Re: [RFC][PATCH 08/10] sched/fair: Implement delayed dequeue

Peter Zijlstra
2024-05-15T11:48:43Z

On Wed, May 15, 2024 at 11:36:49AM +0200, Peter Zijlstra wrote:
> On Fri, May 10, 2024 at 03:49:46PM +0100, Luis Machado wrote:
> > Just a quick update on this. While investigating this behavior, I
> > spotted very high loadavg values on an idle system. For instance:
> >
> > load average: 4733.84, 4721.24, 4680.33
> >
> > I wonder if someone else also spotted this.
>
> Hadn't spotted it, but now that you mention it, I can definitely see it.
>
> Let me go prod with something sharp. Thanks!

What's the point of making notes if you then don't read them... *sigh*.

Does this help?

---
kernel/sched/core.c | 23 ++++++++++++-----------
kernel/sched/fair.c | 4 ++--
kernel/sched/sched.h | 8 ++++++++
3 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5ffd7e047393..43f061bcfe54 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2154,14 +2154,18 @@ void activate_task(struct rq *rq, struct task_struct *p, int flags)

void deactivate_task(struct rq *rq, struct task_struct *p, int flags)
{
- bool sleep = flags & DEQUEUE_SLEEP;
+ SCHED_WARN_ON(flags & DEQUEUE_SLEEP);

- if (dequeue_task(rq, p, flags)) {
- WRITE_ONCE(p->on_rq, sleep ? 0 : TASK_ON_RQ_MIGRATING);
- ASSERT_EXCLUSIVE_WRITER(p->on_rq);
- } else {
- SCHED_WARN_ON(!sleep); /* only sleep can fail */
- }
+ dequeue_task(rq, p, flags);
+
+ WRITE_ONCE(p->on_rq, TASK_ON_RQ_MIGRATING);
+ ASSERT_EXCLUSIVE_WRITER(p->on_rq);
+}
+
+static void block_task(struct rq *rq, struct task_struct *p, int flags)
+{
+ if (dequeue_task(rq, p, DEQUEUE_SLEEP | flags))
+ __block_task(rq, p);
}

static inline int __normal_prio(int policy, int rt_prio, int nice)
@@ -6693,9 +6697,6 @@ static void __sched notrace __schedule(unsigned int sched_mode)
!(prev_state & TASK_NOLOAD) &&
!(prev_state & TASK_FROZEN);

- if (prev->sched_contributes_to_load)
- rq->nr_uninterruptible++;
-
/*
* __schedule() ttwu()
* prev_state = prev->state; if (p->on_rq && ...)
@@ -6707,7 +6708,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
*
* After this, schedule() must not care about p->state any more.
*/
- deactivate_task(rq, prev, DEQUEUE_SLEEP | DEQUEUE_NOCLOCK);
+ block_task(rq, prev, DEQUEUE_SLEEP | DEQUEUE_NOCLOCK);

if (prev->in_iowait) {
atomic_inc(&rq->nr_iowait);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 536eabcb1a71..596a5fabe490 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7032,8 +7032,8 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
util_est_update(&rq->cfs, p, task_sleep);
hrtick_update(rq);

- /* Fix-up what deactivate_task() skipped. */
- WRITE_ONCE(p->on_rq, 0);
+ /* Fix-up what block_task() skipped. */
+ __block_task(rq, p);
}
}

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 12841d8195c5..48e5f49d9bc2 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2560,6 +2560,14 @@ static inline void sub_nr_running(struct rq *rq, unsigned count)
sched_update_tick_dependency(rq);
}

+static inline void __block_task(struct rq *rq, struct task_struct *p)
+{
+ WRITE_ONCE(p->on_rq, 0);
+ ASSERT_EXCLUSIVE_WRITER(p->on_rq);
+ if (p->sched_contributes_to_load)
+ rq->nr_uninterruptible++;
+}
+
extern void activate_task(struct rq *rq, struct task_struct *p, int flags);
extern void deactivate_task(struct rq *rq, struct task_struct *p, int flags);