[RFC PATCH 02/13] sched: Introduce sched_class::pick_task()

vpillai
2020-03-04T17:00:50Z

From: Peter Zijlstra <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>
---
kernel/sched/deadline.c | 16 ++++++++++++++--
kernel/sched/fair.c | 34 +++++++++++++++++++++++++++++++---
kernel/sched/idle.c | 6 ++++++
kernel/sched/rt.c | 14 ++++++++++++--
kernel/sched/sched.h | 3 +++
kernel/sched/stop_task.c | 13 +++++++++++--
6 files changed, 77 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index ded147f84382..ee7fd8611ee4 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1773,7 +1773,7 @@ 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)
+static struct task_struct *pick_task_dl(struct rq *rq)
{
struct sched_dl_entity *dl_se;
struct dl_rq *dl_rq = &rq->dl;
@@ -1785,7 +1785,18 @@ static struct task_struct *pick_next_task_dl(struct rq *rq)
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, true);
+
+ return p;
+}
+
+static struct task_struct *pick_next_task_dl(struct rq *rq)
+{
+ struct task_struct *p;
+
+ p = pick_task_dl(rq);
+ if (p)
+ set_next_task_dl(rq, p, true);
+
return p;
}

@@ -2442,6 +2453,7 @@ const struct sched_class dl_sched_class = {

#ifdef CONFIG_SMP
.balance = balance_dl,
+ .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 3b218753bf7a..5eaaf0c4d9ad 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4228,7 +4228,7 @@ 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) {
@@ -4246,13 +4246,13 @@ 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);
@@ -6642,6 +6642,33 @@ 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 (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);
+}
+
struct task_struct *
pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
{
@@ -10771,6 +10798,7 @@ const struct sched_class fair_sched_class = {

#ifdef CONFIG_SMP
.balance = balance_fair,
+ .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 f8653290de95..46c18e3dab13 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -397,6 +397,11 @@ static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool fir
schedstat_inc(rq->sched_goidle);
}

+static struct task_struct *pick_task_idle(struct rq *rq)
+{
+ return rq->idle;
+}
+
struct task_struct *pick_next_task_idle(struct rq *rq)
{
struct task_struct *next = rq->idle;
@@ -469,6 +474,7 @@ const struct sched_class idle_sched_class = {

#ifdef CONFIG_SMP
.balance = balance_idle,
+ .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 fc7d6706b209..d044baedc617 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1567,7 +1567,7 @@ 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)
+static struct task_struct *pick_task_rt(struct rq *rq)
{
struct task_struct *p;

@@ -1575,7 +1575,16 @@ static struct task_struct *pick_next_task_rt(struct rq *rq)
return NULL;

p = _pick_next_task_rt(rq);
- set_next_task_rt(rq, p, true);
+
+ return p;
+}
+
+static struct task_struct *pick_next_task_rt(struct rq *rq)
+{
+ struct task_struct *p = pick_task_rt(rq);
+ if (p)
+ set_next_task_rt(rq, p, true);
+
return p;
}

@@ -2368,6 +2377,7 @@ const struct sched_class rt_sched_class = {

#ifdef CONFIG_SMP
.balance = balance_rt,
+ .pick_task = pick_task_rt,
.select_task_rq = select_task_rq_rt,
.set_cpus_allowed = set_cpus_allowed_common,
.rq_online = rq_online_rt,
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index a306008a12f7..a8335e3078ab 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1724,6 +1724,9 @@ struct sched_class {

#ifdef CONFIG_SMP
int (*balance)(struct rq *rq, struct task_struct *prev, struct rq_flags *rf);
+
+ 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 4c9e9975684f..0611348edb28 100644
--- a/kernel/sched/stop_task.c
+++ b/kernel/sched/stop_task.c
@@ -34,15 +34,23 @@ static void set_next_task_stop(struct rq *rq, struct task_struct *stop, bool fir
stop->se.exec_start = rq_clock_task(rq);
}

-static struct task_struct *pick_next_task_stop(struct rq *rq)
+static struct task_struct *pick_task_stop(struct rq *rq)
{
if (!sched_stop_runnable(rq))
return NULL;

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

+static struct task_struct *pick_next_task_stop(struct rq *rq)
+{
+ struct task_struct *p = pick_task_stop(rq);
+ if (p)
+ set_next_task_stop(rq, p, true);
+
+ return p;
+}
+
static void
enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
{
@@ -130,6 +138,7 @@ const struct sched_class stop_sched_class = {

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