mm/vmscan: add tracepoint and reason for kswapd_failures reset

mm/vmscan: add tracepoint and reason for kswapd_failures reset

Currently, kswapd_failures is reset in multiple places (kswapd, direct
reclaim, PCP freeing, memory-tiers), but there's no way to trace when and
why it was reset, making it difficult to debug memory reclaim issues.

This patch:

1. Introduce kswapd_clear_hopeless() as a wrapper function to
   centralize kswapd_failures reset logic.

2. Introduce kswapd_test_hopeless() to encapsulate hopeless node
   checks, replacing all open-coded kswapd_failures comparisons.

3. Add kswapd_clear_hopeless_reason enum to distinguish reset sources:
   - KSWAPD_CLEAR_HOPELESS_KSWAPD: reset from kswapd context
   - KSWAPD_CLEAR_HOPELESS_DIRECT: reset from direct reclaim
   - KSWAPD_CLEAR_HOPELESS_PCP: reset from PCP page freeing
   - KSWAPD_CLEAR_HOPELESS_OTHER: reset from other paths

4. Add tracepoints for better observability:
   - mm_vmscan_kswapd_clear_hopeless: traces each reset with reason
   - mm_vmscan_kswapd_reclaim_fail: traces each kswapd reclaim failure

Test results:

$ trace-cmd record -e vmscan:mm_vmscan_kswapd_clear_hopeless -e vmscan:mm_vmscan_kswapd_reclaim_fail
$ # generate memory pressure
$ trace-cmd report
cpus=4
 kswapd0-71    [000]    27.216563: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=1
 kswapd0-71    [000]    27.217169: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=2
 kswapd0-71    [000]    27.217764: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=3
 kswapd0-71    [000]    27.218353: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=4
 kswapd0-71    [000]    27.218993: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=5
 kswapd0-71    [000]    27.219744: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=6
 kswapd0-71    [000]    27.220488: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=7
 kswapd0-71    [000]    27.221206: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=8
 kswapd0-71    [000]    27.221806: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=9
 kswapd0-71    [000]    27.222634: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=10
 kswapd0-71    [000]    27.223286: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=11
 kswapd0-71    [000]    27.223894: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=12
 kswapd0-71    [000]    27.224712: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=13
 kswapd0-71    [000]    27.225424: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=14
 kswapd0-71    [000]    27.226082: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=15
 kswapd0-71    [000]    27.226810: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=16
 kswapd1-72    [002]    27.386869: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=1
 kswapd1-72    [002]    27.387435: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=2
 kswapd1-72    [002]    27.388016: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=3
 kswapd1-72    [002]    27.388586: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=4
 kswapd1-72    [002]    27.389155: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=5
 kswapd1-72    [002]    27.389723: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=6
 kswapd1-72    [002]    27.390292: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=7
 kswapd1-72    [002]    27.392364: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=8
 kswapd1-72    [002]    27.392934: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=9
 kswapd1-72    [002]    27.393504: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=10
 kswapd1-72    [002]    27.394073: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=11
 kswapd1-72    [002]    27.394899: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=12
 kswapd1-72    [002]    27.395472: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=13
 kswapd1-72    [002]    27.396055: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=14
 kswapd1-72    [002]    27.396628: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=15
 kswapd1-72    [002]    27.397199: mm_vmscan_kswapd_reclaim_fail: nid=1 failures=16
kworker/u18:0-40    [002]    27.410151: mm_vmscan_kswapd_clear_hopeless: nid=0 reason=DIRECT
 kswapd0-71    [000]    27.439454: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=1
 kswapd0-71    [000]    27.440048: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=2
 kswapd0-71    [000]    27.440634: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=3
 kswapd0-71    [000]    27.441211: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=4
 kswapd0-71    [000]    27.441787: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=5
 kswapd0-71    [000]    27.442363: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=6
 kswapd0-71    [000]    27.443030: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=7
 kswapd0-71    [000]    27.443725: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=8
 kswapd0-71    [000]    27.444315: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=9
 kswapd0-71    [000]    27.444898: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=10
 kswapd0-71    [000]    27.445476: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=11
 kswapd0-71    [000]    27.446053: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=12
 kswapd0-71    [000]    27.446646: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=13
 kswapd0-71    [000]    27.447230: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=14
 kswapd0-71    [000]    27.447812: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=15
 kswapd0-71    [000]    27.448391: mm_vmscan_kswapd_reclaim_fail: nid=0 failures=16
 ann-423   [003]    28.028285: mm_vmscan_kswapd_clear_hopeless: nid=0 reason=PCP

Link: https://lkml.kernel.org/r/20260120024402.387576-3-jiayuan.chen@linux.dev
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>	[tracing]
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>