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Subject: Re: [PATCH] cpu-topology: warn if NUMA configurations conflicts with
 lower layer
From:   Valentin Schneider <valentin.schneider@arm.com>
To:     "Zengtao (B)" <prime.zeng@hisilicon.com>,
        Sudeep Holla <sudeep.holla@arm.com>
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        Morten Rasmussen <morten.rasmussen@arm.com>
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Date:   Fri, 3 Jan 2020 12:14:35 +0000
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On 03/01/2020 10:57, Valentin Schneider wrote:
> I'm juggling with other things atm, but let me have a think and see if we
> couldn't detect that in the scheduler itself.
> 

Something like this ought to catch your case; might need to compare group
spans rather than pure group pointers.

---
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 6ec1e595b1d4..c4151e11afcd 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1120,6 +1120,13 @@ build_sched_groups(struct sched_domain *sd, int cpu)
 
 		sg = get_group(i, sdd);
 
+		/* sg's are inited as self-looping. If 'last' is not self
+		 * looping, we set it in a previous visit. No further visit
+		 * should change the link order, if we do then the topology
+		 * description is terminally broken.
+		 */
+		BUG_ON(last && last->next != last && last->next != sg);
+
 		cpumask_or(covered, covered, sched_group_span(sg));
 
 		if (!first)