Re: [PATCH 1/1] mm/khugepaged: bypassing unnecessary scans with MMF_DISABLE_THP check

[Lance Yang]

Thanks for taking the time to look into this!

Thanks, Yang and Andrew!

Best,
Lance

On Fri, Feb 23, 2024 at 5:11 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, 22 Feb 2024 12:23:21 -0800 Yang Shi <shy828301@xxxxxxxxx> 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.
>