linux-kernel - Re: [RFC v3] sched/topology: fix kernel crash when a CPU is hotplugged in a memoryless node

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Message-Id: <20190318110610.GB4450@linux.vnet.ibm.com>
Date:   Mon, 18 Mar 2019 16:36:10 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Laurent Vivier <lvivier@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Borislav Petkov <bp@...e.de>,
        David Gibson <david@...son.dropbear.id.au>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        Michael Bringmann <mwb@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC v3] sched/topology: fix kernel crash when a CPU is
 hotplugged in a memoryless node

* Laurent Vivier <lvivier@...hat.com> [2019-03-15 12:12:45]:

> 
> Another way to avoid the nodes overlapping for the offline nodes at
> startup is to ensure the default values don't define a distance that
> merge all offline nodes into node 0.
> 
> A powerpc specific patch can workaround the kernel crash by doing this:
> 
> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index 87f0dd0..3ba29bb 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -623,6 +623,7 @@ static int __init parse_numa_properties(void)
>         struct device_node *memory;
>         int default_nid = 0;
>         unsigned long i;
> +       int nid, dist;
> 
>         if (numa_enabled == 0) {
>                 printk(KERN_WARNING "NUMA disabled by user\n");
> @@ -636,6 +637,10 @@ static int __init parse_numa_properties(void)
> 
>         dbg("NUMA associativity depth for CPU/Memory: %d\n",
> min_common_depth);
> 
> +       for (nid = 0; nid < MAX_NUMNODES; nid ++)
> +               for (dist = 0; dist < MAX_DISTANCE_REF_POINTS; dist++)
> +                       distance_lookup_table[nid][dist] = nid;
> +

The only reason, this would have worked in the specific case, is because we
are overriding the distance_lookup_table with a unique distance.
So node_distance for any other node other than itself will return max
distance which is 40 in this case. (since distance_ref_points_depth is 2)

I am not sure if  this will work if the node distance between the two nodes
happens to be 20.

>         /*
>          * Even though we connect cpus to numa domains later in SMP
>          * init, we need to know the node ids now. This is because
> 

-- 
Thanks and Regards
Srikar Dronamraju