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

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Date:	Mon, 10 Dec 2007 11:21:57 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Gautham R Shenoy <ego@...ibm.com>
Cc:	Jiri Slaby <jirislaby@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: broken suspend (sched related) [Was: 2.6.24-rc4-mm1]


* Gautham R Shenoy <ego@...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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