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Date: Thu, 22 Feb 2024 13:11:51 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Yang Shi <shy828301@gmail.com>
Cc: Lance Yang <ioworker0@gmail.com>, mhocko@suse.com, zokeefe@google.com,
 david@redhat.com, songmuchun@bytedance.com, peterx@redhat.com,
 minchan@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] mm/khugepaged: bypassing unnecessary scans with
 MMF_DISABLE_THP check
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On Thu, 22 Feb 2024 12:23:21 -0800 Yang Shi <shy828301@gmail.com> wrote:

> > 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?

I added it.

--- a/txt/mm-khugepaged-bypassing-unnecessary-scans-with-mmf_disable_thp-check.txt
+++ b/txt/mm-khugepaged-bypassing-unnecessary-scans-with-mmf_disable_thp-check.txt
@@ -9,6 +9,24 @@ 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.
 
+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%
+
+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.
+
 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.