Received-SPF: pass (google.com: domain of linux-kernel+bounces-76029-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Precedence: bulk
MIME-Version: 1.0
References: <20240129054551.57728-1-ioworker0@gmail.com> <CAK1f24keWtJNVv37r2vNsqnmMLRMvF-F76WR5RD_Y-BbAgEaYQ@mail.gmail.com>
In-Reply-To: <CAK1f24keWtJNVv37r2vNsqnmMLRMvF-F76WR5RD_Y-BbAgEaYQ@mail.gmail.com>
From: Lance Yang <ioworker0@gmail.com>
Date: Thu, 22 Feb 2024 15:51:29 +0800
Message-ID: <CAK1f24nk19eciysFqvTd=rqpiKePhstDWEEQ_mvT89WCDNrWNQ@mail.gmail.com>
Subject: Re: [PATCH 1/1] mm/khugepaged: bypassing unnecessary scans with
 MMF_DISABLE_THP check
To: 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
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

Hey,

On an Intel Core i5 CPU, the time taken by
khugepaged to scan the address space of
the process, which has been set with the
MMF_DISABLE_THP flag after being added
to the mm_slots list, is as follows (shorter is better):

VMA Count |   Old   |   New   |  Change
---------------------------------------
    50     |   23us  |    9us  |  -60.9%
   100     |   32us  |    9us  |  -71.9%
   200     |   44us  |    9us  |  -79.5%
   400     |   75us  |    9us  |  -88.0%
   800     |   98us  |    9us  |  -90.8%

IIUC, once the count of VMAs for the process
exceeds page_to_scan, khugepaged needs to
wait for scan_sleep_millisecs ms before scanning
the next process. IMO, unnecessary scans could
actually be skipped with a very inexpensive
mm->flags check in this case.

Best,
Lance

On Wed, Jan 31, 2024 at 5:30=E2=80=AFPM Lance Yang <ioworker0@gmail.com> wr=
ote:
>
> Updating the change log.
[...]

> On Mon, Jan 29, 2024 at 1:46=E2=80=AFPM Lance Yang <ioworker0@gmail.com> =
wrote:
> >
> > 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.
[...]
> > Signed-off-by: Lance Yang <ioworker0@gmail.com>