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Date: Sun, 13 Jul 2008 11:41:16 +0200
From: Ingo Molnar <mingo@...e.hu>
To: Vegard Nossum <vegard.nossum@...il.com>
Cc: Dmitry Adamushko <dmitry.adamushko@...il.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
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>,
Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: current linux-2.6.git: cpusets completely broken
* Vegard Nossum <vegard.nossum@...il.com> wrote:
> On Sun, Jul 13, 2008 at 2:10 AM, Dmitry Adamushko
> <dmitry.adamushko@...il.com> wrote:
> > Subject: fix cpuset_handle_cpuhp()
> >
> > The following commit
> >
> > ---
> > commit f18f982abf183e91f435990d337164c7a43d1e6d
> > Author: Max Krasnyansky <maxk@...lcomm.com>
> > Date: Thu May 29 11:17:01 2008 -0700
> >
> > sched: CPU hotplug events must not destroy scheduler domains created by
> > the cpusets
> > ---
> >
> > [ Note, with this commit arch_update_cpu_topology is not called any more for CPUSETS. But it's just a nop.
> > The whole scheme should be probably reworked later. ]
> >
> >
> > introduced a hotplug-related problem as described below:
> >
> > [ Basically the fix below just emulates the 'old' behavior of update_sched_domains().
> > We call rebuild_sched_domains() for the same hotplug-events as it was called (and is still called
> > for !CPUSETS case) in update_sched_domains(). ]
> >
> >
> > 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. has been
> > completed, effectively 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()
> >
> > 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.
> >
> > __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.
> >
> >
> > Signed-off-by: Dmitry Adamushko <dmitry.adamushko@...il.com>
>
> Tested-by: Vegard Nossum <vegard.nossum@...il.com>
>
> Works :-)
thanks! I've tidied up the changelog and queued it up into
tip/sched/urgent. I'd prefer this more conservative patch so late in the
cycle, but i'll also queue up the more intrusive real fix from Linus and
Dmitry in sched/devel.
Linus, if you've not applied it already, you can pull Dmitry's fix from:
git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip.git sched-fixes-for-linus
shortlog, diffstat and diff below.
Thanks,
Ingo
------------------>
Dmitry Adamushko (1):
cpusets, hotplug, scheduler: fix scheduler domain breakage
kernel/cpuset.c | 24 ++++++++++++++++++------
1 files changed, 18 insertions(+), 6 deletions(-)
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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