linux-kernel - Re: [RESEND PATCH] mm: align larger anonymous mappings on THP boundaries

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Date: Tue, 7 May 2024 13:34:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, Yang Shi <shy828301@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>,
 Yang Shi <yang@...amperecomputing.com>, riel@...riel.com, cl@...ux.com,
 akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Ze Zuo <zuoze1@...wei.com>
Subject: Re: [RESEND PATCH] mm: align larger anonymous mappings on THP
 boundaries

On 07.05.24 13:26, Ryan Roberts wrote:
> On 07/05/2024 12:14, Ryan Roberts wrote:
>> On 07/05/2024 12:13, David Hildenbrand wrote:
>>>
>>>> https://github.com/intel/lmbench/blob/master/src/lat_mem_rd.c#L95
>>>>
>>>>> suggest. If you want to try something semi-randomly; it might be useful to rule
>>>>> out the arm64 contpte feature. I don't see how that would be interacting here if
>>>>> mTHP is disabled (is it?). But its new for 6.9 and arm64 only. Disable with
>>>>> ARM64_CONTPTE (needs EXPERT) at compile time.
>>>> I don't enabled mTHP, so it should be not related about ARM64_CONTPTE,
>>>> but will have a try.
>>>
>>> cont-pte can get active if we're just lucky when allocating pages in the right
>>> order, correct Ryan?
>>
>> No it shouldn't do; it requires the pages to be in the same folio.

Ah, my memory comes back. That's also important for folio_pte_batch() to 
currently work as expected I think. We could change that, though, and 
let cont-pte batch across folios.

>>
> 
> That said, if we got lucky in allocating the "right" pages, then we will end up
> doing an extra function call and a bit of maths per every 16 PTEs in order to
> figure out that the span is not contained by a single folio, before backing out
> of an attempt to fold. That would probably be just about measurable.
> 
> But the regression doesn't kick in until 96K, which is the step after 64K. I'd
> expect to see the regression on 64K too if that was the issue. The cacheline is
> 64K so I suspect it could be something related to the cache?
> 

-- 
Cheers,

David / dhildenb