[PATCH v2 29/35] sched: handle preempt=voluntary under PREEMPT_AUTO

Ankur Arora
2024-05-28T00:37:08Z

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.

Comparing a cyclictest workload with a background kernel load of
'stress-ng --mmap', shows that both the average and the maximum
latencies improve:

# stress-ng --mmap 0 &
# cyclictest --mlockall --smp --priority=80 --interval=200 --distance=0 -q -D 300

Min ( %stdev ) Act ( %stdev ) Avg ( %stdev ) Max ( %stdev )

PREEMPT_AUTO, preempt=voluntary 1.73 ( +- 25.43% ) 62.16 ( +- 303.39% ) 14.92 ( +- 17.96% ) 2778.22 ( +- 15.04% )
PREEMPT_DYNAMIC, preempt=voluntary 1.83 ( +- 20.76% ) 253.45 ( +- 233.21% ) 18.70 ( +- 15.88% ) 2992.45 ( +- 15.95% )

The table above shows the aggregated latencies across all CPUs.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Ziljstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Originally-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
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 c25cccc09b65..2bc3ae21a9d0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1052,6 +1052,9 @@ static resched_t resched_opt_translate(struct task_struct *curr,
if (preempt_model_preemptible())
return RESCHED_NOW;

+ if (preempt_model_voluntary() && opt == RESCHED_PRIORITY)
+ return RESCHED_NOW;
+
if (is_idle_task(curr))
return RESCHED_NOW;

@@ -2289,7 +2292,7 @@ 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
@@ -8989,11 +8992,11 @@ 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:
@@ -9003,9 +9006,10 @@ 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 107c5fc2b7bb..ee8e99a9a677 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2468,6 +2468,7 @@ enum resched_opt {
RESCHED_DEFAULT,
RESCHED_FORCE,
RESCHED_TICK,
+ RESCHED_PRIORITY,
};

extern void __resched_curr(struct rq *rq, enum resched_opt opt);
@@ -2482,6 +2483,11 @@ 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