Re: [PATCH v2 3/3] sched/fair: break out of newidle balancing if an RT task appears

[Mike Galbraith]

On Fri, 2021-05-07 at 13:03 +0200, Peter Zijlstra wrote:
>
> newidle_balance() already has 'stop, you're spending too much time'
> controls; you've failed to explain how those are falling short and why
> they cannot be improved.

Greetings,

I bet a nickle he's meeting contention on a more painful scale than the
2212.429350 sample from my little i4790 desktop box, as master.today+rt
does.. nothing remotely RT like, just plays youtube as I instead watch
trace output.

homer:..debug/tracing # tail -20 trace
 MediaPl~back #1-17837   [003] d...2..  2212.292335: load_balance: NEWIDLE rq:2 lock acquired
  Gecko_IOThread-4001    [000] d...2..  2212.315030: load_balance: NEWIDLE rq:3 lock acquired
         firefox-3980    [007] d...1..  2212.315030: load_balance: NEWIDLE rq:3 locked - aborting
           Timer-4089    [007] d...2..  2212.317260: load_balance: NEWIDLE rq:0 lock acquired
           Timer-4089    [007] d...2..  2212.317319: load_balance: NEWIDLE rq:0 lock acquired
     ksoftirqd/6-51      [006] d...2..  2212.317358: load_balance: NEWIDLE rq:0 lock acquired
           Timer-4089    [007] d...2..  2212.317474: load_balance: NEWIDLE rq:0 lock acquired
 MediaPl~back #2-17839   [002] d...1..  2212.345438: load_balance: NEWIDLE rq:3 locked - aborting
 Chrome_~dThread-16830   [006] d...2..  2212.345438: load_balance: NEWIDLE rq:3 lock acquired
 AudioIP~ent RPC-17665   [003] d...2..  2212.404999: load_balance: NEWIDLE rq:5 lock acquired
 ImageBridgeChld-16855   [001] d...2..  2212.429350: load_balance: NEWIDLE rq:6 lock acquired
 MediaPl~back #1-17837   [000] d...1..  2212.429351: load_balance: NEWIDLE rq:6 locked - aborting
 Chrome_~dThread-16830   [002] d...1..  2212.429356: load_balance: NEWIDLE rq:6 locked - aborting
               X-2157    [003] d...2..  2212.461351: load_balance: NEWIDLE rq:6 lock acquired
           <...>-4043    [006] d...2..  2212.480451: load_balance: NEWIDLE rq:2 lock acquired
     ksoftirqd/0-12      [000] d...2..  2212.505545: load_balance: NEWIDLE rq:3 lock acquired
     ksoftirqd/0-12      [000] d...2..  2212.505550: load_balance: NEWIDLE rq:3 lock acquired
     threaded-ml-4399    [001] d...2..  2212.517943: load_balance: NEWIDLE rq:7 lock acquired
      pulseaudio-1917    [002] d...2..  2212.575245: load_balance: NEWIDLE rq:6 lock acquired
 IPDL Background-4028    [004] d...2..  2212.581085: load_balance: NEWIDLE rq:5 lock acquired
homer:..debug/tracing #

homer:..debug/tracing # cat trace | grep lock | wc -l
218165
homer:..debug/tracing # cat trace | grep abort | wc -l
40081

I still carry the below in my local RT trees, to which I added those
trace_printk()s.  It was born some years ago on a 64 core box.

---
 kernel/sched/fair.c     |   11 +++++++++++
 kernel/sched/features.h |    4 ++++
 kernel/sched/sched.h    |   10 ++++++++++
 3 files changed, 25 insertions(+)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7200,6 +7200,12 @@ done: __maybe_unused;
 	if (!rf)
 		return NULL;

+#ifdef CONFIG_PREEMPT_RT
+	/* RT tasks have better things to do than load balancing. */
+	if (prev && prev->sched_class != &fair_sched_class)
+		return NULL;
+#endif
+
 	new_tasks = newidle_balance(rq, rf);

 	/*
@@ -9669,7 +9675,12 @@ static int load_balance(int this_cpu, st
 		env.loop_max  = min(sysctl_sched_nr_migrate, busiest->nr_running);

 more_balance:
+#ifdef CONFIG_PREEMPT_RT
+		if (!rq_lock_trylock_irqsave(busiest, &rf))
+			goto out;
+#else
 		rq_lock_irqsave(busiest, &rf);
+#endif
 		update_rq_clock(busiest);

 		/*
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -86,7 +86,11 @@ SCHED_FEAT(RT_PUSH_IPI, true)
 #endif

 SCHED_FEAT(RT_RUNTIME_SHARE, false)
+#ifndef CONFIG_PREEMPT_RT
 SCHED_FEAT(LB_MIN, false)
+#else
+SCHED_FEAT(LB_MIN, true)
+#endif
 SCHED_FEAT(ATTACH_AGE_LOAD, true)

 SCHED_FEAT(WA_IDLE, true)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1322,6 +1322,16 @@ rq_lock_irqsave(struct rq *rq, struct rq
 	rq_pin_lock(rq, rf);
 }

+static inline int
+rq_lock_trylock_irqsave(struct rq *rq, struct rq_flags *rf)
+	__acquires(rq->lock)
+{
+	if (!raw_spin_trylock_irqsave(&rq->lock, rf->flags))
+		return 0;
+	rq_pin_lock(rq, rf);
+	return 1;
+}
+
 static inline void
 rq_lock_irq(struct rq *rq, struct rq_flags *rf)
 	__acquires(rq->lock)