[PATCH v2 0/3] mm/mglru: add tracepoints for scan and aging paths

[Ridong Chen]

From: Ridong Chen <chenridong@xxxxxxxxxx>

Background
==========

MGLRU currently has no tracepoints of its own. The scan and evict paths
reuse the classic-LRU tracepoints (trace_mm_vmscan_lru_isolate() and
trace_mm_vmscan_lru_shrink_inactive()), which predate MGLRU and carry no
generation, sequence, memcg or swappiness context. A trace of a running
system therefore cannot tell which memcg a given scan belongs to, how far
reclaim has progressed through the generations, or when a new generation
is created - so how MGLRU actually operates is effectively invisible.

Implementation
==============

Patch 1 is a cleanup that factors the per-generation page-count
summation into a helper, lru_gen_seq_nr_pages(), reused by patch 3.

Patch 2 adds mm_mglru_scan_folios on the scan path, and patch 3 adds
mm_mglru_inc_max_seq on the aging path. Both carry the memcg id and the
generation window (min_seq/max_seq), live at MGLRU-specific layers with
no classic-LRU counterpart, and are guarded so the hot paths stay
zero-cost when disabled.

Effect
======

Paired, the two tracepoints make the full aging-to-eviction window
observable per memcg: aging advances max_seq (the leading edge), scanning
consumes the oldest generations, and the min_seq/max_seq pair on each
event shows how the generation window moves over time.

A sample trace, with the classic-LRU tracepoints left enabled to show
how they interleave:

  mm_vmscan_lru_isolate: classzone=4 order=0 nr_requested=36 nr_scanned=31 nr_skipped=0 nr_taken=29 lru=inactive_file
  mm_mglru_scan_folios: memcg_id=73 classzone=4 order=0 nr_requested=36 nr_scanned=31 nr_sorted=2 nr_skipped=0 nr_taken=29 lru=inactive_file max_seq=3 tier=3 min_seq=0
  mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=31 nr_reclaimed=29 nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activate_anon=0 nr_activate_file=0 nr_ref_keep=0 nr_unmap_fail=0 priority=5 flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
  mm_mglru_inc_max_seq: memcg_id=73 max_seq=4 anon_min_seq=1 file_min_seq=1 nr_anon={0x0,0x0,0x1,0x0} nr_file={0x0,0x427,0x6,0x4a}
  mm_vmscan_lru_isolate: classzone=4 order=0 nr_requested=5 nr_scanned=5 nr_skipped=0 nr_taken=1 lru=inactive_file
  mm_mglru_scan_folios: memcg_id=73 classzone=4 order=0 nr_requested=5 nr_scanned=5 nr_sorted=4 nr_skipped=0 nr_taken=1 lru=inactive_file max_seq=4 tier=3 min_seq=1
  mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=5 nr_reclaimed=1 nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activate_anon=0 nr_activate_file=0 nr_ref_keep=0 nr_unmap_fail=0 priority=5 flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC

The mm_mglru_scan_folios lines carry the memcg id and the generation
window (max_seq/min_seq) that the bare mm_vmscan_lru_isolate lines above
them cannot - those cannot even say which memcg they came from. The
mm_mglru_inc_max_seq line then shows a new generation being created
(max_seq 3 -> 4) with the per-generation page counts for both types.

---
v2: address Sashiko AI review [1] - pass struct lruvec * into the tracepoints
    and do the memcg lookup inside TP_fast_assign.

[1] https://sashiko.dev/#/patchset/20260911072848.2346073-1-ridong.chen@xxxxxxxxx?part=2

Ridong Chen (3):
  mm/mglru: factor out lru_gen_seq_nr_pages()
  mm/mglru: add tracepoint for scan_folios()
  mm/mglru: add tracepoint for inc_max_seq()

 include/trace/events/vmscan.h | 101 ++++++++++++++++++++++++++++++++++
 mm/vmscan.c                   |  53 +++++++++++++++---
 2 files changed, 146 insertions(+), 8 deletions(-)

-- 
2.34.1