Re: [PATCH v2 1/3] arm64/numa: export memory_add_physaddr_to_nid as EXPORT_SYMBOL_GPL

[David Hildenbrand <david@redhat.com>]

On 08.07.20 09:50, Dan Williams wrote:
> On Wed, Jul 8, 2020 at 12:22 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 08.07.20 07:27, Mike Rapoport wrote:
>>> On Tue, Jul 07, 2020 at 03:05:48PM -0700, Dan Williams wrote:
>>>> On Tue, Jul 7, 2020 at 11:01 AM Mike Rapoport <rppt@linux.ibm.com> wrote:
>>>>>
>>>>> On Tue, Jul 07, 2020 at 02:26:08PM +0200, David Hildenbrand wrote:
>>>>>> On 07.07.20 14:13, Mike Rapoport wrote:
>>>>>>> On Tue, Jul 07, 2020 at 01:54:54PM +0200, Michal Hocko wrote:
>>>>>>>> On Tue 07-07-20 13:59:15, Jia He wrote:
>>>>>>>>> This exports memory_add_physaddr_to_nid() for module driver to use.
>>>>>>>>>
>>>>>>>>> memory_add_physaddr_to_nid() is a fallback option to get the nid in case
>>>>>>>>> NUMA_NO_NID is detected.
>>>>>>>>>
>>>>>>>>> Suggested-by: David Hildenbrand <david@redhat.com>
>>>>>>>>> Signed-off-by: Jia He <justin.he@arm.com>
>>>>>>>>> ---
>>>>>>>>>  arch/arm64/mm/numa.c | 5 +++--
>>>>>>>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/arch/arm64/mm/numa.c b/arch/arm64/mm/numa.c
>>>>>>>>> index aafcee3e3f7e..7eeb31740248 100644
>>>>>>>>> --- a/arch/arm64/mm/numa.c
>>>>>>>>> +++ b/arch/arm64/mm/numa.c
>>>>>>>>> @@ -464,10 +464,11 @@ void __init arm64_numa_init(void)
>>>>>>>>>
>>>>>>>>>  /*
>>>>>>>>>   * We hope that we will be hotplugging memory on nodes we already know about,
>>>>>>>>> - * such that acpi_get_node() succeeds and we never fall back to this...
>>>>>>>>> + * such that acpi_get_node() succeeds. But when SRAT is not present, the node
>>>>>>>>> + * id may be probed as NUMA_NO_NODE by acpi, Here provide a fallback option.
>>>>>>>>>   */
>>>>>>>>>  int memory_add_physaddr_to_nid(u64 addr)
>>>>>>>>>  {
>>>>>>>>> - pr_warn("Unknown node for memory at 0x%llx, assuming node 0\n", addr);
>>>>>>>>>   return 0;
>>>>>>>>>  }
>>>>>>>>> +EXPORT_SYMBOL_GPL(memory_add_physaddr_to_nid);
>>>>>>>>
>>>>>>>> Does it make sense to export a noop function? Wouldn't make more sense
>>>>>>>> to simply make it static inline somewhere in a header? I haven't checked
>>>>>>>> whether there is an easy way to do that sanely bu this just hit my eyes.
>>>>>>>
>>>>>>> We'll need to either add a CONFIG_ option or arch specific callback to
>>>>>>> make both non-empty (x86, powerpc, ia64) and empty (arm64, sh)
>>>>>>> implementations coexist ...
>>>>>>
>>>>>> Note: I have a similar dummy (return 0) patch for s390x lying around here.
>>>>>
>>>>> Then we'll call it a tie - 3:3 ;-)
>>>>
>>>> So I'd be happy to jump on the train of people wanting to export the
>>>> ARM stub for this (and add a new ARM stub for phys_to_target_node()),
>>>> but Will did have a plausibly better idea that I have been meaning to
>>>> circle back to:
>>>>
>>>> http://lore.kernel.org/r/20200325111039.GA32109@willie-the-truck
>>>>
>>>> ...i.e. iterate over node data to do the lookup. This would seem to
>>>> work generically for multiple archs unless I am missing something?
>>
>> IIRC, only memory assigned to/onlined to a ZONE is represented in the
>> pgdat node span. E.g., not offline memory blocks.
> 
> So this dovetails somewhat with Will's idea. What if we populated
> node_data for "offline" ranges? I started there, but then saw
> ARCH_KEEP_MEMBLOCK and thought it would be safer to just teach
> phys_to_target_node() to use that rather than update other code paths
> to expect node_data might not always reflect online data.

We currently need a somewhat-accurate pgdat node span to detect when to
offline a node. See try_offline_node(). This works fairly reliable.

Shrinking the node span is currently fairly easy for !ZONE_DEVICE
memory, because we can rely on pfn_to_online_page() + pfn_to_nid(pfn).
See e.g., find_biggest_section_pfn().

If we glue growing/shrinking the node span to adding/removing of memory
(instead of e.g., onlining/offlining), we can no longer base shrinking
on memmap data. We would have to get the information ("how far can I
shrink the node span, is it empty?") from somewhere else. E.g.,
for_each_memory_block() - but that one does not cover ZONE_DEVICE. And
there are memory blocks which cover multiple nodes, in which case we
only store one of them ... unreliable.

This certainly needs more thought :/

> 
>> Esp., when hotplugging + onlining consecutive memory, there won't really
>> be any intersections in most cases if I am not wrong. It would not be
>> "intersection" but rather "closest fit".
>>
>> With overlapping nodes it's even more unclear. Which one to pick?
> 
> In the overlap case you get what you get. Some signal is better than
> the noise of a dummy function. The consequences of picking the wrong
> node might be that the kernel can't properly associate a memory range
> to its performance data tables in firmware, but then again firmware
> messed up with an overlapping node definition in the first instance.

I'd be curious if what we are trying to optimize here is actually worth
optimizing. IOW, is there a well-known scenario where the dummy value on
arm64 would be problematic and is worth the effort?

I mean, in all performance relevant setups (ignoring
hv_balloon/xen-balloon/prove_store(), which also use
memory_add_physaddr_to_nid()), we should have a proper PXM/node
specified by the hardware on memory hotadd. The fallback of
memory_add_physaddr_to_nid() is not relevant in these scenarios.

-- 
Thanks,

David / dhildenb
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