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

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Message-ID: <CACZaFFN8Y1dKYJPeNcnE+9hfBYNFLCJMi4TAyKuQ70JvjcrZPQ@mail.gmail.com>
Date: Sun, 8 Feb 2026 21:23:30 +0800
From: Vernon Yang <vernon2gm@...il.com>
To: Dev Jain <dev.jain@....com>
Cc: "David Hildenbrand (Arm)" <david@...nel.org>, 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 Sun, Feb 8, 2026 at 5:05 PM Dev Jain <dev.jain@....com> 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.

$ gcc --version
gcc (GCC) 15.2.1 20250808 (Red Hat 15.2.1-1)

Above is my gcc version. However, I performed the assembly again without
any optimization :(

I suspect that I might have messed up the environment earlier, failing to
compile the newly modified code successfully, which resulted is assembly
old_khugepaged.o.