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From: Lance Yang <ioworker0@gmail.com>
Date: Thu, 22 Feb 2024 15:43:20 +0800
Message-ID: <CAK1f24=D1e=tKCjMMG1cr3bT4mow2ZinOOsYmwkv8oD2LHQbBw@mail.gmail.com>
Subject: Re: [PATCH 1/1] mm/khugepaged: bypassing unnecessary scans with
 MMF_DISABLE_THP check
To: Andrew Morton <akpm@linux-foundation.org>
Cc: mhocko@suse.com, zokeefe@google.com, david@redhat.com, 
	songmuchun@bytedance.com, shy828301@gmail.com, peterx@redhat.com, 
	minchan@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org
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Hey Andrew,

Thanks for taking time to review!

I appreciate your suggestion and will be
supplementing with test results shortly.

Best,
Lance

On Thu, Feb 22, 2024 at 6:12=E2=80=AFAM Andrew Morton <akpm@linux-foundatio=
n.org> wrote:
>
> On Wed, 31 Jan 2024 17:30:11 +0800 Lance Yang <ioworker0@gmail.com> wrote=
:
>
> > Updating the change log.
> >
> > khugepaged scans the entire address space in the
> > background for each given mm, looking for
> > opportunities to merge sequences of basic pages
> > into huge pages. However, when an mm is inserted
> > to the mm_slots list, and the MMF_DISABLE_THP
> > flag is set later, this scanning process becomes
> > unnecessary for that mm and can be skipped to
> > avoid redundant operations, especially in scenarios
> > with a large address space.
> >
> > This commit introduces a check before each scanning
> > process to test the MMF_DISABLE_THP flag for the
> > given mm; if the flag is set, the scanning process is
> > bypassed, thereby improving the efficiency of khugepaged.
> >
> > This optimization is not a correctness issue but rather an
> > enhancement to save expensive checks on each VMA
> > when userspace cannot prctl itself before spawning
> > into the new process.
> >
> > On some servers within our company, we deploy a
> > daemon responsible for monitoring and updating local
> > applications. Some applications prefer not to use THP,
> > so the daemon calls prctl to disable THP before fork/exec.
> > Conversely, for other applications, the daemon calls prctl
> > to enable THP before fork/exec.
> >
> > Ideally, the daemon should invoke prctl after the fork,
> > but its current implementation follows the described
> > approach. In the Go standard library, there is no direct
> > encapsulation of the fork system call; instead, fork and
> > execve are combined into one through syscall.ForkExec.
>
> I pasted the above into the v1 patch's changelog.
>
> However I'm not seeing a good level of reviewer enthusiasm.  Pertially
> because of the lack of quantitative testing results.  Is is possible to
> generate such results, to give people an overall feel of the
> desirability of this change?
>