linux-kernel - Re: CONFIG_NO_HZ_FULL + CONFIG_PREEMPT_RT

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Date:	Fri, 20 Dec 2013 16:41:46 +0100
From:	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Mike Galbraith <bitbucket@...ine.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: CONFIG_NO_HZ_FULL + CONFIG_PREEMPT_RT_FULL = nogo

On 07.11.13, Thomas Gleixner wrote:
> below should address that issue. If that works on mainline we can
> adapt it for RT (needs a trylock(&base->lock) there).

okay, so this is waht I'm going to take for -RT:

diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index 79a7a35..bdbf77db 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -461,9 +461,8 @@ extern int schedule_hrtimeout_range_clock(ktime_t *expires,
 		unsigned long delta, const enum hrtimer_mode mode, int clock);
 extern int schedule_hrtimeout(ktime_t *expires, const enum hrtimer_mode mode);
 
-/* Soft interrupt function to run the hrtimer queues: */
+/* Called from the periodic timer tick */
 extern void hrtimer_run_queues(void);
-extern void hrtimer_run_pending(void);
 
 /* Bootup initialization: */
 extern void __init hrtimers_init(void);
diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index c383841..7aa442e 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -1694,30 +1694,6 @@ static void run_hrtimer_softirq(struct softirq_action *h)
 }
 
 /*
- * Called from timer softirq every jiffy, expire hrtimers:
- *
- * For HRT its the fall back code to run the softirq in the timer
- * softirq context in case the hrtimer initialization failed or has
- * not been done yet.
- */
-void hrtimer_run_pending(void)
-{
-	if (hrtimer_hres_active())
-		return;
-
-	/*
-	 * This _is_ ugly: We have to check in the softirq context,
-	 * whether we can switch to highres and / or nohz mode. The
-	 * clocksource switch happens in the timer interrupt with
-	 * xtime_lock held. Notification from there only sets the
-	 * check bit in the tick_oneshot code, otherwise we might
-	 * deadlock vs. xtime_lock.
-	 */
-	if (tick_check_oneshot_change(!hrtimer_is_hres_enabled()))
-		hrtimer_switch_to_hres();
-}
-
-/*
  * Called from hardirq context every jiffy
  */
 void hrtimer_run_queues(void)
@@ -1730,6 +1706,13 @@ void hrtimer_run_queues(void)
 	if (hrtimer_hres_active())
 		return;
 
+	/*
+	 * Check whether we can switch to highres mode.
+	 */
+	if (tick_check_oneshot_change(!hrtimer_is_hres_enabled())
+	    && hrtimer_switch_to_hres())
+		return;
+
 	for (index = 0; index < HRTIMER_MAX_CLOCK_BASES; index++) {
 		base = &cpu_base->clock_base[index];
 		if (!timerqueue_getnext(&base->active))
diff --git a/kernel/timer.c b/kernel/timer.c
index b06c647..46467be 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1443,8 +1443,6 @@ static void run_timer_softirq(struct softirq_action *h)
 	irq_work_run();
 #endif
 
-	hrtimer_run_pending();
-
 	if (time_after_eq(jiffies, base->timer_jiffies))
 		__run_timers(base);
 }
@@ -1454,8 +1452,27 @@ static void run_timer_softirq(struct softirq_action *h)
  */
 void run_local_timers(void)
 {
+	struct tvec_base *base = __this_cpu_read(tvec_bases);
+
 	hrtimer_run_queues();
-	raise_softirq(TIMER_SOFTIRQ);
+	/*
+	 * We can access this lockless as we are in the timer
+	 * interrupt. If there are no timers queued, nothing to do in
+	 * the timer softirq.
+	 */
+	if (!spin_do_trylock(&base->lock)) {
+		raise_softirq(TIMER_SOFTIRQ);
+		return;
+	}
+	if (!base->active_timers)
+		goto out;
+
+	/* Check whether the next pending timer has expired */
+	if (time_before_eq(base->next_timer, jiffies))
+		raise_softirq(TIMER_SOFTIRQ);
+out:
+	rt_spin_unlock_after_trylock_in_irq(&base->lock);
+
 }
 
 #ifdef __ARCH_WANT_SYS_ALARM
-- 
1.8.5.1

Sebastian
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