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Message-ID: <20150312110911.1a93c8e8@gandalf.local.home>
Date: Thu, 12 Mar 2015 11:09:11 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Mike Galbraith <umgwanakikbuti@...il.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
linux-rt-users <linux-rt-users@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>,
John Kacur <jkacur@...hat.com>
Subject: Re: [rfc patch] rt,nohz_full: fix nohz_full for PREEMPT_RT_FULL
On Thu, 12 Mar 2015 10:38:13 +0100
Mike Galbraith <umgwanakikbuti@...il.com> wrote:
> This works, let's see is someone has a better idea...
>
> 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@...il.com>
> ---
> kernel/sched/core.c | 6 +++++-
> kernel/softirq.c | 17 +++++++++++++++++
> kernel/time/hrtimer.c | 9 +++++++++
> kernel/time/tick-sched.c | 16 +++++++++++++++-
> 4 files changed, 46 insertions(+), 2 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -783,12 +783,16 @@ static inline bool got_nohz_idle_kick(vo
> #ifdef CONFIG_NO_HZ_FULL
> bool sched_can_stop_tick(void)
> {
> + int softirqd = IS_ENABLED(CONFIG_PREEMPT_RT_FULL);
> +
> + softirqd &= current == this_cpu_ksoftirqd();
Ug, binary and logical ANDs should not be combined. Just looks nasty.
What about:
softirqd = !!(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.
> */
> - if (this_rq()->nr_running > 1)
> + if (this_rq()->nr_running - softirqd > 1)
There should also be a comment explaining this hack.
> 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 +222,14 @@ void __tick_nohz_full_check(void)
>
> static void nohz_full_kick_work_func(struct irq_work *work)
> {
> + unsigned long flags;
> +
> + local_save_flags(flags);
> + /* ksoftirqd processes softirqs with interrupts enabled */
> + if (current == this_cpu_ksoftirqd())
> + local_irq_disable();
Why the funkiness above? Why not just call local_irq_save()?
-- Steve
> __tick_nohz_full_check();
> + local_irq_restore(flags);
> }
>
> static DEFINE_PER_CPU(struct irq_work, nohz_full_kick_work) = {
> @@ -879,7 +886,14 @@ 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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