From bfe19ab466d325b3dd78e44a2546a2d9b6205dfa Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Tue, 5 Apr 2022 03:07:51 +0200 Subject: [PATCH 13/64] rcutorture: Also force sched priority to timersd on boosting test. 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 Cc: Sebastian Andrzej Siewior Cc: Thomas Gleixner Signed-off-by: Frederic Weisbecker Link: https://lkml.kernel.org/r/20220405010752.1347437-1-frederic@kernel.org Signed-off-by: Sebastian Andrzej Siewior --- include/linux/interrupt.h | 1 + kernel/rcu/rcutorture.c | 6 ++++++ kernel/softirq.c | 2 +- 3 files changed, 8 insertions(+), 1 deletion(-) Index: linux-6.1.96-rt35/include/linux/interrupt.h =================================================================== --- linux-6.1.96-rt35.orig/include/linux/interrupt.h +++ linux-6.1.96-rt35/include/linux/interrupt.h @@ -610,6 +610,7 @@ extern void raise_softirq_irqoff(unsigne 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); Index: linux-6.1.96-rt35/kernel/rcu/rcutorture.c =================================================================== --- linux-6.1.96-rt35.orig/kernel/rcu/rcutorture.c +++ linux-6.1.96-rt35/kernel/rcu/rcutorture.c @@ -2364,6 +2364,12 @@ static int rcutorture_booster_init(unsig 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. */ Index: linux-6.1.96-rt35/kernel/softirq.c =================================================================== --- linux-6.1.96-rt35.orig/kernel/softirq.c +++ linux-6.1.96-rt35/kernel/softirq.c @@ -642,7 +642,7 @@ 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)