Commit a735d43c authored by Tejun Heo's avatar Tejun Heo
Browse files

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



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: default avatarTejun Heo <tj@kernel.org>
Suggested-by: default avatarPeter Zijlstra <peterz@infradead.org>
Acked-by: default avatarDavid Vernet <void@manifault.com>
parent 7799140b

kernel/sched/core.c

+1 −3
Original line number Diff line number Diff line
@@ -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);

kernel/sched/ext.c

+1 −40
Original line number Diff line number Diff line
@@ -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,

kernel/sched/sched.h

+1 −1
Original line number Diff line number Diff line
@@ -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);