linux-kernel - Re: [PATCH 1/1] mm/khugepaged: bypassing unnecessary scans with MMF_DISABLE

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Date: Thu, 22 Feb 2024 12:23:21 -0800
From: Yang Shi <shy828301@...il.com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, mhocko@...e.com, zokeefe@...gle.com, 
	david@...hat.com, songmuchun@...edance.com, peterx@...hat.com, 
	minchan@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/khugepaged: bypassing unnecessary scans with
 MMF_DISABLE_THP check

On Wed, Feb 21, 2024 at 11:51 PM Lance Yang <ioworker0@...il.com> wrote:
>
> 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.

Thanks for following up on this, can you please capture all the
information in the commit log?

>
> Best,
> Lance
>
> On Wed, Jan 31, 2024 at 5:30 PM Lance Yang <ioworker0@...il.com> wrote:
> >
> > Updating the change log.
> [...]
>
> > On Mon, Jan 29, 2024 at 1:46 PM Lance Yang <ioworker0@...il.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@...il.com>