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Subject[RFC PATCH v2 09/17] sched: Introduce sched_class::pick_task()
Date
From: "Peter Zijlstra (Intel)" <peterz@infradead.org>

Because sched_class::pick_next_task() also implies
sched_class::set_next_task() (and possibly put_prev_task() and
newidle_balance) it is not state invariant. This makes it unsuitable
for remote task selection.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Vineeth Remanan Pillai <vpillai@digitalocean.com>
Signed-off-by: Julien Desfossez <jdesfossez@digitalocean.com>
---

Changes in v2
-------------
- Fixes a NULL pointer dereference crash
- Subhra Mazumdar
- Tim Chen

---
kernel/sched/deadline.c | 21 ++++++++++++++++-----
kernel/sched/fair.c | 39 ++++++++++++++++++++++++++++++++++++---
kernel/sched/idle.c | 10 +++++++++-
kernel/sched/rt.c | 21 ++++++++++++++++-----
kernel/sched/sched.h | 2 ++
kernel/sched/stop_task.c | 21 ++++++++++++++++-----
6 files changed, 95 insertions(+), 19 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 249310e68592..010234908cc0 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1723,15 +1723,12 @@ static struct sched_dl_entity *pick_next_dl_entity(struct rq *rq,
return rb_entry(left, struct sched_dl_entity, rb_node);
}

-static struct task_struct *
-pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
+static struct task_struct *pick_task_dl(struct rq *rq)
{
struct sched_dl_entity *dl_se;
struct task_struct *p;
struct dl_rq *dl_rq;

- WARN_ON_ONCE(prev || rf);
-
dl_rq = &rq->dl;

if (unlikely(!dl_rq->dl_nr_running))
@@ -1742,7 +1739,19 @@ pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)

p = dl_task_of(dl_se);

- set_next_task_dl(rq, p);
+ return p;
+}
+
+static struct task_struct *
+pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
+{
+ struct task_struct *p;
+
+ WARN_ON_ONCE(prev || rf);
+
+ p = pick_task_dl(rq);
+ if (p)
+ set_next_task_dl(rq, p);

return p;
}
@@ -2389,6 +2398,8 @@ const struct sched_class dl_sched_class = {
.set_next_task = set_next_task_dl,

#ifdef CONFIG_SMP
+ .pick_task = pick_task_dl,
+
.select_task_rq = select_task_rq_dl,
.migrate_task_rq = migrate_task_rq_dl,
.set_cpus_allowed = set_cpus_allowed_dl,
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c055bad249a9..45d86b862750 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4132,7 +4132,7 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
* Avoid running the skip buddy, if running something else can
* be done without getting too unfair.
*/
- if (cfs_rq->skip == se) {
+ if (cfs_rq->skip && cfs_rq->skip == se) {
struct sched_entity *second;

if (se == curr) {
@@ -4150,13 +4150,13 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
/*
* Prefer last buddy, try to return the CPU to a preempted task.
*/
- if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
+ if (left && cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
se = cfs_rq->last;

/*
* Someone really wants this to run. If it's not unfair, run it.
*/
- if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
+ if (left && cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
se = cfs_rq->next;

clear_buddies(cfs_rq, se);
@@ -6937,6 +6937,37 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
set_last_buddy(se);
}

+static struct task_struct *
+pick_task_fair(struct rq *rq)
+{
+ struct cfs_rq *cfs_rq = &rq->cfs;
+ struct sched_entity *se;
+
+ if (!cfs_rq->nr_running)
+ return NULL;
+
+ do {
+ struct sched_entity *curr = cfs_rq->curr;
+
+ se = pick_next_entity(cfs_rq, NULL);
+
+ if (!(se || curr))
+ return NULL;
+
+ if (curr) {
+ if (se && curr->on_rq)
+ update_curr(cfs_rq);
+
+ if (!se || entity_before(curr, se))
+ se = curr;
+ }
+
+ cfs_rq = group_cfs_rq(se);
+ } while (cfs_rq);
+
+ return task_of(se);
+}
+
static struct task_struct *
pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
{
@@ -10648,6 +10679,8 @@ const struct sched_class fair_sched_class = {
.set_next_task = set_next_task_fair,

#ifdef CONFIG_SMP
+ .pick_task = pick_task_fair,
+
.select_task_rq = select_task_rq_fair,
.migrate_task_rq = migrate_task_rq_fair,

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 7ece8e820b5d..e7f38da60373 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -373,6 +373,12 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl
resched_curr(rq);
}

+static struct task_struct *
+pick_task_idle(struct rq *rq)
+{
+ return rq->idle;
+}
+
static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
{
}
@@ -386,11 +392,12 @@ static void set_next_task_idle(struct rq *rq, struct task_struct *next)
static struct task_struct *
pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
{
- struct task_struct *next = rq->idle;
+ struct task_struct *next;

if (prev)
put_prev_task(rq, prev);

+ next = pick_task_idle(rq);
set_next_task_idle(rq, next);

return next;
@@ -458,6 +465,7 @@ const struct sched_class idle_sched_class = {
.set_next_task = set_next_task_idle,

#ifdef CONFIG_SMP
+ .pick_task = pick_task_idle,
.select_task_rq = select_task_rq_idle,
.set_cpus_allowed = set_cpus_allowed_common,
#endif
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 79f2e60516ef..81557224548c 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1548,20 +1548,29 @@ static struct task_struct *_pick_next_task_rt(struct rq *rq)
return rt_task_of(rt_se);
}

-static struct task_struct *
-pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
+static struct task_struct *pick_task_rt(struct rq *rq)
{
struct task_struct *p;
struct rt_rq *rt_rq = &rq->rt;

- WARN_ON_ONCE(prev || rf);
-
if (!rt_rq->rt_queued)
return NULL;

p = _pick_next_task_rt(rq);

- set_next_task_rt(rq, p);
+ return p;
+}
+
+static struct task_struct *
+pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
+{
+ struct task_struct *p;
+
+ WARN_ON_ONCE(prev || rf);
+
+ p = pick_task_rt(rq);
+ if (p)
+ set_next_task_rt(rq, p);

return p;
}
@@ -2364,6 +2373,8 @@ const struct sched_class rt_sched_class = {
.set_next_task = set_next_task_rt,

#ifdef CONFIG_SMP
+ .pick_task = pick_task_rt,
+
.select_task_rq = select_task_rq_rt,

.set_cpus_allowed = set_cpus_allowed_common,
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 460dd04e76af..a024dd80eeb3 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1682,6 +1682,8 @@ struct sched_class {
void (*set_next_task)(struct rq *rq, struct task_struct *p);

#ifdef CONFIG_SMP
+ struct task_struct * (*pick_task)(struct rq *rq);
+
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 new_cpu);

diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
index 7e1cee4e65b2..fb6c436cba6c 100644
--- a/kernel/sched/stop_task.c
+++ b/kernel/sched/stop_task.c
@@ -29,20 +29,30 @@ static void set_next_task_stop(struct rq *rq, struct task_struct *stop)
}

static struct task_struct *
-pick_next_task_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
+pick_task_stop(struct rq *rq)
{
struct task_struct *stop = rq->stop;

- WARN_ON_ONCE(prev || rf);
-
if (!stop || !task_on_rq_queued(stop))
return NULL;

- set_next_task_stop(rq, stop);
-
return stop;
}

+static struct task_struct *
+pick_next_task_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
+{
+ struct task_struct *p;
+
+ WARN_ON_ONCE(prev || rf);
+
+ p = pick_task_stop(rq);
+ if (p)
+ set_next_task_stop(rq, p);
+
+ return p;
+}
+
static void
enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
{
@@ -129,6 +139,7 @@ const struct sched_class stop_sched_class = {
.set_next_task = set_next_task_stop,

#ifdef CONFIG_SMP
+ .pick_task = pick_task_stop,
.select_task_rq = select_task_rq_stop,
.set_cpus_allowed = set_cpus_allowed_common,
#endif
--
2.17.1
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