mm: khugepaged: add trace_mm_khugepaged_scan event

Patch series "Improve khugepaged scan logic", v8.

This series improves the khugepaged scan logic and reduces CPU consumption
by prioritizing scanning tasks 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 or SCAN_PTE_MAPPED_HUGEPAGE [5], just skip it.

For the second case, if the user has explicitly informed us via
MADV_FREE that these folios 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 18522.51 ( 0.00%) 18333.64 * 1.02%*
Amean syst-32 1137.96 ( 0.00%) 1113.79 * 2.12%*
Amean elsp-32 666.04 ( 0.00%) 659.44 * 0.99%*
BAmean-95 user-32 18520.01 ( 0.00%) 18323.57 ( 1.06%)
BAmean-95 syst-32 1137.68 ( 0.00%) 1110.50 ( 2.39%)
BAmean-95 elsp-32 665.92 ( 0.00%) 659.06 ( 1.03%)
BAmean-99 user-32 18520.01 ( 0.00%) 18323.57 ( 1.06%)
BAmean-99 syst-32 1137.68 ( 0.00%) 1110.50 ( 2.39%)
BAmean-99 elsp-32 665.92 ( 0.00%) 659.06 ( 1.03%)

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_FREE). 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 patch (of 4):

Add mm_khugepaged_scan event to track the total time for full scan and the
total number of pages scanned of khugepaged.

Link: https://lkml.kernel.org/r/20260221093918.1456187-2-vernon2gm@gmail.com
Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Liam Howlett <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>