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Date: Thu, 04 Dec 2008 11:17:10 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...e.hu>,
linux-kernel <linux-kernel@...r.kernel.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
oliver@...tkopp.net
Subject: Re: [RFC PATCH] hrtimer: removing all ur callback modes
On Tue, 2008-11-25 at 12:43 +0100, Peter Zijlstra wrote:
> Hi,
>
> This is an attempt at removing some of the hrtimer complexity by
> reducing the number of callback modes to 1.
>
> This means that all hrtimer callback functions will be ran from HARD-irq
> context.
>
> I went through all the 30 odd hrtimer callback functions in the kernel
> and saw only one that I'm not quite sure of, which is the one in
> net/can/bcm.c - hence I'm CC-ing the folks responsible for that code.
>
> Furthermore, the hrtimer core now calls callbacks directly with IRQs
> disabled in case you try to enqueue an expired timer. If this timer is a
> periodic timer (which should use hrtimer_forward() to advance its time)
> then it might be possible to end up in an inf. recursive loop due to the
> fact that hrtimer_forward() doesn't round up to the next timer
> granularity, and therefore keeps on calling the callback - obviously
> this needs a fix.
>
> Aside from that, this seems to compile and actually boot on my dual core
> test box - although I'm sure there are some bugs in, me not hitting any
> makes me certain :-)
>
> Not-Quite-Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Ingo, this addition fixes the hotplug issue on my machine
---
hrtimer.c | 65 +++++++++++++++++++++++++++++++++++---------------------------
1 file changed, 37 insertions(+), 28 deletions(-)
Index: linux-2.6/kernel/hrtimer.c
===================================================================
--- linux-2.6.orig/kernel/hrtimer.c
+++ linux-2.6/kernel/hrtimer.c
@@ -1496,7 +1496,7 @@ static void __cpuinit init_hrtimers_cpu(
#ifdef CONFIG_HOTPLUG_CPU
static void migrate_hrtimer_list(struct hrtimer_clock_base *old_base,
- struct hrtimer_clock_base *new_base, int dcpu)
+ struct hrtimer_clock_base *new_base)
{
struct hrtimer *timer;
struct rb_node *node;
@@ -1514,40 +1514,34 @@ static void migrate_hrtimer_list(struct
__remove_hrtimer(timer, old_base, HRTIMER_STATE_MIGRATE, 0);
timer->base = new_base;
/*
- * Enqueue the timer. Allow reprogramming of the event device
+ * Enqueue the timers on the new cpu, but do not reprogram
+ * the timer as that would enable a deadlock between
+ * hrtimer_enqueue_reprogramm() running the timer and us still
+ * holding a nested base lock.
+ *
+ * Instead we tickle the hrtimer interrupt after the migration
+ * is done, which will run all expired timers and re-programm
+ * the timer device.
*/
- enqueue_hrtimer(timer, new_base, 1);
+ enqueue_hrtimer(timer, new_base, 0);
-#ifdef CONFIG_HIGH_RES_TIMERS
- /*
- * Happens with high res enabled when the timer was
- * already expired and the callback mode is
- * HRTIMER_CB_IRQSAFE_UNLOCKED (hrtimer_sleeper). The
- * enqueue code does not move them to the soft irq
- * pending list for performance/latency reasons, but
- * in the migration state, we need to do that
- * otherwise we end up with a stale timer.
- */
- if (timer->state == HRTIMER_STATE_MIGRATE) {
- /* XXX: running on offline cpu */
- __run_hrtimer(timer);
- }
-#endif
/* Clear the migration state bit */
timer->state &= ~HRTIMER_STATE_MIGRATE;
}
}
-static void migrate_hrtimers(int cpu)
+static int migrate_hrtimers(int scpu)
{
struct hrtimer_cpu_base *old_base, *new_base;
- int i;
+ int dcpu, i;
- BUG_ON(cpu_online(cpu));
- old_base = &per_cpu(hrtimer_bases, cpu);
+ BUG_ON(cpu_online(scpu));
+ old_base = &per_cpu(hrtimer_bases, scpu);
new_base = &get_cpu_var(hrtimer_bases);
- tick_cancel_sched_timer(cpu);
+ dcpu = smp_processor_id();
+
+ tick_cancel_sched_timer(scpu);
/*
* The caller is globally serialized and nobody else
* takes two locks at once, deadlock is not possible.
@@ -1557,32 +1551,47 @@ static void migrate_hrtimers(int cpu)
for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
migrate_hrtimer_list(&old_base->clock_base[i],
- &new_base->clock_base[i], cpu);
+ &new_base->clock_base[i]);
}
spin_unlock(&old_base->lock);
spin_unlock_irq(&new_base->lock);
put_cpu_var(hrtimer_bases);
+
+ return dcpu;
+}
+
+static void tickle_timers(void *arg)
+{
+ hrtimer_peek_ahead_timers();
}
+
#endif /* CONFIG_HOTPLUG_CPU */
static int __cpuinit hrtimer_cpu_notify(struct notifier_block *self,
unsigned long action, void *hcpu)
{
- unsigned int cpu = (long)hcpu;
+ int dcpu = -1, scpu = (long)hcpu;
switch (action) {
case CPU_UP_PREPARE:
case CPU_UP_PREPARE_FROZEN:
- init_hrtimers_cpu(cpu);
+ init_hrtimers_cpu(scpu);
break;
#ifdef CONFIG_HOTPLUG_CPU
case CPU_DEAD:
case CPU_DEAD_FROZEN:
- clockevents_notify(CLOCK_EVT_NOTIFY_CPU_DEAD, &cpu);
- migrate_hrtimers(cpu);
+ clockevents_notify(CLOCK_EVT_NOTIFY_CPU_DEAD, &scpu);
+ dcpu = migrate_hrtimers(scpu);
+ break;
+
+ case CPU_POST_DEAD:
+ if (dcpu == -1)
+ break;
+
+ smp_call_function_single(dcpu, tickle_timers, NULL, 0);
break;
#endif
--
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