Re: [PATCH 4/9] sched: Clean up idle task SMP logic

Peter Zijlstra
2014-01-23T11:38:23Z

On Tue, Jan 21, 2014 at 06:27:11PM +0100, Vincent Guittot wrote:
> On 21 January 2014 12:17, Peter Zijlstra <peterz@infradead.org> wrote:

> If i have correctly followed the new function path that is introduced
> by the patchset, idle_enter_fair is called after idle_balance whereas
> it must be called before in order to update the
> runnable_avg_sum/period of the rq before evaluating the interest of
> pulling cfs task

Its idle_exit_fair, that's moved from pre_schedule to put_prev_task and
thus indeed has crossed idle_balance.

Yeah, I can leave that pre_schedule thing in place, however all that has
be thinking.

Ideally we'd do something like the below; but I must admit to still
being slightly confused about the idle_{enter,exit}_fair() calls, their
comment doesn't seem to clarify things.

Steve, I don't think I wrecked rt/deadline by pulling in the
pre_schedule call into pick_next_task(), but then, who knows :-)

The only thing I really don't like is having that double conditional for
the direct fair_sched_class call, but I didn't see a way around that,
other than dropping that entirely. Then again, we cut out a conditional
and indirect call by getting rid of pre_schedule() -- so it might just
balance out.

---
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2146,13 +2146,6 @@ static void finish_task_switch(struct rq

#ifdef CONFIG_SMP

-/* assumes rq->lock is held */
-static inline void pre_schedule(struct rq *rq, struct task_struct *prev)
-{
- if (prev->sched_class->pre_schedule)
- prev->sched_class->pre_schedule(rq, prev);
-}
-
/* rq->lock is NOT held, but preemption is disabled */
static inline void post_schedule(struct rq *rq)
{
@@ -2170,10 +2163,6 @@ static inline void post_schedule(struct

#else

-static inline void pre_schedule(struct rq *rq, struct task_struct *p)
-{
-}
-
static inline void post_schedule(struct rq *rq)
{
}
@@ -2571,7 +2560,8 @@ pick_next_task(struct rq *rq, struct tas
* Optimization: we know that if all tasks are in
* the fair class we can call that function directly:
*/
- if (likely(rq->nr_running == rq->cfs.h_nr_running))
+ if (likely(prev->sched_class == &fair_sched_class &&
+ rq->nr_running == rq->cfs.h_nr_running))
return fair_sched_class.pick_next_task(rq, prev);

for_each_class(class) {
@@ -2581,14 +2571,6 @@ pick_next_task(struct rq *rq, struct tas
}
}

- /*
- * If there is a task balanced on this cpu, pick the next task,
- * otherwise fall in the optimization by picking the idle task
- * directly.
- */
- if (idle_balance(rq))
- goto again;
-
rq->idle_stamp = rq_clock(rq);

return idle_sched_class.pick_next_task(rq, prev);
@@ -2682,8 +2664,6 @@ static void __sched __schedule(void)
switch_count = &prev->nvcsw;
}

- pre_schedule(rq, prev);
-
if (prev->on_rq || rq->skip_clock_update < 0)
update_rq_clock(rq);

--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -997,6 +997,9 @@ struct task_struct *pick_next_task_dl(st

dl_rq = &rq->dl;

+ if (dl_task(prev))
+ pull_dl_task(rq);
+
if (unlikely(!dl_rq->dl_nr_running))
return NULL;

@@ -1430,8 +1433,6 @@ static int pull_dl_task(struct rq *this_
static void pre_schedule_dl(struct rq *rq, struct task_struct *prev)
{
/* Try to pull other tasks here */
- if (dl_task(prev))
- pull_dl_task(rq);
}

static void post_schedule_dl(struct rq *rq)
@@ -1626,7 +1627,6 @@ const struct sched_class dl_sched_class
.set_cpus_allowed = set_cpus_allowed_dl,
.rq_online = rq_online_dl,
.rq_offline = rq_offline_dl,
- .pre_schedule = pre_schedule_dl,
.post_schedule = post_schedule_dl,
.task_woken = task_woken_dl,
#endif
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2581,7 +2581,8 @@ void idle_exit_fair(struct rq *this_rq)
update_rq_runnable_avg(this_rq, 0);
}

-#else
+#else /* CONFIG_SMP */
+
static inline void update_entity_load_avg(struct sched_entity *se,
int update_cfs_rq) {}
static inline void update_rq_runnable_avg(struct rq *rq, int runnable) {}
@@ -2593,7 +2594,7 @@ static inline void dequeue_entity_load_a
int sleep) {}
static inline void update_cfs_rq_blocked_load(struct cfs_rq *cfs_rq,
int force_update) {}
-#endif
+#endif /* CONFIG_SMP */

static void enqueue_sleeper(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
@@ -4700,10 +4701,11 @@ pick_next_task_fair(struct rq *rq, struc
struct sched_entity *se;
struct task_struct *p;

+again:
+#ifdef CONFIG_FAIR_GROUP_SCHED
if (!cfs_rq->nr_running)
- return NULL;
+ goto idle;

-#ifdef CONFIG_FAIR_GROUP_SCHED
if (prev->sched_class != &fair_sched_class)
goto simple;

@@ -4769,11 +4771,10 @@ pick_next_task_fair(struct rq *rq, struc

return p;
simple:
-#endif
cfs_rq = &rq->cfs;
-
+#endif
if (!cfs_rq->nr_running)
- return NULL;
+ goto idle;

prev->sched_class->put_prev_task(rq, prev);

@@ -4789,6 +4790,13 @@ pick_next_task_fair(struct rq *rq, struc
hrtick_start_fair(rq, p);

return p;
+
+idle:
+ idle_exit_fair();
+ if (idle_balance()) /* drops rq->lock */
+ goto again;
+
+ return NULL;
}

/*
--- a/kernel/sched/idle_task.c
+++ b/kernel/sched/idle_task.c
@@ -13,18 +13,8 @@ select_task_rq_idle(struct task_struct *
{
return task_cpu(p); /* IDLE tasks as never migrated */
}
-
-static void pre_schedule_idle(struct rq *rq, struct task_struct *prev)
-{
- idle_exit_fair(rq);
- rq_last_tick_reset(rq);
-}
-
-static void post_schedule_idle(struct rq *rq)
-{
- idle_enter_fair(rq);
-}
#endif /* CONFIG_SMP */
+
/*
* Idle tasks are unconditionally rescheduled:
*/
@@ -40,8 +30,7 @@ pick_next_task_idle(struct rq *rq, struc

schedstat_inc(rq, sched_goidle);
#ifdef CONFIG_SMP
- /* Trigger the post schedule to do an idle_enter for CFS */
- rq->post_schedule = 1;
+ idle_enter_fair(rq);
#endif
return rq->idle;
}
@@ -61,6 +50,10 @@ dequeue_task_idle(struct rq *rq, struct

static void put_prev_task_idle(struct rq *rq, struct task_struct *prev)
{
+#ifdef CONFIG_SMP
+ idle_exit_fair(rq);
+ rq_last_tick_reset(rq);
+#endif
}

static void task_tick_idle(struct rq *rq, struct task_struct *curr, int queued)
@@ -104,8 +97,6 @@ const struct sched_class idle_sched_clas

#ifdef CONFIG_SMP
.select_task_rq = select_task_rq_idle,
- .pre_schedule = pre_schedule_idle,
- .post_schedule = post_schedule_idle,
#endif

.set_curr_task = set_curr_task_idle,
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1330,6 +1330,10 @@ pick_next_task_rt(struct rq *rq, struct
struct task_struct *p;
struct rt_rq *rt_rq = &rq->rt;

+ /* Try to pull RT tasks here if we lower this rq's prio */
+ if (rq->rt.highest_prio.curr > prev->prio)
+ pull_rt_task(rq);
+
if (!rt_rq->rt_nr_running)
return NULL;

@@ -1720,13 +1724,6 @@ static int pull_rt_task(struct rq *this_
return ret;
}

-static void pre_schedule_rt(struct rq *rq, struct task_struct *prev)
-{
- /* Try to pull RT tasks here if we lower this rq's prio */
- if (rq->rt.highest_prio.curr > prev->prio)
- pull_rt_task(rq);
-}
-
static void post_schedule_rt(struct rq *rq)
{
push_rt_tasks(rq);
@@ -2003,7 +2000,6 @@ const struct sched_class rt_sched_class
.set_cpus_allowed = set_cpus_allowed_rt,
.rq_online = rq_online_rt,
.rq_offline = rq_offline_rt,
- .pre_schedule = pre_schedule_rt,
.post_schedule = post_schedule_rt,
.task_woken = task_woken_rt,
.switched_from = switched_from_rt,
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1136,7 +1136,6 @@ struct sched_class {
int (*select_task_rq)(struct task_struct *p, int task_cpu, int sd_flag, int flags);
void (*migrate_task_rq)(struct task_struct *p, int next_cpu);

- void (*pre_schedule) (struct rq *this_rq, struct task_struct *task);
void (*post_schedule) (struct rq *this_rq);
void (*task_waking) (struct task_struct *task);
void (*task_woken) (struct rq *this_rq, struct task_struct *task);