linux-kernel - Re: [PATCH v2 6/9] x86/clear_huge

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Date:   Wed, 13 Sep 2023 12:13:55 +0530
From:   Raghavendra K T <raghavendra.kt@....com>
To:     Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org
Cc:     akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        willy@...radead.org, mgorman@...e.de, peterz@...radead.org,
        rostedt@...dmis.org, tglx@...utronix.de, jon.grimm@....com,
        bharata@....com, boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v2 6/9] x86/clear_huge_page: multi-page clearing

On 8/31/2023 12:19 AM, Ankur Arora wrote:
> clear_pages_rep(), clear_pages_erms() clear using string instructions.
> While clearing extents of more than a single page, we can use these
> more effectively by explicitly advertising the region-size to the
> processor.
> 
> This can be used as a hint by the processor-uarch to optimize the
> clearing (ex. to avoid polluting one or more levels of the data-cache.)
> 
> As a secondary benefit, string instructions are typically microcoded,
> and so it's a good idea to amortize the cost of the decode across larger
> regions.
> 
> Accordingly, clear_huge_page() now does huge-page clearing in three
> parts: the neighbourhood of the faulting address, the left, and the
> right region of the neighbourhood.
> 
> The local neighbourhood is cleared last to keep its cachelines hot.
> 
[...]
> 
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> ---
>   arch/x86/mm/hugetlbpage.c | 54 +++++++++++++++++++++++++++++++++++++++
>   1 file changed, 54 insertions(+)
> 

Hello Ankur,

Just thinking loud here (w.r.t THP).

V3 patchset with uarch changes had changes in THP path too, where
one could explicitly give hints or non-caching hints. and they are
  passed down to call incoherent clearing.

IMO, those changes logically belong to uarch optimizations.. right?