Re: broken suspend (sched related) [Was: 2.6.24-rc4-mm1]

Gautham R Shenoy
2007-12-10T11:23:36Z

On Mon, Dec 10, 2007 at 11:21:57AM +0100, Ingo Molnar wrote:
>
> * Gautham R Shenoy <ego@in.ibm.com> wrote:
>
> > > i'm wondering, what's the proper CPU-hotplug safe sequence here
> > > then? I'm picking a CPU number from cpu_online_map, and that CPU
> > > could go away while i'm still using it, right? What's saving us
> > > here?
> >
> > In this particular case, we are trying to see if any task on a
> > particular cpu has not been scheduled for a really long time. If we do
> > this check on a cpu which has gone offline, then a) If the tasks have
> > not been migrated on to another cpu yet, we will still perform that
> > check and yell if something has been holding any task for a
> > sufficiently long time. b) If the tasks have been migrated off, then
> > we have nothing to check.
>
> say we've got 100 CPUs, so we've got 100 watchdog tasks running - one
> for each CPU. Checking for hung tasks is a global operation not a
> per-CPU operation (we iterate over the global tasklist), hence only one
> CPU should really be calling this function. That online-cpus logic
> achieves this by picking a single CPU. Perhaps it would be better to
> keep a hung_task_checker_cpu variable that is driven from a
> CPU-hotplug-down notifier? That way if a CPU is brought down we can
> update hung_task_checker_cpu to another, still-online CPU. (this would
> also be faster, because event-driven)

Do you mean something like this?

From: Gautham R Shenoy <ego@in.ibm.com>

softlockup: update check_cpu during cpu-hotplug

Update the check_cpu value during a cpu-hotplug operation
so that we don't check for hung tasks on a cpu which is about
to go offline.

Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linuxtronix.de>
Cc: Jiri Slaby <jirislaby@gmail.com>
---

kernel/softlockup.c | 14 ++++++++++++--
1 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/kernel/softlockup.c b/kernel/softlockup.c
index 576eb9c..b1a8c7c 100644
--- a/kernel/softlockup.c
+++ b/kernel/softlockup.c
@@ -194,6 +194,9 @@ static void check_hung_uninterruptible_tasks(int this_cpu)
read_unlock(&tasklist_lock);
}

+
+static int check_cpu = -1;
+
/*
* The watchdog thread - runs every second and touches the timestamp.
*/
@@ -219,8 +222,6 @@ static int watchdog(void *__bind_cpu)
/*
* Only do the hung-tasks check on one CPU:
*/
- check_cpu = any_online_cpu(cpu_online_map);
-
if (this_cpu != check_cpu)
continue;

@@ -255,6 +256,7 @@ cpu_callback(struct notifier_block *nfb, unsigned long action, void *hcpu)
break;
case CPU_ONLINE:
case CPU_ONLINE_FROZEN:
+ check_cpu = any_online_cpu(cpu_online_map);
wake_up_process(per_cpu(watchdog_task, hotcpu));
break;
#ifdef CONFIG_HOTPLUG_CPU
@@ -265,6 +267,14 @@ cpu_callback(struct notifier_block *nfb, unsigned long action, void *hcpu)
/* Unbind so it can run. Fall thru. */
kthread_bind(per_cpu(watchdog_task, hotcpu),
any_online_cpu(cpu_online_map));
+ case CPU_DOWN_PREPARE:
+ case CPU_DOWN_PREPARE_FROZEN:
+ if (hotcpu == check_cpu) {
+ cpumask_t temp_cpu_online_map = cpu_online_map;
+ cpu_clear(hotcpu, temp_cpu_online_map);
+ check_cpu = any_online_cpu(temp_cpu_online_map);
+ }
+ break;
case CPU_DEAD:
case CPU_DEAD_FROZEN:
p = per_cpu(watchdog_task, hotcpu);

>
> Ingo

Thanks and Regards
gautham.
--
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"