linux-kernel - Re: [PATCH v1] drivers/base/memory.c: Don't store end_section

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Message-ID: <f8767e9a-034d-dca6-05e6-dc6bbcb4d005@redhat.com>
Date:   Thu, 1 Aug 2019 09:00:45 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v1] drivers/base/memory.c: Don't store end_section_nr in
 memory blocks

On 01.08.19 08:13, Michal Hocko wrote:
> On Wed 31-07-19 16:43:58, David Hildenbrand wrote:
>> On 31.07.19 16:37, Michal Hocko wrote:
>>> On Wed 31-07-19 16:21:46, David Hildenbrand wrote:
>>> [...]
>>>>> Thinking about it some more, I believe that we can reasonably provide
>>>>> both APIs controlable by a command line parameter for backwards
>>>>> compatibility. It is the hotplug code to control sysfs APIs.  E.g.
>>>>> create one sysfs entry per add_memory_resource for the new semantic.
>>>>
>>>> Yeah, but the real question is: who needs it. I can only think about
>>>> some DIMM scenarios (some, not all). I would be interested in more use
>>>> cases. Of course, to provide and maintain two APIs we need a good reason.
>>>
>>> Well, my 3TB machine that has 7 movable nodes could really go with less
>>> than
>>> $ find /sys/devices/system/memory -name "memory*" | wc -l
>>> 1729>
>>
>> The question is if it would be sufficient to increase the memory block
>> size even further for these kinds of systems (e.g., via a boot parameter
>> - I think we have that on uv systems) instead of having blocks of
>> different sizes. Say, 128GB blocks because you're not going to hotplug
>> 128MB DIMMs into such a system - at least that's my guess ;)
> 
> The system has
> [    0.000000] ACPI: SRAT: Node 1 PXM 1 [mem 0x10000000000-0x17fffffffff]
> [    0.000000] ACPI: SRAT: Node 2 PXM 2 [mem 0x80000000000-0x87fffffffff]
> [    0.000000] ACPI: SRAT: Node 3 PXM 3 [mem 0x90000000000-0x97fffffffff]
> [    0.000000] ACPI: SRAT: Node 4 PXM 4 [mem 0x100000000000-0x107fffffffff]
> [    0.000000] ACPI: SRAT: Node 5 PXM 5 [mem 0x110000000000-0x117fffffffff]
> [    0.000000] ACPI: SRAT: Node 6 PXM 6 [mem 0x180000000000-0x183fffffffff]
> [    0.000000] ACPI: SRAT: Node 7 PXM 7 [mem 0x190000000000-0x191fffffffff]
> 
> hotplugable memory. I would love to have those 7 memory blocks to work
> with. Any smaller grained split is just not helping as the platform will
> not be able to hotremove it anyway.
> 

So the smallest granularity in your system is indeed 128GB (btw, nice
system, I wish I had something like that), the biggest one 512GB.

Using a memory block size of 128GB would imply on a 3TB system 24 memory
blocks - which is tolerable IMHO. Especially, performance-wise there
shouldn't be a real difference to 7 blocks. Hotunplug triggered via ACPI
will take care of offlining the right DIMMs.

Of course, 7 blocks would be nicer, but as discussed, not possible with
the current ABI.

What we could do right now is finally make "cat
/sys/devices/system/memory/memory99/phys_device" indicate on x86-64 to
which DIMM an added memory range belongs (if applicable). For now, it's
only used on s390x. We could store for each memory block the
"phys_index" - a.k.a. section number of the lowest memory block of a
add_memory() range.

This would at least allow user space to identify all memory blocks that
logically belong together (DIMM) without ABI changes.

-- 

Thanks,

David / dhildenb