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sched_ext: Simplify UP support by enabling sched_class->balance() in UP

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On SMP, SCX performs dispatch from sched_class->balance(). As balance() was
not available in UP, it instead called the internal balance function from
put_prev_task_scx() and pick_next_task_scx() to emulate the effect, which is
rather nasty.

Enabling sched_class->balance() on UP shouldn't cause any meaningful
overhead. Enable balance() on UP and drop the ugly workaround.

Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: David Vernet <void@manifault.com>
This commit is contained in:
Tejun Heo 2024-08-06 09:40:11 -10:00
parent 7799140b6a
commit a735d43c7f
3 changed files with 3 additions and 44 deletions
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4
kernel/sched/core.c
View File

@ -5857,7 +5857,6 @@ static inline void schedule_debug(struct task_struct *prev, bool preempt)
static void put_prev_task_balance(struct rq *rq, struct task_struct *prev,
struct rq_flags *rf)
{
#ifdef CONFIG_SMP
const struct sched_class *start_class = prev->sched_class;
const struct sched_class *class;
@ -5880,10 +5879,9 @@ static void put_prev_task_balance(struct rq *rq, struct task_struct *prev,
* a runnable task of @class priority or higher.
*/
for_active_class_range(class, start_class, &idle_sched_class) {
if (class->balance(rq, prev, rf))
if (class->balance && class->balance(rq, prev, rf))
break;
}
#endif
put_prev_task(rq, prev);

41
kernel/sched/ext.c
View File

@ -2616,7 +2616,6 @@ static int balance_one(struct rq *rq, struct task_struct *prev, bool local)
return has_tasks;
}
#ifdef CONFIG_SMP
static int balance_scx(struct rq *rq, struct task_struct *prev,
struct rq_flags *rf)
{
@ -2650,7 +2649,6 @@ static int balance_scx(struct rq *rq, struct task_struct *prev,
return ret;
}
#endif
static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
{
@ -2719,37 +2717,6 @@ static void process_ddsp_deferred_locals(struct rq *rq)
static void put_prev_task_scx(struct rq *rq, struct task_struct *p)
{
#ifndef CONFIG_SMP
/*
* UP workaround.
*
* Because SCX may transfer tasks across CPUs during dispatch, dispatch
* is performed from its balance operation which isn't called in UP.
* Let's work around by calling it from the operations which come right
* after.
*
* 1. If the prev task is on SCX, pick_next_task() calls
* .put_prev_task() right after. As .put_prev_task() is also called
* from other places, we need to distinguish the calls which can be
* done by looking at the previous task's state - if still queued or
* dequeued with %SCX_DEQ_SLEEP, the caller must be pick_next_task().
* This case is handled here.
*
* 2. If the prev task is not on SCX, the first following call into SCX
* will be .pick_next_task(), which is covered by calling
* balance_scx() from pick_next_task_scx().
*
* Note that we can't merge the first case into the second as
* balance_scx() must be called before the previous SCX task goes
* through put_prev_task_scx().
*
* @rq is pinned and can't be unlocked. As UP doesn't transfer tasks
* around, balance_one() doesn't need to.
*/
if (p->scx.flags & (SCX_TASK_QUEUED | SCX_TASK_DEQD_FOR_SLEEP))
balance_one(rq, p, true);
#endif
update_curr_scx(rq);
/* see dequeue_task_scx() on why we skip when !QUEUED */
@ -2807,12 +2774,6 @@ static struct task_struct *pick_next_task_scx(struct rq *rq)
{
struct task_struct *p;
#ifndef CONFIG_SMP
/* UP workaround - see the comment at the head of put_prev_task_scx() */
if (unlikely(rq->curr->sched_class != &ext_sched_class))
balance_one(rq, rq->curr, true);
#endif
p = first_local_task(rq);
if (!p)
return NULL;
@ -3673,6 +3634,7 @@ DEFINE_SCHED_CLASS(ext) = {
.wakeup_preempt = wakeup_preempt_scx,
.balance = balance_scx,
.pick_next_task = pick_next_task_scx,
.put_prev_task = put_prev_task_scx,
@ -3681,7 +3643,6 @@ DEFINE_SCHED_CLASS(ext) = {
.switch_class = switch_class_scx,
#ifdef CONFIG_SMP
.balance = balance_scx,
.select_task_rq = select_task_rq_scx,
.task_woken = task_woken_scx,
.set_cpus_allowed = set_cpus_allowed_scx,

2
kernel/sched/sched.h
View File

@ -2361,6 +2361,7 @@ struct sched_class {
void (*wakeup_preempt)(struct rq *rq, struct task_struct *p, int flags);
int (*balance)(struct rq *rq, struct task_struct *prev, struct rq_flags *rf);
struct task_struct *(*pick_next_task)(struct rq *rq);
void (*put_prev_task)(struct rq *rq, struct task_struct *p);
@ -2369,7 +2370,6 @@ struct sched_class {
void (*switch_class)(struct rq *rq, struct task_struct *next);
#ifdef CONFIG_SMP
int (*balance)(struct rq *rq, struct task_struct *prev, struct rq_flags *rf);
int (*select_task_rq)(struct task_struct *p, int task_cpu, int flags);
struct task_struct * (*pick_task)(struct rq *rq);