linux-kernel - Re: [PATCH mm-new v6 2/5] mm: khugepaged: refine scan progress number

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Message-ID: <78b9bb1c-21aa-435f-a697-ebbfbe604a5a@arm.com>
Date: Sun, 8 Feb 2026 14:35:11 +0530
From: Dev Jain <dev.jain@....com>
To: Vernon Yang <vernon2gm@...il.com>,
 "David Hildenbrand (Arm)" <david@...nel.org>
Cc: akpm@...ux-foundation.org, lorenzo.stoakes@...cle.com, ziy@...dia.com,
 baohua@...nel.org, lance.yang@...ux.dev, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Vernon Yang <yanglincheng@...inos.cn>
Subject: Re: [PATCH mm-new v6 2/5] mm: khugepaged: refine scan progress number


On 06/02/26 4:42 pm, Vernon Yang wrote:
> On Fri, Feb 06, 2026 at 10:02:48AM +0100, David Hildenbrand (Arm) wrote:
>> On 2/5/26 15:25, Dev Jain wrote:
>>> On 05/02/26 5:41 pm, David Hildenbrand (arm) wrote:
>>>> On 2/5/26 07:08, Vernon Yang wrote:
>>>>> On Thu, Feb 5, 2026 at 5:35 AM David Hildenbrand (arm)
>>>>> <david@...nel.org> wrote:
>>>>>
>>>>> I guess, your meaning is "min(_pte - pte + 1, HPAGE_PMD_NR)", not max().
>>>> Yes!
>>>>
>>>>>
>>>>> I'm also worried that the compiler can't optimize this since the body of
>>>>> the loop is complex, as with Dev's opinion [1].
>>>> Why do we even have to optimize this? :)
>>>>
>>>> Premature ... ? :)
>>>
>>> I mean .... we don't, but the alternate is a one liner using max().
>> I'm fine with the max(), but it still seems like adding complexity to
>> optimize something that is nowhere prove to really be a problem.
> Hi David, Dev,
>
> I use "*cur_progress += 1" at the beginning of the loop, the compiler
> optimize that. Assembly as follows:
>
> 60c1:	4d 29 ca        sub    %r9,%r10		// r10 is _pte, r9 is pte, r10 = _pte - pte
> 60c4:	b8 00 02 00 00  mov    $0x200,%eax	// eax = HPAGE_PMD_NR
> 60c9:	44 89 5c 24 10  mov    %r11d,0x10(%rsp)	//
> 60ce:	49 c1 fa 03     sar    $0x3,%r10	//
> 60d2:	49 83 c2 01     add    $0x1,%r10	// r10 += 1
> 60d6:	49 39 c2        cmp    %rax,%r10	// r10 = min(r10, eax)
> 60d9:	4c 0f 4f d0     cmovg  %rax,%r10	//
> 60dd:	44 89 55 00     mov    %r10d,0x0(%rbp)	// *cur_progress = r10
>
> To make the code simpler, Let us use "*cur_progress += 1".

Wow! Wasn't expecting that. What's your gcc version? I checked with
gcc 11.4.0 (looks pretty old) with both x86 and arm64, and it couldn't
optimize.

> --
> Thanks,
> Vernon