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

[Ingo Molnar <mingo@elte.hu>]

* 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
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