linux-kernel - Re: [PATCH v2 1/3] arm64/numa: export memory_add_physaddr_to_nid as EXPORT_SYMBOL

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Date:   Wed, 8 Jul 2020 11:39:51 +0300
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Michal Hocko <mhocko@...nel.org>, Jia He <justin.he@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>,
        Chuhong Yuan <hslester96@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Kaly Xin <Kaly.Xin@....com>
Subject: Re: [PATCH v2 1/3] arm64/numa: export memory_add_physaddr_to_nid as
 EXPORT_SYMBOL_GPL

On Wed, Jul 08, 2020 at 10:26:41AM +0200, David Hildenbrand wrote:
> On 08.07.20 09:50, Dan Williams wrote:
> > On Wed, Jul 8, 2020 at 12:22 AM David Hildenbrand <david@...hat.com> 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@...hat.com>
> >>>>>>>>> Signed-off-by: Jia He <justin.he@....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.

> 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?

Well, it started with Michal's comment above that EXPORT_SYMBOL_GPL()
for a stub might be an overkill.

I think Jia's suggestion [1] with addition of a comment that explains
why and when the stub will be used, can work for both
memory_add_physaddr_to_nid() and phys_to_target_node().

But on more theoretical/fundmanetal level, I think we lack a generic
abstraction similar to e.g. x86 'struct numa_meminfo' that serves as
translaton of firmware supplied information into data that can be used
by the generic mm without need to reimplement it for each and every
arch.

[1] https://lore.kernel.org/lkml/AM6PR08MB406907F9F2B13DA6DC893AD9F7670@AM6PR08MB4069.eurprd08.prod.outlook.com

> 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
> 

-- 
Sincerely yours,
Mike.