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

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Date:	Sat, 12 Jul 2008 13:14:45 +0200
From:	Dmitry Adamushko <dmitry.adamushko@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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, 2008-07-12 at 12:45 +0200, Dmitry Adamushko wrote:
> 2008/7/12 Dmitry Adamushko <dmitry.adamushko@...il.com>:
> > 2008/7/12 Linus Torvalds <torvalds@...ux-foundation.org>:
> >>
> >>
> >> On Sat, 12 Jul 2008, Vegard Nossum wrote:
> >>>
> >>> Can somebody else please test/ack/review it too? This should eventually
> >>> go into 2.6.26 if it doesn't break anything else.
> >>
> >> And Dmitry, _please_ also explain what was going on. Why did things break
> >> from calling common_cpu_mem_hotplug_unplug() too much? That function is
> >> called pretty randomly anyway (for just about any random CPU event), so
> >> why did it fail in some circumstances?
> >
> > Upon CPU_DOWN_PREPARE, update_sched_domains() ->
> > detach_destroy_domains(&cpu_online_map) ;
> > does the following:
> >
> > /*
> >  * Force a reinitialization of the sched domains hierarchy. The domains
> >  * and groups cannot be updated in place without racing with the balancing
> >  * code, so we temporarily attach all running cpus to the NULL domain
> >  * which will prevent rebalancing while the sched domains are recalculated.
> >  */
> >
> > The sched-domains should be rebuilt when a CPU_DOWN ops. is completed,
> > effectivelly either upon CPU_DEAD{_FROZEN} (upon success) or
> > CPU_DOWN_FAILED{_FROZEN} (upon failure -- restore the things to their
> > initial state). That's what update_sched_domains() also does but only
> > for !CPUSETS case.
> >
> > 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. ]
> 
> I had in mind something like this:
> 
> [ yes, probably the patch makes things somewhat uglier. I tried to bring a minimal amount of changes so far, just to emulate the 'old' behavior of update_sched_domains().
> I guess, common_cpu_mem_hotplug_unplug() needs to be split up into cpu- and mem-hotplug parts to make it cleaner ]
> 
> (not tested yet)
> 
> ---

argh, this one compiles (will test shortly).


Signed-off-by: Dmitry Adamushko <dmitry.adamushko@...il.com>


diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 9fceb97..798b3ab 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1882,7 +1882,7 @@ static void scan_for_empty_cpusets(const struct
cpuset *root)
  * in order to minimize text size.
  */
 
-static void common_cpu_mem_hotplug_unplug(void)
+static void common_cpu_mem_hotplug_unplug(int rebuild_sd)
 {
 	cgroup_lock();
 
@@ -1894,7 +1894,8 @@ static void common_cpu_mem_hotplug_unplug(void)
 	 * Scheduler destroys domains on hotplug events.
 	 * Rebuild them based on the current settings.
 	 */
-	rebuild_sched_domains();
+	if (rebuild_sd)
+		rebuild_sched_domains();
 
 	cgroup_unlock();
 }
@@ -1912,11 +1913,22 @@ 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)
+	switch (phase) {
+	case CPU_UP_CANCELED:
+	case CPU_UP_CANCELED_FROZEN:
+	case CPU_DOWN_FAILED:
+	case CPU_DOWN_FAILED_FROZEN:
+	case CPU_ONLINE:
+	case CPU_ONLINE_FROZEN:
+	case CPU_DEAD:
+	case CPU_DEAD_FROZEN:
+		common_cpu_mem_hotplug_unplug(1);
+		break;
+	default:
 		return NOTIFY_DONE;
+	}
 
-	common_cpu_mem_hotplug_unplug();
-	return 0;
+	return NOTIFY_OK;
 }
 
 #ifdef CONFIG_MEMORY_HOTPLUG
@@ -1929,7 +1941,7 @@ static int cpuset_handle_cpuhp(struct
notifier_block *unused_nb,
 
 void cpuset_track_online_nodes(void)
 {
-	common_cpu_mem_hotplug_unplug();
+	common_cpu_mem_hotplug_unplug(0);
 }
 #endif
 



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