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From a10aa54ef224bbd46ca1777e4b9fe960360961cc Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <frederic@kernel.org>
Date: Tue, 5 Apr 2022 03:07:51 +0200
Subject: [PATCH 13/62] rcutorture: Also force sched priority to timersd on
boosting test.
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.1/older/patches-6.1.69-rt21.tar.xz
ksoftirqd is statically boosted to the priority level right above the
one of rcu_torture_boost() so that timers, which torture readers rely on,
get a chance to run while rcu_torture_boost() is polling.
However timers processing got split from ksoftirqd into their own kthread
(timersd) that isn't boosted. It has the same SCHED_FIFO low prio as
rcu_torture_boost() and therefore timers can't preempt it and may
starve.
The issue can be triggered in practice on v5.17.1-rt17 using:
./kvm.sh --allcpus --configs TREE04 --duration 10m --kconfig "CONFIG_EXPERT=y CONFIG_PREEMPT_RT=y"
Fix this with statically boosting timersd just like is done with
ksoftirqd in commit
ea6d962e80b61 ("rcutorture: Judge RCU priority boosting on grace periods, not callbacks")
Suggested-by: Mel Gorman <mgorman@suse.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lkml.kernel.org/r/20220405010752.1347437-1-frederic@kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
include/linux/interrupt.h | 1 +
kernel/rcu/rcutorture.c | 6 ++++++
kernel/softirq.c | 2 +-
3 files changed, 8 insertions(+), 1 deletion(-)
diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 0efba74a835c..f459b0f27c94 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -610,6 +610,7 @@ extern void raise_softirq_irqoff(unsigned int nr);
extern void raise_softirq(unsigned int nr);
#ifdef CONFIG_PREEMPT_RT
+DECLARE_PER_CPU(struct task_struct *, timersd);
extern void raise_timer_softirq(void);
extern void raise_hrtimer_softirq(void);
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 503c2aa845a4..dcd8c0e44c00 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -2363,6 +2363,12 @@ static int rcutorture_booster_init(unsigned int cpu)
WARN_ON_ONCE(!t);
sp.sched_priority = 2;
sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
+#ifdef CONFIG_PREEMPT_RT
+ t = per_cpu(timersd, cpu);
+ WARN_ON_ONCE(!t);
+ sp.sched_priority = 2;
+ sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
+#endif
}
/* Don't allow time recalculation while creating a new task. */
diff --git a/kernel/softirq.c b/kernel/softirq.c
index ed6d7c41aa17..1892af494cdd 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -638,7 +638,7 @@ static inline void tick_irq_exit(void)
}
#ifdef CONFIG_PREEMPT_RT
-static DEFINE_PER_CPU(struct task_struct *, timersd);
+DEFINE_PER_CPU(struct task_struct *, timersd);
static DEFINE_PER_CPU(unsigned long, pending_timer_softirq);
static unsigned int local_pending_timers(void)
--
2.43.0
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