diff options
Diffstat (limited to 'kernels/linux-libre-rt/timers-do-not-raise-softirq-unconditionally.patch')
| -rw-r--r-- | kernels/linux-libre-rt/timers-do-not-raise-softirq-unconditionally.patch | 156 |
1 files changed, 156 insertions, 0 deletions
diff --git a/kernels/linux-libre-rt/timers-do-not-raise-softirq-unconditionally.patch b/kernels/linux-libre-rt/timers-do-not-raise-softirq-unconditionally.patch new file mode 100644 index 000000000..d8f01f2d6 --- /dev/null +++ b/kernels/linux-libre-rt/timers-do-not-raise-softirq-unconditionally.patch @@ -0,0 +1,156 @@ +From 35b6173e6176fc978c635f9e07f1778eff7b76e7 Mon Sep 17 00:00:00 2001 +From: Thomas Gleixner <tglx@linutronix.de> +Date: Thu, 7 Nov 2013 12:21:11 +0100 +Subject: [PATCH] timers: do not raise softirq unconditionally + +Mike, + +On Thu, 7 Nov 2013, Mike Galbraith wrote: + +> On Thu, 2013-11-07 at 04:26 +0100, Mike Galbraith wrote: +> > On Wed, 2013-11-06 at 18:49 +0100, Thomas Gleixner wrote: +> +> > > I bet you are trying to work around some of the side effects of the +> > > occasional tick which is still necessary despite of full nohz, right? +> > +> > Nope, I wanted to check out cost of nohz_full for rt, and found that it +> > doesn't work at all instead, looked, and found that the sole running +> > task has just awakened ksoftirqd when it wants to shut the tick down, so +> > that shutdown never happens. +> +> Like so in virgin 3.10-rt. Box is x3550 M3 booted nowatchdog +> rcu_nocbs=1-3 nohz_full=1-3, and CPUs1-3 are completely isolated via +> cpusets as well. + +well, that very same problem is in mainline if you add "threadirqs" to +the command line. But we can be smart about this. The untested patch +below should address that issue. If that works on mainline we can +adapt it for RT (needs a trylock(&base->lock) there). + +Though it's not a full solution. It needs some thought versus the +softirq code of timers. Assume we have only one timer queued 1000 +ticks into the future. So this change will cause the timer softirq not +to be called until that timer expires and then the timer softirq is +going to do 1000 loops until it catches up with jiffies. That's +anything but pretty ... + +What worries me more is this one: + + pert-5229 [003] d..h1.. 684.482618: softirq_raise: vec=9 [action=RCU] + +The CPU has no callbacks as you shoved them over to cpu 0, so why is +the RCU softirq raised? + +Thanks, + + tglx +------------------ +Message-id: <alpine.DEB.2.02.1311071158350.23353@ionos.tec.linutronix.de> +|CONFIG_NO_HZ_FULL + CONFIG_PREEMPT_RT_FULL = nogo +Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> +--- + include/linux/hrtimer.h | 3 +-- + kernel/hrtimer.c | 31 +++++++------------------------ + kernel/timer.c | 23 ++++++++++++++++++++--- + 3 files changed, 28 insertions(+), 29 deletions(-) + +--- a/include/linux/hrtimer.h ++++ b/include/linux/hrtimer.h +@@ -461,9 +461,8 @@ extern int schedule_hrtimeout_range_cloc + 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); +--- a/kernel/hrtimer.c ++++ b/kernel/hrtimer.c +@@ -1694,30 +1694,6 @@ static void run_hrtimer_softirq(struct s + } + + /* +- * 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)) +--- a/kernel/timer.c ++++ b/kernel/timer.c +@@ -1439,8 +1439,6 @@ static void run_timer_softirq(struct sof + { + struct tvec_base *base = __this_cpu_read(tvec_bases); + +- hrtimer_run_pending(); +- + if (time_after_eq(jiffies, base->timer_jiffies)) + __run_timers(base); + } +@@ -1450,8 +1448,27 @@ static void run_timer_softirq(struct sof + */ + 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 |