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Re: [Xen-devel] Shutdown panic in disable_nonboot_cpus after cpupool-numa-split

On 07/07/2014 04:08 PM, Stefan Bader wrote:
On 07.07.2014 15:03, Jürgen Groß wrote:
On 07/07/2014 02:49 PM, Stefan Bader wrote:
On 07.07.2014 14:38, Jürgen Groß wrote:
On 07/07/2014 02:00 PM, Andrew Cooper wrote:
On 07/07/14 12:33, Stefan Bader wrote:
I recently noticed that I get a  panic (rebooting the system) on shutdown in
   > cases. This happened only on my AMD system and also not all the time.
   > realized that it is related to the use of using cpupool-numa-split
   > (libxl with xen-4.4 maybe, but not 100% sure 4.3 as well).
   > What happens is that on shutdown the hypervisor runs
disable_nonboot_cpus which
   > call cpu_down for each online cpu. There is a BUG_ON in the code for
the case of
   > cpu_down returning -EBUSY. This happens in my case as soon as the
first cpu that
   > has been moved to pool-1 by cpupool-numa-split is attempted. The error is
   > returned by running the notifier_call_chain and I suspect that ends
up calling
   > cpupool_cpu_remove which always returns EBUSY for cpus not in pool0.
   > I am not sure which end needs to be fixed but looping over all online
cpus in
   > disable_nonboot_cpus sounds plausible. So maybe the check for pool-0 in
   > cpupool_cpu_remove is wrong...?
   > -Stefan

Hmm yes - this looks completely broken.

cpupool_cpu_remove() only has a single caller which is from cpu_down(),
and will unconditionally fail for cpus outside of the default pool.

It is not obvious at all how this is supposed to work, and the comment
beside cpupool_cpu_remove() doesn't help.

Can you try the following (only compile tested) patch, which looks
plausibly like it might DTRT.  The for_each_cpupool() is a little nastly
but there appears to be no cpu_to_cpupool mapping available.

Your patch has the disadvantage to support hot-unplug of the last cpu in
a cpupool. The following should work, however:

Disadvantage and support sounded a bit confusing. But I think it means
hot-unplugging the last cpu of a pool is bad and should not be working.


diff --git a/xen/common/cpupool.c b/xen/common/cpupool.c
index 4a0e569..73249d3 100644
--- a/xen/common/cpupool.c
+++ b/xen/common/cpupool.c
@@ -471,12 +471,24 @@ static void cpupool_cpu_add(unsigned int cpu)
   static int cpupool_cpu_remove(unsigned int cpu)
-    int ret = 0;
+    int ret = -EBUSY;
+    struct cpupool **c;

-    if ( !cpumask_test_cpu(cpu, cpupool0->cpu_valid))
-        ret = -EBUSY;
+    if ( cpumask_test_cpu(cpu, cpupool0->cpu_valid) )
+        ret = 0;
+    {
+        for_each_cpupool(c)
+        {
+            if ( cpumask_test_cpu(cpu, (*c)->cpu_suspended ) )

The rest seems to keep the semantics the same as before (though does that mean
unplugging the last cpu of pool-0 is ok?) But why testing for suspended here to
succeed (and not valid)?

Testing valid would again enable to remove the last cpu of a cpupool in
case of hotplugging. cpu_suspended is set if all cpus are to be removed
due to shutdown, suspend to ram/disk, ...

Ah, ok. Thanks for the detail explanation. So I was trying this change in
parallel and can confirm that it gets rid of the panic on shutdown. But when I
try to offline any cpu in pool1 (if echoing 0 into /sys/devices/xen_cpu/xen_cpu?
is the correct method) I always get EBUSY. IOW I cannot hot-unplug any cpu that
is in a pool other than 0. It does only work after removing it from pool1, then
add it to pool0 and then echo 0 into online.

That's how it was designed some years ago. I don't want to change the
behavior in the hypervisor. Adding some tool support could make sense,


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