[RFC][PATCH 07/16] sched: Allow put_prev_task() to drop rq->lock

Peter Zijlstra
2019-02-18T17:41:43Z

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(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5592,7 +5592,7 @@ static void calc_load_migrate(struct 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)
{
}

--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1773,13 +1773,25 @@ pick_next_task_dl(struct rq *rq, struct
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);
+ }
}

/*
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7080,7 +7080,7 @@ 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;
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -373,7 +373,7 @@ static void check_preempt_curr_idle(stru
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)
{
}

--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1593,7 +1593,7 @@ pick_next_task_rt(struct rq *rq, struct
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 *
*/
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
--- 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)
--- a/kernel/sched/stop_task.c
+++ b/kernel/sched/stop_task.c
@@ -59,7 +59,7 @@ static void yield_task_stop(struct rq *r
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;