[PATCH v2 0/4] Improve khugepaged scan logic

[Vernon Yang <vernon2gm@gmail.com>]

Hi all,

This series is improve the khugepaged scan logic, reduce CPU consumption,
prioritize scanning task that access memory frequently.

The following data is traced by bpftrace[1] on a desktop system. After
the system has been left idle for 10 minutes upon booting, a lot of
SCAN_PMD_MAPPED or SCAN_NO_PTE_TABLE are observed during a full scan by
khugepaged.

@scan_pmd_status[1]: 1           ## SCAN_SUCCEED
@scan_pmd_status[6]: 2           ## SCAN_EXCEED_SHARED_PTE
@scan_pmd_status[3]: 142         ## SCAN_PMD_MAPPED
@scan_pmd_status[2]: 178         ## SCAN_NO_PTE_TABLE
total progress size: 674 MB
Total time         : 419 seconds ## include khugepaged_scan_sleep_millisecs

The khugepaged has below phenomenon: the khugepaged list is scanned in a
FIFO manner, as long as the task is not destroyed,
1. the task no longer has memory that can be collapsed into hugepage,
   continues scan it always.
2. the task at the front of the khugepaged scan list is cold, they are
   still scanned first.
3. everyone scan at intervals of khugepaged_scan_sleep_millisecs
   (default 10s). If we always scan the above two cases first, the valid
   scan will have to wait for a long time.

For the first case, when the memory is either SCAN_PMD_MAPPED or
SCAN_NO_PTE_TABLE, just skip it.

For the second case, if the user has explicitly informed us via
MADV_COLD/MADV_FREE that this vma is cold or will be freed,
just skip it only.

The below is some performance test results.

kernbench results (testing on x86_64 machine):

                     baseline w/o patches   test w/ patches
Amean     user-32    18586.99 (   0.00%)    18562.36 *   0.13%*
Amean     syst-32     1133.61 (   0.00%)     1126.02 *   0.67%*
Amean     elsp-32      668.05 (   0.00%)      667.13 *   0.14%*
BAmean-95 user-32    18585.23 (   0.00%)    18559.71 (   0.14%)
BAmean-95 syst-32     1133.22 (   0.00%)     1125.49 (   0.68%)
BAmean-95 elsp-32      667.94 (   0.00%)      667.08 (   0.13%)
BAmean-99 user-32    18585.23 (   0.00%)    18559.71 (   0.14%)
BAmean-99 syst-32     1133.22 (   0.00%)     1125.49 (   0.68%)
BAmean-99 elsp-32      667.94 (   0.00%)      667.08 (   0.13%)

Create three task[2]: hot1 -> cold -> hot2. After all three task are
created, each allocate memory 128MB. the hot1/hot2 task continuously
access 128 MB memory, while the cold task only accesses its memory
briefly andthen call madvise(MADV_COLD). Here are the performance test
results:
(Throughput bigger is better, other smaller is better)

Testing on x86_64 machine:

| task hot2           | without patch | with patch    |  delta  |
|---------------------|---------------|---------------|---------|
| total accesses time |  3.14 sec     |  2.93 sec     | -6.69%  |
| cycles per access   |  4.96         |  2.21         | -55.44% |
| Throughput          |  104.38 M/sec |  111.89 M/sec | +7.19%  |
| dTLB-load-misses    |  284814532    |  69597236     | -75.56% |

Testing on qemu-system-x86_64 -enable-kvm:

| task hot2           | without patch | with patch    |  delta  |
|---------------------|---------------|---------------|---------|
| total accesses time |  3.35 sec     |  2.96 sec     | -11.64% |
| cycles per access   |  7.29         |  2.07         | -71.60% |
| Throughput          |  97.67 M/sec  |  110.77 M/sec | +13.41% |
| dTLB-load-misses    |  241600871    |  3216108      | -98.67% |

This series is based on Linux v6.19-rc2.

Thank you very much for your comments and discussions :)


[1] https://github.com/vernon2gh/app_and_module/blob/main/khugepaged/khugepaged_mm.bt
[2] https://github.com/vernon2gh/app_and_module/blob/main/khugepaged/app.c

V1 -> V2:
- Rename full to full_scan_finished, pickup Acked-by.
- Just skip SCAN_PMD_MAPPED/NO_PTE_TABLE memory, not remove mm.
- Set VM_NOHUGEPAGE flag when MADV_COLD/MADV_FREE to just skip, not move mm.
- Again test performance at the v6.19-rc2.

Vernon Yang (4):
  mm: khugepaged: add trace_mm_khugepaged_scan event
  mm: khugepaged: just skip when the memory has been collapsed
  mm: khugepaged: set VM_NOHUGEPAGE flag when MADV_COLD/MADV_FREE
  mm: khugepaged: set to next mm direct when mm has
    MMF_DISABLE_THP_COMPLETELY

 include/trace/events/huge_memory.h | 24 +++++++++++++++++++++
 mm/khugepaged.c                    | 34 ++++++++++++++++++++++++------
 mm/madvise.c                       | 17 ++++++++++-----
 3 files changed, 63 insertions(+), 12 deletions(-)

--
2.51.0