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Subject: Re: [PATCH v2] sched/topology: fix kernel crash when a CPU is
 hotplugged in a memoryless node
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Borislav Petkov <bp@suse.de>,
        David Gibson <david@gibson.dropbear.id.au>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Nathan Fontenot <nfont@linux.vnet.ibm.com>,
        Michael Bringmann <mwb@linux.vnet.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, Ingo Molnar <mingo@redhat.com>
References: <20190220165520.5241-1-lvivier@redhat.com>
 <20190220170841.GM32494@hirez.programming.kicks-ass.net>
From:   Laurent Vivier <lvivier@redhat.com>
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Message-ID: <11997145-4718-ed17-6085-54be18bf85ba@redhat.com>
Date:   Wed, 20 Feb 2019 18:57:01 +0100
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On 20/02/2019 18:08, Peter Zijlstra wrote:
> On Wed, Feb 20, 2019 at 05:55:20PM +0100, Laurent Vivier wrote:
>> index 3f35ba1d8fde..372278605f0d 100644
>> --- a/kernel/sched/topology.c
>> +++ b/kernel/sched/topology.c
>> @@ -1651,6 +1651,7 @@ void sched_init_numa(void)
>>  	 */
>>  	tl[i++] = (struct sched_domain_topology_level){
>>  		.mask = sd_numa_mask,
>> +		.flags = SDTL_OVERLAP,
> 
> This makes no sense what so ever. The numa identify node should not have
> overlap with other domains.
> 
> Are you sure this is not because of the utterly broken powerpc nonsense
> where they move CPUs between nodes?

No, I'm not sure. This why I've Cc: powerpc folks. My conclusion is only
based on the before/after changes.

I've tested some patches from powerpc ML, but they don't fix this problem:
  powerpc/numa: Perform full re-add of CPU for PRRN/VPHN topology update
  powerpc/pseries: Perform full re-add of CPU for topology update
post-migration

So the only reason I can see to have a corrupted sched_group list is the
sched_domain_span() fonction doesn't return a correct cpumask for the
domain once a new CPU is added.

Thanks,
Laurent