Re: [RFC,1/2] powerpc/numa: fix cpu_to_node() usage during boot

Hello,

On Fri, Jul 10, 2015 at 09:15:47AM -0700, Nishanth Aravamudan wrote:

On 08.07.2015 [16:16:23 -0700], Nishanth Aravamudan wrote:

quoted

On 08.07.2015 [14:00:56 +1000], Michael Ellerman wrote:

quoted

On Thu, 2015-02-07 at 23:02:02 UTC, Nishanth Aravamudan wrote:

quoted

Much like on x86, now that powerpc is using USE_PERCPU_NUMA_NODE_ID, we
have an ordering issue during boot with early calls to cpu_to_node().

"now that .." implies we changed something and broke this. What commit was
it that changed the behaviour?

Well, that's something I'm trying to still unearth. In the commits
before and after adding USE_PERCPU_NUMA_NODE_ID (8c272261194d
"powerpc/numa: Enable USE_PERCPU_NUMA_NODE_ID"), the dmesg reports:

pcpu-alloc: [0] 0 1 2 3 4 5 6 7

Ok, I did a bisection, and it seems like prior to commit
1a4d76076cda69b0abf15463a8cebc172406da25 ("percpu: implement
asynchronous chunk population"), we emitted the above, e.g.:

pcpu-alloc: [0] 0 1 2 3 4 5 6 7

And after that commit, we emitted:

pcpu-alloc: [0] 0 1 2 3 [0] 4 5 6 7

I'm not exactly sure why that changed, but I'm still
reading/understanding the commit. Tejun might be able to explain.

Tejun, for reference, I noticed on Power systems since the
above-mentioned commit, pcpu-alloc is not reflecting the topology of the
system correctly -- that is, the pcpu areas are all on node 0
unconditionally (based up on pcpu-alloc's output). Prior to that, there
was just one group, it seems like, which completely ignored the NUMA
topology.

Is this just an ordering thing that changed with the introduction of the
async code?

It's just each unit growing and percpu allocator deciding to split
them into separate allocation units.  Before it was serving all cpus
in a single alloc unit as they looked like they belong to the same
NUMA node and small enough to fit into one alloc unit.  In the latter,
the async one added more reserve space, so the allocator is deciding
to split them into two alloc units while assigning them to the same
group as the NUMA info wasn't still there.

Thanks.

-- 
tejun