Re: 4.12-rc ppc64 4k-page needs costly allocations

[Mathieu Malaterre <malat@debian.org>]

On Wed, May 31, 2017 at 8:44 PM, Hugh Dickins <hughd@google.com> wrote:
> [ Merging two mails into one response ]
>
> On Wed, 31 May 2017, Christoph Lameter wrote:
>> On Tue, 30 May 2017, Hugh Dickins wrote:
>> > SLUB: Unable to allocate memory on node -1, gfp=0x14000c0(GFP_KERNEL)
>> >   cache: pgtable-2^12, object size: 32768, buffer size: 65536, default order: 4, min order: 4
>> >   pgtable-2^12 debugging increased min order, use slub_debug=O to disable.
>>
>> > I did try booting with slub_debug=O as the message suggested, but that
>> > made no difference: it still hoped for but failed on order:4 allocations.
>>
>> I am curious as to what is going on there. Do you have the output from
>> these failed allocations?
>
> I thought the relevant output was in my mail.  I did skip the Mem-Info
> dump, since that just seemed noise in this case: we know memory can get
> fragmented.  What more output are you looking for?
>
>>
>> > I wanted to try removing CONFIG_SLUB_DEBUG, but didn't succeed in that:
>> > it seemed to be a hard requirement for something, but I didn't find what.
>>
>> CONFIG_SLUB_DEBUG does not enable debugging. It only includes the code to
>> be able to enable it at runtime.
>
> Yes, I thought so.
>
>>
>> > I did try CONFIG_SLAB=y instead of SLUB: that lowers these allocations to
>> > the expected order:3, which then results in OOM-killing rather than direct
>> > allocation failure, because of the PAGE_ALLOC_COSTLY_ORDER 3 cutoff.  But
>> > makes no real difference to the outcome: swapping loads still abort early.
>>
>> SLAB uses order 3 and SLUB order 4??? That needs to be tracked down.
>>
>> Ahh. Ok debugging increased the object size to an order 4. This should be
>> order 3 without debugging.
>
> But it was still order 4 when booted with slub_debug=O, which surprised me.
> And that surprises you too?  If so, then we ought to dig into it further.
>
>>
>> Why are the slab allocators used to create slab caches for large object
>> sizes?
>
> There may be more optimal ways to allocate, but I expect that when
> the ppc guys are writing the code to handle both 4k and 64k page sizes,
> kmem caches offer the best span of possibility without complication.
>
>>
>> > Relying on order:3 or order:4 allocations is just too optimistic: ppc64
>> > with 4k pages would do better not to expect to support a 128TB userspace.
>>
>> I thought you had these huge 64k page sizes?
>
> ppc64 does support 64k page sizes, and they've been the default for years;
> but since 4k pages are still supported, I choose to use those (I doubt
> I could ever get the same load going with 64k pages).

4k is pretty much required on ppc64 when it comes to nouveau:

https://bugs.freedesktop.org/show_bug.cgi?id=94757

2cts

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