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SubjectRe: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs hotplug-rollback
On Tue, Apr 13, 2021 at 08:51:23AM +0200, Peter Zijlstra wrote:

> > I'm afraid I don't follow; we're replacing a read of rq->balance_push with
> > cpu_dying(), and those are still written on the same side of the
> > synchronize_rcu(). What am I missing?
>
> Yeah, I'm not sure anymnore either; I tried to work out why I'd done
> that but upon closer examination everything fell flat.
>
> Let me try again today :-)

Can't make sense of what I did.. I've removed that hunk. Patch now looks
like this.

---
Subject: sched: Use cpu_dying() to fix balance_push vs hotplug-rollback
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu Jan 21 16:09:32 CET 2021

Use the new cpu_dying() state to simplify and fix the balance_push()
vs CPU hotplug rollback state.

Specifically, we currently rely on notifiers sched_cpu_dying() /
sched_cpu_activate() to terminate balance_push, however if the
cpu_down() fails when we're past sched_cpu_deactivate(), it should
terminate balance_push at that point and not wait until we hit
sched_cpu_activate().

Similarly, when cpu_up() fails and we're going back down, balance_push
should be active, where it currently is not.

So instead, make sure balance_push is enabled between
sched_cpu_deactivate() and sched_cpu_activate() (eg. when
!cpu_active()), and gate it's utility with cpu_dying().

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/sched/core.c | 26 +++++++++++++++-----------
kernel/sched/sched.h | 1 -
2 files changed, 15 insertions(+), 12 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1811,7 +1811,7 @@ static inline bool is_cpu_allowed(struct
return cpu_online(cpu);

/* Regular kernel threads don't get to stay during offline. */
- if (cpu_rq(cpu)->balance_push)
+ if (cpu_dying(cpu))
return false;

/* But are allowed during online. */
@@ -7639,6 +7639,9 @@ static DEFINE_PER_CPU(struct cpu_stop_wo

/*
* Ensure we only run per-cpu kthreads once the CPU goes !active.
+ *
+ * This is active/set between sched_cpu_deactivate() / sched_cpu_activate().
+ * But only effective when the hotplug motion is down.
*/
static void balance_push(struct rq *rq)
{
@@ -7646,12 +7649,19 @@ static void balance_push(struct rq *rq)

lockdep_assert_held(&rq->lock);
SCHED_WARN_ON(rq->cpu != smp_processor_id());
+
/*
* Ensure the thing is persistent until balance_push_set(.on = false);
*/
rq->balance_callback = &balance_push_callback;

/*
+ * Only active while going offline.
+ */
+ if (!cpu_dying(rq->cpu))
+ return;
+
+ /*
* Both the cpu-hotplug and stop task are in this case and are
* required to complete the hotplug process.
*
@@ -7704,7 +7714,6 @@ static void balance_push_set(int cpu, bo
struct rq_flags rf;

rq_lock_irqsave(rq, &rf);
- rq->balance_push = on;
if (on) {
WARN_ON_ONCE(rq->balance_callback);
rq->balance_callback = &balance_push_callback;
@@ -7829,8 +7838,8 @@ int sched_cpu_activate(unsigned int cpu)
struct rq_flags rf;

/*
- * Make sure that when the hotplug state machine does a roll-back
- * we clear balance_push. Ideally that would happen earlier...
+ * Clear the balance_push callback and prepare to schedule
+ * regular tasks.
*/
balance_push_set(cpu, false);

@@ -8015,12 +8024,6 @@ int sched_cpu_dying(unsigned int cpu)
}
rq_unlock_irqrestore(rq, &rf);

- /*
- * Now that the CPU is offline, make sure we're welcome
- * to new tasks once we come back up.
- */
- balance_push_set(cpu, false);
-
calc_load_migrate(rq);
update_max_interval();
hrtick_clear(rq);
@@ -8205,7 +8208,7 @@ void __init sched_init(void)
rq->sd = NULL;
rq->rd = NULL;
rq->cpu_capacity = rq->cpu_capacity_orig = SCHED_CAPACITY_SCALE;
- rq->balance_callback = NULL;
+ rq->balance_callback = &balance_push_callback;
rq->active_balance = 0;
rq->next_balance = jiffies;
rq->push_cpu = 0;
@@ -8252,6 +8255,7 @@ void __init sched_init(void)

#ifdef CONFIG_SMP
idle_thread_set_boot_cpu();
+ balance_push_set(smp_processor_id(), false);
#endif
init_sched_fair_class();

--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -983,7 +983,6 @@ struct rq {
unsigned long cpu_capacity_orig;

struct callback_head *balance_callback;
- unsigned char balance_push;

unsigned char nohz_idle_balance;
unsigned char idle_balance;
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