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Date: Wed, 11 Mar 2015 07:18:56 +0100
From: Mike Galbraith <umgwanakikbuti@...il.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-rt-users <linux-rt-users@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>, rostedt@...dmis.org,
John Kacur <jkacur@...hat.com>
Subject: Re: [patch-3.18.7-rt1] sched/context-tracking: fix PREEMPT_LAZY
explosions
On Tue, 2015-03-10 at 10:35 +0100, Mike Galbraith wrote:
> On Mon, 2015-03-09 at 15:36 +0100, Mike Galbraith wrote:
> > On Mon, 2015-03-09 at 14:45 +0100, Sebastian Andrzej Siewior wrote:
> > > * Mike Galbraith | 2015-02-18 12:21:54 [+0100]:
> > >
> > > >On Mon, 2015-02-16 at 12:18 +0100, Sebastian Andrzej Siewior wrote:
> > > >
> > > >> Known issues:
> > > >>
> > > >> - lazy preempt on x86_64 leads to a crash with some load.
> > > >
> > > >The below still works for me. (it doesn't make nohz_full actually work
> > > >in rt, but at least folks who want to tinker with it can do so)
> > > So your box still crashes without it?
> > >
> > > The thing is I tried to reproduce it and it does not occur anymore.
> > > Neither in KVM nor on real HW.
> >
> > Heh, well that's interesting, I just presumed it would still explode
> > because you said it would. I'll beat on it any let you know if I'm
> > carrying a placebo patch or not :)
>
> Yup, someone made kaboom go away. I was able to easily blow my box out
> of the water by running tbench + kbuild with nohz_full active. No more.
>
> nohz_full doesn't work though, due to an otherwise solo task trying to
> shut the tick down having just awakened ksoftirqd.. but no more kaboom.
FWIW, the hack below made nohz_full functional in my trees, and it still
doesn't explode, so seems it really really did get fixed up.
---
kernel/sched/core.c | 2 +-
kernel/softirq.c | 25 +++++++++++++++++++------
kernel/time/hrtimer.c | 10 +++++++++-
kernel/time/tick-sched.c | 15 ++++++++++++++-
4 files changed, 43 insertions(+), 9 deletions(-)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -810,7 +810,7 @@ bool sched_can_stop_tick(void)
* 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 - task_is_softirqd(current) > 1)
return false;
return true;
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -517,19 +517,32 @@ static void unlock_softirq(int which)
static void do_single_softirq(int which, int need_rcu_bh_qs)
{
- unsigned long old_flags = current->flags;
+ struct task_struct *p = current;
+ unsigned long old_flags = p->flags;
- current->flags &= ~PF_MEMALLOC;
+ p->flags &= ~PF_MEMALLOC;
vtime_account_irq_enter(current);
- current->flags |= PF_IN_SOFTIRQ;
+ p->flags |= PF_IN_SOFTIRQ;
lockdep_softirq_enter();
local_irq_enable();
handle_softirq(which);
local_irq_disable();
lockdep_softirq_exit();
- current->flags &= ~PF_IN_SOFTIRQ;
- vtime_account_irq_enter(current);
- tsk_restore_flags(current, old_flags, PF_MEMALLOC);
+ p->flags &= ~PF_IN_SOFTIRQ;
+ vtime_account_irq_enter(p);
+ tsk_restore_flags(p, old_flags, PF_MEMALLOC);
+#if defined(CONFIG_NO_HZ_FULL) && defined(CONFIG_PREEMPT_RT_FULL)
+ /*
+ * If a task calls irq_exit()->invoke_softirq(), it won't
+ * be able to shut the tick down because it just woke us.
+ * Try to shut it down now.
+ */
+ if (tick_nohz_full_cpu(raw_smp_processor_id()) &&
+ task_is_softirqd(p) && which == TIMER_SOFTIRQ &&
+ sched_can_stop_tick()) {
+ tick_nohz_irq_exit();
+ }
+#endif
}
/*
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1141,7 +1141,6 @@ hrtimer_start(struct hrtimer *timer, kti
}
EXPORT_SYMBOL_GPL(hrtimer_start);
-
/**
* hrtimer_try_to_cancel - try to deactivate a timer
* @timer: hrtimer to stop
@@ -1162,6 +1161,15 @@ int hrtimer_try_to_cancel(struct hrtimer
if (!hrtimer_callback_running(timer))
ret = remove_hrtimer(timer, base);
+#if defined(CONFIG_PREEMPT_RT_FULL) && defined(CONFIG_NO_HZ_FULL)
+ else {
+ extern enum hrtimer_restart tick_sched_timer(struct hrtimer *timer);
+ int cpu = raw_smp_processor_id();
+
+ if (tick_nohz_full_cpu(cpu) && timer->function == tick_sched_timer)
+ ret = HRTIMER_STATE_CALLBACK;
+ }
+#endif
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 (task_is_softirqd(current))
+ local_irq_disable_rt();
__tick_nohz_full_check();
+ local_irq_restore(flags);
}
static DEFINE_PER_CPU(struct irq_work, nohz_full_kick_work) = {
@@ -879,6 +886,12 @@ ktime_t tick_nohz_get_sleep_length(void)
static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
{
+#if defined(CONFIG_PREEMPT_RT_FULL) && defined(CONFIG_NO_HZ_FULL)
+ if (tick_nohz_full_cpu(raw_smp_processor_id()) &&
+ hrtimer_cancel(&ts->sched_timer) == HRTIMER_STATE_CALLBACK)
+ return;
+ else
+#endif
hrtimer_cancel(&ts->sched_timer);
hrtimer_set_expires(&ts->sched_timer, ts->last_tick);
@@ -1108,7 +1121,7 @@ void tick_irq_enter(void)
* We rearm the timer until we get disabled by the idle code.
* Called with interrupts disabled.
*/
-static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
+enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
{
struct tick_sched *ts =
container_of(timer, struct tick_sched, sched_timer);
--
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