sched/core: Further clarify sched_class::set_next_task()

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>

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index a8a0803..08bdee0 100644 (file)
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1743,13 +1743,16 @@ static void start_hrtick_dl(struct rq *rq, struct task_struct *p)
 }
 #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 task_struct *prev, struct rq_flags *rf)
        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;
 }
 

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 69a81a5..2b7034e 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10151,7 +10151,7 @@ static void switched_to_fair(struct rq *rq, struct task_struct *p)
  * 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;
 

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index f65ef1e..131e7c8 100644 (file)
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -385,7 +385,7 @@ static void put_prev_task_idle(struct rq *rq, struct task_struct *prev)
 {
 }
 
-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, struct task_struct *prev, struct rq_flags *rf
        if (prev)
                put_prev_task(rq, prev);
 
-       set_next_task_idle(rq, next);
+       set_next_task_idle(rq, next, true);
 
        return next;
 }

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 9b8adc0..7bf917e 100644 (file)
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1515,13 +1515,16 @@ static void check_preempt_curr_rt(struct rq *rq, struct task_struct *p, int flag
 #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 task_struct *prev, struct rq_flags *rf)
                return NULL;
 
        p = _pick_next_task_rt(rq);
-       set_next_task_rt(rq, p);
+       set_next_task_rt(rq, p, true);
        return p;
 }
 

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 49ed949..e5e2605 100644 (file)
--- 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 rq *rq, struct task_struct *prev)
 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

diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
index c064073..02dc0a8 100644 (file)
--- a/kernel/sched/stop_task.c
+++ b/kernel/sched/stop_task.c
@@ -29,7 +29,7 @@ check_preempt_curr_stop(struct rq *rq, struct task_struct *p, int flags)
        /* 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, struct task_struct *prev, struct rq_flags *rf
        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;
 }