NUMA topology question wrt. d4edc5b6
Srivatsa S. Bhat
srivatsa.bhat at linux.vnet.ibm.com
Fri May 23 06:48:05 EST 2014
[ Adding a few more CC's ]
On 05/22/2014 01:34 AM, Nishanth Aravamudan wrote:
> Hi Srivatsa,
> After d4edc5b6 ("powerpc: Fix the setup of CPU-to-Node mappings during
> CPU online"), cpu_to_node() looks like:
> static inline int cpu_to_node(int cpu)
> int nid;
> nid = numa_cpu_lookup_table[cpu];
> * During early boot, the numa-cpu lookup table might not have been
> * setup for all CPUs yet. In such cases, default to node 0.
> return (nid < 0) ? 0 : nid;
> However, I'm curious if this is correct in all cases. I have seen
> several LPARs that do not have any CPUs on node 0. In fact, because node
> 0 is statically set online in the initialization of the N_ONLINE
> nodemask, 0 is always present to Linux, whether it is present on the
> system. I'm not sure what the best thing to do here is, but I'm curious
> if you have any ideas? I would like to remove the static initialization
> of node 0, as it's confusing to users to see an empty node (particularly
> when it's completely separate in the numbering from other nodes), but
> we trip a panic (refer to:
Ah, I see. I didn't have any particular reason to default it to zero.
I just did that because the existing code before this patch did the same
thing. (numa_cpu_lookup_table is a global array, so it will be initialized
with zeros. So if we access it before populating it via numa_setup_cpu(),
it would return 0. So I retained that behaviour with the above conditional).
Will something like the below [totally untested] patch solve the boot-panic?
I understand that as of today first_online_node will still pick 0 since
N_ONLINE is initialized statically, but with your proposed change to that
init code, I guess the following patch should avoid the boot panic.
[ But note that first_online_node is hard-coded to 0, if MAX_NUMNODES is = 1.
So we'll have to fix that if powerpc can have a single node system whose node
is numbered something other than 0. Can that happen as well? ]
And regarding your question about what is the best way to fix this whole Linux
MM's assumption about node0, I'm not really sure.. since I am not really aware
of the extent to which the MM subsystem is intertwined with this assumption
and what it would take to cure that :-(
Srivatsa S. Bhat
diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h
index c920215..58e6469 100644
@@ -18,6 +18,7 @@ struct device_node;
#define RECLAIM_DISTANCE 10
static inline int cpu_to_node(int cpu)
@@ -30,7 +31,7 @@ static inline int cpu_to_node(int cpu)
* During early boot, the numa-cpu lookup table might not have been
* setup for all CPUs yet. In such cases, default to node 0.
- return (nid < 0) ? 0 : nid;
+ return (nid < 0) ? first_online_node : nid;
#define parent_node(node) (node)
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