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Subject[rfc patch v2] rt,nohz_full: fix nohz_full for PREEMPT_RT_FULL
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First of all, a task being ticked and trying to shut the tick down will
fail to do so due to having just awakened ksoftirqd, so let ksoftirqd
try to do that after SOFTIRQ_TIMER processing. Secondly, should the
tick be shut down, we may livelock in hrtimer-cancel() because in -rt
a callback may be running. Break the loop, and let tick_nohz_restart()
know that the timer is busy so it can bail.

Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
---
kernel/sched/core.c | 8 +++++++-
kernel/softirq.c | 17 +++++++++++++++++
kernel/time/hrtimer.c | 9 +++++++++
kernel/time/tick-sched.c | 14 +++++++++++++-
4 files changed, 46 insertions(+), 2 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -783,12 +783,18 @@ static inline bool got_nohz_idle_kick(vo
#ifdef CONFIG_NO_HZ_FULL
bool sched_can_stop_tick(void)
{
+ int ksoftirqd = !!(IS_ENABLED(CONFIG_PREEMPT_RT_FULL) &&
+ current == this_cpu_ksoftirqd());
+
/*
* More than one running task need preemption.
* nr_running update is assumed to be visible
* after IPI is sent from wakers.
+ *
+ * NOTE, RT: ksoftirqd tries to stop the tick for
+ * tasks as they exit irq, ergo subtracts itself.
*/
- if (this_rq()->nr_running > 1)
+ if (this_rq()->nr_running - ksoftirqd > 1)
return false;

return true;
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -498,6 +498,22 @@ static void unlock_softirq(int which)
local_unlock(local_softirq_locks[which]);
}

+/*
+ * Let ksoftirqd try to shut down the tick when awakened via
+ * timer_interrupt->irq_exit()->invoke_softirq(), as the task
+ * then calling tick_nohz_irq_exit() WILL fail to do so due
+ * to that very wakeup having made rq->nr_running > 1.
+ */
+static void tick_nohz_sirq_timer_exit(int which)
+{
+ if (!IS_ENABLED(CONFIG_NO_HZ_FULL))
+ return;
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
+ return;
+ if (which != TIMER_SOFTIRQ || current != this_cpu_ksoftirqd())
+ return;
+ tick_nohz_irq_exit();
+}
static void do_single_softirq(int which, int need_rcu_bh_qs)
{
unsigned long old_flags = current->flags;
@@ -513,6 +529,7 @@ static void do_single_softirq(int which,
current->flags &= ~PF_IN_SOFTIRQ;
vtime_account_irq_enter(current);
tsk_restore_flags(current, old_flags, PF_MEMALLOC);
+ tick_nohz_sirq_timer_exit(which);
}

/*
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1162,6 +1162,15 @@ int hrtimer_try_to_cancel(struct hrtimer

if (!hrtimer_callback_running(timer))
ret = remove_hrtimer(timer, base);
+ else if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL) &&
+ tick_nohz_full_cpu(raw_smp_processor_id())) {
+ /*
+ * Let tick_nohz_restart() know that the timer is
+ * active lest we spin forever in hrtimer_cancel().
+ */
+ if (in_irq() && timer->irqsafe)
+ ret = HRTIMER_STATE_CALLBACK;
+ }

unlock_hrtimer_base(timer, &flags);

--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -222,7 +222,12 @@ void __tick_nohz_full_check(void)

static void nohz_full_kick_work_func(struct irq_work *work)
{
+ unsigned long flags;
+
+ /* ksoftirqd processes sirqs with interrupts enabled */
+ local_irq_save(flags);
__tick_nohz_full_check();
+ local_irq_restore(flags);
}

static DEFINE_PER_CPU(struct irq_work, nohz_full_kick_work) = {
@@ -879,7 +884,14 @@ ktime_t tick_nohz_get_sleep_length(void)

static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
{
- hrtimer_cancel(&ts->sched_timer);
+ int cpu = raw_smp_processor_id();
+
+ /* In an RT kernel, a callback may be running. If so, bail */
+ if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL) && tick_nohz_full_cpu(cpu)) {
+ if (hrtimer_cancel(&ts->sched_timer) == HRTIMER_STATE_CALLBACK)
+ return;
+ } else
+ hrtimer_cancel(&ts->sched_timer);
hrtimer_set_expires(&ts->sched_timer, ts->last_tick);

while (1) {




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