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Date: Sat, 22 Mar 2008 15:30:00 +0100 (CET)
From: Thomas Gleixner <tglx@...utronix.de>
To: Gabriel C <crazy@...galware.org>
cc: Gabriel C <nix.or.die@...glemail.com>,
"Rafael J. Wysocki" <rjw@...k.pl>,
LKML <linux-kernel@...r.kernel.org>,
Adrian Bunk <bunk@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Natalie Protasevich <protasnb@...il.com>,
andi-bz@...stfloor.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: 2.6.25-rc5-git6: Reported regressions from 2.6.24
On Sat, 22 Mar 2008, Gabriel C wrote:
> Now some time later CPU1 gets woken by an interrupt/IPI and runs the
> > timer wheel. At this point the pm_timer which is the reference clock
> > has already wrapped around, so the watchdog thinks that there is a
> > huge time difference and marks the TSC unstable.
> >
> > Aside of that watchdog issue this also affects the other users of
> > add_timer_on(): e.g. queue_delayed_work_on().
> >
> > Can you please apply the patch below and verify it with Andi's
> > watchdog patch applied ?
>
>
> Did that , git head , Andi's + your patch but TSC is still marked unstable.
Doh, stupid me. We do not reevaluate the timer wheel, when we just
wake up via the smp_reschedule IPI when the resched flag on the other
CPU is not set. That's a separate vector which is not going through
irq_enter() / irq_exit().
Does the patch below solve the problem ?
Thanks,
tglx
---
include/linux/tick.h | 4 +++
kernel/time/tick-sched.c | 50 +++++++++++++++++++++++++++++++++++++++++++++++
kernel/timer.c | 14 ++++++++++++-
3 files changed, 67 insertions(+), 1 deletion(-)
Index: linux-2.6/include/linux/tick.h
===================================================================
--- linux-2.6.orig/include/linux/tick.h
+++ linux-2.6/include/linux/tick.h
@@ -111,6 +111,8 @@ extern void tick_nohz_update_jiffies(voi
extern ktime_t tick_nohz_get_sleep_length(void);
extern void tick_nohz_stop_idle(int cpu);
extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time);
+extern int tick_nohz_cpu_needs_wakeup(int cpu);
+extern void tick_nohz_rescan_timers_on(int cpu);
# else
static inline void tick_nohz_stop_sched_tick(void) { }
static inline void tick_nohz_restart_sched_tick(void) { }
@@ -123,6 +125,8 @@ static inline ktime_t tick_nohz_get_slee
}
static inline void tick_nohz_stop_idle(int cpu) { }
static inline u64 get_cpu_idle_time_us(int cpu, u64 *unused) { return 0; }
+static inline int tick_nohz_cpu_needs_wakeup(int cpu) { return 0; }
+static inline void tick_nohz_rescan_timers_on(int cpu) { }
# endif /* !NO_HZ */
#endif
Index: linux-2.6/kernel/time/tick-sched.c
===================================================================
--- linux-2.6.orig/kernel/time/tick-sched.c
+++ linux-2.6/kernel/time/tick-sched.c
@@ -183,6 +183,56 @@ u64 get_cpu_idle_time_us(int cpu, u64 *l
}
/**
+ * tick_nohz_cpu_needs_wakeup - check possible wakeup of cpu in add_timer_on()
+ *
+ * when add_timer_on() happens on a CPU which is in a long idle sleep,
+ * then we need to wake it up so the timer wheel gets reevaluated.
+ *
+ * Note: we use idle_cpu() which checks the idle state lockless, but
+ * we are ordered against the other cpu which might be on the way to
+ * idle by the timer base lock, which we hold.
+ */
+int tick_nohz_cpu_needs_wakeup(int cpu)
+{
+ return tick_nohz_enabled && idle_cpu(cpu) &&
+ (cpu != smp_processor_id());
+}
+
+/*
+ * Rescan the timer wheel, when
+ *
+ * - the CPU is idle
+ * - the CPU is not processing an interupt
+ * - the need_resched flag is off
+ */
+static void tick_nohz_rescan_timers(void *unused)
+{
+ int cpu = smp_processor_id();
+
+ if (!idle_cpu(cpu) || in_interrupt() || need_resched())
+ return;
+
+ tick_nohz_stop_idle(cpu);
+ tick_nohz_update_jiffies();
+ tick_nohz_stop_sched_tick();
+}
+
+/**
+ * tick_nohz_rescan_timers_on - reevaluate the idle sleep time of a CPU
+ *
+ * When a CPU is idle and a timer got added to this CPU timer wheel
+ * via add_timer_on() then we need to make sure that the CPU
+ * reevaluates the timer wheel. Otherwise the timer might be delayed
+ * for a real long time.
+ */
+void tick_nohz_rescan_timers_on(int cpu)
+{
+ if (tick_nohz_enabled && idle_cpu(cpu))
+ smp_call_function_single(cpu, tick_nohz_rescan_timers, NULL,
+ 0, 0);
+}
+
+/**
* tick_nohz_stop_sched_tick - stop the idle tick from the idle task
*
* When the next event is more than a tick into the future, stop the idle tick
Index: linux-2.6/kernel/timer.c
===================================================================
--- linux-2.6.orig/kernel/timer.c
+++ linux-2.6/kernel/timer.c
@@ -445,15 +445,27 @@ void add_timer_on(struct timer_list *tim
{
struct tvec_base *base = per_cpu(tvec_bases, cpu);
unsigned long flags;
+ int wakeidle;
timer_stats_timer_set_start_info(timer);
BUG_ON(timer_pending(timer) || !timer->function);
spin_lock_irqsave(&base->lock, flags);
timer_set_base(timer, base);
internal_add_timer(base, timer);
+ /*
+ * Check whether the other CPU is idle and needs to be
+ * triggered to reevaluate the timer wheel when nohz is
+ * active. We are protected against the other CPU fiddling
+ * with the timer by holding the timer base lock. This also
+ * makes sure that a CPU on the way to idle can not evaluate
+ * the timer wheel.
+ */
+ wakeidle = tick_nohz_cpu_needs_wakeup(cpu);
spin_unlock_irqrestore(&base->lock, flags);
-}
+ if (wakeidle)
+ tick_nohz_rescan_timers_on(cpu);
+}
/**
* mod_timer - modify a timer's timeout
--
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