[PATCH 26/30] sched: handle preempt=voluntary under PREEMPT_AUTO
From: Ankur Arora
Date: Tue Feb 13 2024 - 01:04:00 EST
The default preemption policy for voluntary preemption under
PREEMPT_AUTO is to schedule eagerly for tasks of higher scheduling
class, and lazily for well-behaved, non-idle tasks.
This is the same policy as preempt=none, with an eager handling of
higher priority scheduling classes.
So, compare a SCHED_DEADLINE workload (stress-ng --cyclic) with a
background kernel load of 'stress-ng --mmap':
# stress-ng --mmap 0 &
# stress-ng --cyclic 1 --timeout 300
PREEMPT_AUTO, preempt=none
stress-ng: info: [8827] setting to a 300 second (5 mins, 0.00 secs) run per stressor
stress-ng: info: [8827] dispatching hogs: 1 cyclic
stress-ng: info: [8828] cyclic: sched SCHED_DEADLINE: 100000 ns delay, 10000 samples
stress-ng: info: [8828] cyclic: mean: 23829.70 ns, mode: 3317 ns
stress-ng: info: [8828] cyclic: min: 2688 ns, max: 5701735 ns, std.dev. 123502.57
stress-ng: info: [8828] cyclic: latency percentiles:
stress-ng: info: [8828] cyclic: 25.00%: 6289 ns
stress-ng: info: [8828] cyclic: 50.00%: 13945 ns
stress-ng: info: [8828] cyclic: 75.00%: 25335 ns
stress-ng: info: [8828] cyclic: 90.00%: 34500 ns
stress-ng: info: [8828] cyclic: 95.40%: 41470 ns
stress-ng: info: [8828] cyclic: 99.00%: 85602 ns
stress-ng: info: [8828] cyclic: 99.50%: 301099 ns
stress-ng: info: [8828] cyclic: 99.90%: 1798633 ns
stress-ng: info: [8828] cyclic: 99.99%: 5701735 ns
stress-ng: info: [8827] successful run completed in 300.01s (5 mins, 0.01 secs)
PREEMPT_AUTO, preempt=voluntary
stress-ng: info: [8883] setting to a 300 second (5 mins, 0.00 secs) run per stressor
stress-ng: info: [8883] dispatching hogs: 1 cyclic
stress-ng: info: [8884] cyclic: sched SCHED_DEADLINE: 100000 ns delay, 10000 samples
stress-ng: info: [8884] cyclic: mean: 14169.08 ns, mode: 3355 ns
stress-ng: info: [8884] cyclic: min: 2570 ns, max: 2234939 ns, std.dev. 66056.95
stress-ng: info: [8884] cyclic: latency percentiles:
stress-ng: info: [8884] cyclic: 25.00%: 3665 ns
stress-ng: info: [8884] cyclic: 50.00%: 5409 ns
stress-ng: info: [8884] cyclic: 75.00%: 16009 ns
stress-ng: info: [8884] cyclic: 90.00%: 24392 ns
stress-ng: info: [8884] cyclic: 95.40%: 28827 ns
stress-ng: info: [8884] cyclic: 99.00%: 39153 ns
stress-ng: info: [8884] cyclic: 99.50%: 168643 ns
stress-ng: info: [8884] cyclic: 99.90%: 1186267 ns
stress-ng: info: [8884] cyclic: 99.99%: 2234939 ns
stress-ng: info: [8883] successful run completed in 300.01s (5 mins, 0.01 secs)
So, the latency improves significantly -- even at the 25th percentile
threshold.
This configuration also compares quite favourably to voluntary
preemption under PREEMPT_DYNAMIC.
PREEMPT_DYNAMIC, preempt=voluntary
stress-ng: info: [12252] setting to a 300 second (5 mins, 0.00 secs) run per stressor
stress-ng: info: [12252] dispatching hogs: 1 cyclic
stress-ng: info: [12253] cyclic: sched SCHED_DEADLINE: 100000 ns delay, 10000 samples
stress-ng: info: [12253] cyclic: mean: 19973.46 ns, mode: 3560 ns
stress-ng: info: [12253] cyclic: min: 2541 ns, max: 2751830 ns, std.dev. 68891.71
stress-ng: info: [12253] cyclic: latency percentiles:
stress-ng: info: [12253] cyclic: 25.00%: 4800 ns
stress-ng: info: [12253] cyclic: 50.00%: 12458 ns
stress-ng: info: [12253] cyclic: 75.00%: 25220 ns
stress-ng: info: [12253] cyclic: 90.00%: 35404 ns
stress-ng: info: [12253] cyclic: 95.40%: 43135 ns
stress-ng: info: [12253] cyclic: 99.00%: 61053 ns
stress-ng: info: [12253] cyclic: 99.50%: 98159 ns
stress-ng: info: [12253] cyclic: 99.90%: 1164407 ns
stress-ng: info: [12253] cyclic: 99.99%: 2751830 ns
stress-ng: info: [12252] successful run completed in 300.01s (5 mins, 0.01 secs)
And, as you would expect, we perform identically to preempt=full
with PREEMPT_DYNAMIC (ignoring the outliers at 99.99%.)
PREEMPT_DYNAMIC, preempt=full
stress-ng: info: [12206] setting to a 300 second (5 mins, 0.00 secs) run per stressor
stress-ng: info: [12206] dispatching hogs: 1 cyclic
stress-ng: info: [12207] cyclic: sched SCHED_DEADLINE: 100000 ns delay, 10000 samples
stress-ng: info: [12207] cyclic: mean: 16647.05 ns, mode: 3535 ns
stress-ng: info: [12207] cyclic: min: 2548 ns, max: 4093382 ns, std.dev. 116825.95
stress-ng: info: [12207] cyclic: latency percentiles:
stress-ng: info: [12207] cyclic: 25.00%: 3568 ns
stress-ng: info: [12207] cyclic: 50.00%: 4755 ns
stress-ng: info: [12207] cyclic: 75.00%: 15187 ns
stress-ng: info: [12207] cyclic: 90.00%: 24083 ns
stress-ng: info: [12207] cyclic: 95.40%: 29314 ns
stress-ng: info: [12207] cyclic: 99.00%: 40102 ns
stress-ng: info: [12207] cyclic: 99.50%: 366571 ns
stress-ng: info: [12207] cyclic: 99.90%: 2752037 ns
stress-ng: info: [12207] cyclic: 99.99%: 4093382 ns
stress-ng: info: [12206] successful run completed in 300.01s (5 mins, 0.01 secs)
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Ziljstra <peterz@xxxxxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Originally-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
kernel/sched/core.c | 12 ++++++++----
kernel/sched/sched.h | 6 ++++++
2 files changed, 14 insertions(+), 4 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index aaa87d5fecdd..aa31f23f43f4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1056,6 +1056,9 @@ static resched_t resched_opt_translate(struct task_struct *curr,
if (preempt_model_preemptible())
return NR_now;
+ if (preempt_model_voluntary() && opt == RESCHED_PRIORITY)
+ return NR_now;
+
if (is_idle_task(curr))
return NR_now;
@@ -2297,7 +2300,7 @@ void wakeup_preempt(struct rq *rq, struct task_struct *p, int flags)
if (p->sched_class == rq->curr->sched_class)
rq->curr->sched_class->wakeup_preempt(rq, p, flags);
else if (sched_class_above(p->sched_class, rq->curr->sched_class))
- resched_curr(rq);
+ resched_curr_priority(rq);
/*
* A queue event has occurred, and we're going to schedule. In
@@ -8974,11 +8977,11 @@ static void __sched_dynamic_update(int mode)
case preempt_dynamic_none:
if (mode != preempt_dynamic_mode)
pr_info("%s: none\n", PREEMPT_MODE);
- preempt_dynamic_mode = mode;
break;
case preempt_dynamic_voluntary:
- preempt_dynamic_mode = preempt_dynamic_undefined;
+ if (mode != preempt_dynamic_mode)
+ pr_info("%s: voluntary\n", PREEMPT_MODE);
break;
case preempt_dynamic_full:
@@ -8988,9 +8991,10 @@ static void __sched_dynamic_update(int mode)
if (mode != preempt_dynamic_mode)
pr_info("%s: full\n", PREEMPT_MODE);
- preempt_dynamic_mode = mode;
break;
}
+
+ preempt_dynamic_mode = mode;
}
#endif /* CONFIG_PREEMPT_AUTO */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c7e7acab1022..197c038b87c6 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2466,6 +2466,7 @@ enum resched_opt {
RESCHED_DEFAULT,
RESCHED_FORCE,
RESCHED_TICK,
+ RESCHED_PRIORITY,
};
extern void __resched_curr(struct rq *rq, enum resched_opt opt);
@@ -2480,6 +2481,11 @@ static inline void resched_curr_tick(struct rq *rq)
__resched_curr(rq, RESCHED_TICK);
}
+static inline void resched_curr_priority(struct rq *rq)
+{
+ __resched_curr(rq, RESCHED_PRIORITY);
+}
+
extern void resched_cpu(int cpu);
extern struct rt_bandwidth def_rt_bandwidth;
--
2.31.1