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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 07/07/14 12:33, Stefan Bader wrote:
> I recently noticed that I get a panic (rebooting the system) on shutdown in some
> cases. This happened only on my AMD system and also not all the time. Finally
> 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.

~Andrew

---8<-----
diff --git a/xen/common/cpupool.c b/xen/common/cpupool.c
index 4a0e569..a1af1ea 100644
--- a/xen/common/cpupool.c
+++ b/xen/common/cpupool.c
@@ -472,12 +472,12 @@ static void cpupool_cpu_add(unsigned int cpu)
 static int cpupool_cpu_remove(unsigned int cpu)
 {
     int ret = 0;
+    struct cpupool **c;
 
     spin_lock(&cpupool_lock);
-    if ( !cpumask_test_cpu(cpu, cpupool0->cpu_valid))
-        ret = -EBUSY;
-    else
-        cpumask_set_cpu(cpu, &cpupool_locked_cpus);
+    for_each_cpupool(c)
+        cpumask_clear_cpu(cpu, (*c)->cpu_valid);
+    cpumask_set_cpu(cpu, &cpupool_locked_cpus);
     spin_unlock(&cpupool_lock);
 
     return ret;

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