
kernel/sched/sched.h | 4 ++--
kernel/sched/stop_task.c | 4 ++--
6 files changed, 17 insertions(+), 11 deletions(-)

--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1743,13 +1743,16 @@ static void start_hrtick_dl(struct rq *r
}
#endif

-static void set_next_task_dl(struct rq *rq, struct task_struct *p)
+static void set_next_task_dl(struct rq *rq, struct task_struct *p, bool first)
{
p->se.exec_start = rq_clock_task(rq);

/* You can't push away the running task */
dequeue_pushable_dl_task(rq, p);

+ if (!first)
+ return;
+
if (hrtick_enabled(rq))
start_hrtick_dl(rq, p);

@@ -1785,7 +1788,7 @@ pick_next_task_dl(struct rq *rq, struct
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);
+ set_next_task_dl(rq, p, true);
return p;
}

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10151,7 +10151,7 @@ static void switched_to_fair(struct rq *
* This routine is mostly called to set cfs_rq->curr field when a task
* migrates between groups/classes.
*/
-static void set_next_task_fair(struct rq *rq, struct task_struct *p)
+static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
{
struct sched_entity *se = &p->se;

--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -385,7 +385,7 @@ static void put_prev_task_idle(struct rq
{
}

-static void set_next_task_idle(struct rq *rq, struct task_struct *next)
+static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
{
update_idle_core(rq);
schedstat_inc(rq->sched_goidle);
@@ -399,7 +399,7 @@ pick_next_task_idle(struct rq *rq, struc
if (prev)
put_prev_task(rq, prev);

- set_next_task_idle(rq, next);
+ set_next_task_idle(rq, next, true);

return next;
}
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1515,13 +1515,16 @@ static void check_preempt_curr_rt(struct
#endif
}

-static inline void set_next_task_rt(struct rq *rq, struct task_struct *p)
+static inline void set_next_task_rt(struct rq *rq, struct task_struct *p, bool first)
{
p->se.exec_start = rq_clock_task(rq);

/* The running task is never eligible for pushing */
dequeue_pushable_task(rq, p);

+ if (!first)
+ return;
+
/*
* If prev task was rt, put_prev_task() has already updated the
* utilization. We only care of the case where we start to schedule a
@@ -1575,7 +1578,7 @@ pick_next_task_rt(struct rq *rq, struct
return NULL;

p = _pick_next_task_rt(rq);
- set_next_task_rt(rq, p);
+ set_next_task_rt(rq, p, true);
return p;
}

--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1728,7 +1728,7 @@ struct sched_class {
struct task_struct *prev,
struct rq_flags *rf);
void (*put_prev_task)(struct rq *rq, struct task_struct *p);
- void (*set_next_task)(struct rq *rq, struct task_struct *p);
+ void (*set_next_task)(struct rq *rq, struct task_struct *p, bool first);

#ifdef CONFIG_SMP
int (*balance)(struct rq *rq, struct task_struct *prev, struct rq_flags *rf);
@@ -1780,7 +1780,7 @@ static inline void put_prev_task(struct
static inline void set_next_task(struct rq *rq, struct task_struct *next)
{
WARN_ON_ONCE(rq->curr != next);
- next->sched_class->set_next_task(rq, next);
+ next->sched_class->set_next_task(rq, next, false);
}

#ifdef CONFIG_SMP
--- a/kernel/sched/stop_task.c
+++ b/kernel/sched/stop_task.c
@@ -29,7 +29,7 @@ check_preempt_curr_stop(struct rq *rq, s
/* we're never preempted */
}

-static void set_next_task_stop(struct rq *rq, struct task_struct *stop)
+static void set_next_task_stop(struct rq *rq, struct task_struct *stop, bool first)
{
stop->se.exec_start = rq_clock_task(rq);
}
@@ -42,7 +42,7 @@ pick_next_task_stop(struct rq *rq, struc
if (!sched_stop_runnable(rq))
return NULL;

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


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