linux-kernel - Re: current linux-2.6.git: cpusets completely broken

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Date:	Sat, 12 Jul 2008 13:10:54 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dmitry Adamushko <dmitry.adamushko@...il.com>
cc:	Vegard Nossum <vegard.nossum@...il.com>,
	Paul Menage <menage@...gle.com>,
	Max Krasnyansky <maxk@...lcomm.com>, Paul Jackson <pj@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, miaox@...fujitsu.com,
	rostedt@...dmis.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: current linux-2.6.git: cpusets completely broken



On Sat, 12 Jul 2008, Dmitry Adamushko wrote:
> 
> With Max's patch, sched-domains' reinitialization is delegated to CPUSETS code:
> 
> cpuset_handle_cpuhp() -> common_cpu_mem_hotplug_unplug() ->
> rebuild_sched_domains()
> 
> which as you've said "called pretty randomly anyway", e.g. for CPU_UP_PREPARE.
> 
> [ ah, then rebuild_sched_domains() should not be there. It should be
> nop for MEMPLUG events I presume - should make another patch. ]

That whole notion of doing the same thing for UP/DOWN/SIDEWAYS looks 
pretty damn odd to me, but whatever.

> Being called for CPU_UP_PREPARE (and if its callback is called after
> update_sched_domains()), it just negates all the work done by
> update_sched_domains() -- i.e. a soon-to-be-offline cpu is included in
> the sched-domains and that makes it visible for the load-balancer
> while the CPU_DOWN ops. is in progress.

This sounds like it could trigger various other problems too, but happily 
hit the BUG_ON() first.

> __migrate_live_tasks() moves the tasks off a 'dead' cpu (it's already
> "offline" when this function is called).
> 
> try_to_wake_up() is called for one of these tasks from another CPU ->
> the load-balancer (wake_idle()) picks up a "dead" CPU and places the
> task on it. Then e.g. BUG_ON(rq->nr_running) detects this a bit later
> -> oops.

Grr. 

Ok, can you re-send the (fixed-up) patch with the explanation and the 
tested-by: from Vegard. It seems that not calling rebuild_sched_domains() 
(by not calling common_cpu_mem_hotplug_unplug()) for CPU_UP_PREPARE was 
the minimal fix.

IOW, the "switch()" statement was just another way of adding 
CPU_UP_PREPARE to the list of things that we don't do anything for, and 
your patch was really just equivalent to the following patch?

Anyway, I'd just like a patch that (a) has been tested and (b) comes with 
a nice subject line and targeted explanation of this issue, so I can 
commit it for 2.6.26.

The patch below is not meant for beign used, it's just to verify that I am 
on the same page with you explanation.

		Linus

---
 kernel/cpuset.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 9fceb97..24f34ce 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1912,7 +1912,8 @@ static void common_cpu_mem_hotplug_unplug(void)
 static int cpuset_handle_cpuhp(struct notifier_block *unused_nb,
 				unsigned long phase, void *unused_cpu)
 {
-	if (phase == CPU_DYING || phase == CPU_DYING_FROZEN)
+	if (phase == CPU_DYING || phase == CPU_DYING_FROZEN ||
+	    phase == CPU_UP_PREPARE || phase == CPU_UP_PREPARE_FROZEN)
 		return NOTIFY_DONE;
 
 	common_cpu_mem_hotplug_unplug();
--
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