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

[Ridong Chen]

On 9/11/2026 6:29 PM, Ridong Chen wrote:
> 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(-)

Hi Steven, Baolin and Baoquan,

Thanks again for reviewing v2.

Following Steven's suggestion in this thread, I first moved the vmscan
trace header under mm/ and exposed struct scan_control to it in a
separate RFC series. Patches [2/3] and [3/3] of v2 have now been
reworked on top of that and posted together with it as one series:

  [PATCH v4 0/9] mm/vmscan: move tracepoints to mm/ and add MGLRU
  tracepoints
  https://lore.kernel.org/all/20261019034426.2354049-1-ridong.chen@xxxxxxxxx/

The two MGLRU tracepoints are [8/9] and [9/9] there. Compared with v2:

- Both events now take only lruvec/sc plus the local counters, and
  derive everything else (memcg id, order, reclaim_idx, lru, max_seq,
  min_seq and the per-generation page counts) in TP_fast_assign()
  (Steven).
- Add nid, derived from lruvec_pgdat(lruvec) (Baoquan).
- Group the int fields at the end of the entry to avoid padding on
  64-bit (Steven).
- Drop v2 [1/3] "mm/mglru: factor out lru_gen_seq_nr_pages()": the
  per-generation page count is now summed inside TP_fast_assign(), so
  the helper has no user left.

Since [8/9] and [9/9] changed substantially, I did not carry over your
Reviewed-by tags. I would really appreciate it if you could take
another look at v4.

--
Best regards
Ridong