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

Ingo Molnar
2007-12-10T10:23:23Z

* 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)

Ingo