* Mike Galbraith | 2015-03-13 05:53:25 [+0100]:
>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.
So with this patch
http://marc.info/?l=linux-rt-users&m=142866940327602&w=2
you could drop the hrtimer hacks. Is this the remaining part all that is
required or we shorten the following patch a little more?
>--- 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/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) = {
--
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