linux-kernel - Re: [PATCH 0/1] mm: Optimizing hugepage zeroing in arm64

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Date:   Fri, 22 Jan 2021 12:45:55 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Will Deacon <will@...nel.org>,
        Prathu Baronia <prathubaronia2011@...il.com>,
        Prathu Baronia <prathu.baronia@...plus.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        linux-kernel@...r.kernel.org, chintan.pandya@...plus.com,
        "glider@...gle.com" <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 0/1] mm: Optimizing hugepage zeroing in arm64

On 2021-01-22 12:13, Catalin Marinas wrote:
> On Thu, Jan 21, 2021 at 06:59:37PM +0000, Robin Murphy wrote:
>> On 2021-01-21 17:46, Will Deacon wrote:
>>> On Thu, Jan 21, 2021 at 10:21:50PM +0530, Prathu Baronia wrote:
>>>> This patch removes the unnecessary kmap calls in the hugepage zeroing path and
>>>> improves the timing by 62%.
>>>>
>>>> I had proposed a similar change in Apr-May'20 timeframe in memory.c where I
>>>> proposed to clear out a hugepage by directly calling a memset over the whole
>>>> hugepage but got the opposition that the change was not architecturally neutral.
>>>>
>>>> Upon revisiting this now I see significant improvement by removing around 2k
>>>> barrier calls from the zeroing path. So hereby I propose an arm64 specific
>>>> definition of clear_user_highpage().
>>>
>>> Given that barrier() is purely a thing for the compiler, wouldn't the same
>>> change yield a benefit on any other architecture without HIGHMEM? In which
>>> case, I think this sort of change belongs in the core code if it's actually
>>> worthwhile.
>>
>> I would have thought it's more the constant manipulation of the preempt and
>> pagefault counts, rather than the compiler barriers between them, that has
>> the impact. Either way, if arm64 doesn't need to be atomic WRT preemption
>> when clearing parts of hugepages then I also can't imagine that anyone else
>> (at least for !HIGHMEM) would either.
> 
> I thought the kmap_local stuff was supposed to fix this unnecessary
> preemption disabling on 64-bit architectures:
> 
> https://lwn.net/Articles/836144/
> 
> I guess it's not there yet.

No, it's there alright - when I pulled up the code to double-check my 
memory of this area, I did notice the kerneldoc and start wondering if 
this should simply be using kmap_local_page() for everyone.

Robin.