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

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Message-ID: <9942f80a-5a99-4e43-8053-8e6707c39252@linux.dev>
Date: Sun, 8 Feb 2026 17:32:23 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Dev Jain <dev.jain@....com>, 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, 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 2026/2/8 17:05, Dev Jain wrote:
> 
> 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.

FWIW, 11.4.0 is newer that the minimum GCC version (8.1) required by
kernel. See Documentation/process/changes.rst

The optimization might just be version-dependent :)