Re: [PATCH RFC 2/2] mm/mglru: add tracepoint for inc_max_seq

[Ridong Chen]

On 9/9/2026 2:45 PM, Barry Song wrote:
> On Mon, Sep 7, 2026 at 12:25 PM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:
> > From: Ridong Chen <chenridong@xxxxxxxxxx>
> >
> > Aging in MGLRU advances max_seq via inc_max_seq(), creating a new
> > youngest generation. There is currently no tracepoint on this path, so
> > the moment a new generation is created, and how the min_seq of each type
> > trails behind it, cannot be observed as it happens.
> >
> > Add mm_mglru_inc_max_seq, emitted right after max_seq is bumped, with
> > the memcg id and the new max_seq alongside the anon and file min_seq.
> > Paired with the mm_mglru_isolate_folios tracepoint it makes the full
> > aging-to-eviction window observable per memcg.
> >
> > Assisted-by: Claude:claude-opus-4-8
> > Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
> > ---
> >   include/trace/events/vmscan.h | 30 ++++++++++++++++++++++++++++++
> >   mm/vmscan.c                   |  4 ++++
> >   2 files changed, 34 insertions(+)
> >
> > diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
> > index a0e3cf75294b..ab93c2f4b318 100644
> > --- a/include/trace/events/vmscan.h
> > +++ b/include/trace/events/vmscan.h
> > @@ -439,6 +439,36 @@ TRACE_EVENT(mm_mglru_isolate_folios,
> >                  __entry->max_seq)
> >   );
> >
> > +TRACE_EVENT(mm_mglru_inc_max_seq,
> > +
> > +       TP_PROTO(u64 memcg_id,
> > +               unsigned long max_seq,
> > +               unsigned long anon_min_seq,
> > +               unsigned long file_min_seq),
> > +
> > +       TP_ARGS(memcg_id, max_seq, anon_min_seq, file_min_seq),
> > +
> > +       TP_STRUCT__entry(
> > +               __field(u64, memcg_id)
> > +               __field(unsigned long, max_seq)
> > +               __field(unsigned long, anon_min_seq)
> > +               __field(unsigned long, file_min_seq)
> > +       ),
> > +
> > +       TP_fast_assign(
> > +               __entry->memcg_id = memcg_id;
> > +               __entry->max_seq = max_seq;
> > +               __entry->anon_min_seq = anon_min_seq;
> > +               __entry->file_min_seq = file_min_seq;
> > +       ),
> > +
> > +       TP_printk("memcg_id=%llu max_seq=%lu anon_min_seq=%lu file_min_seq=%lu",
> > +               __entry->memcg_id,
> > +               __entry->max_seq,
> > +               __entry->anon_min_seq,
> > +               __entry->file_min_seq)
> > +);
> > +
> >   TRACE_EVENT(mm_vmscan_write_folio,
> >
> >          TP_PROTO(struct folio *folio),
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 771fe6827939..3c806a57d113 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -4065,6 +4065,10 @@ static bool inc_max_seq(struct lruvec *lruvec, unsigned long seq, int swappiness
> >          WRITE_ONCE(lrugen->timestamps[next], jiffies);
> >          /* make sure preceding modifications appear */
> >          smp_store_release(&lrugen->max_seq, lrugen->max_seq + 1);
> > +       trace_mm_mglru_inc_max_seq(mem_cgroup_id(lruvec_memcg(lruvec)),
> > +                                  lrugen->max_seq,
> > +                                  lrugen->min_seq[LRU_GEN_ANON],
> > +                                  lrugen->min_seq[LRU_GEN_FILE]);
>
> I suspect we also want to know the number of pages in each generation,
> similar to:
>
> /sys/kernel/debug # cat lru_gen
> memcg 1 /
>   node     0
>            0      25672          0          85
>            1      25672          0        4249
>            2      25672         21           0
>            3      25672          0           0
> memcg 19 /A
>   node     0
>            0      23036          0           0
>            1      23036          0           0
>            2      23036          0           0
>            3      23036          0           0
Thanks.

Will add.

--
Best regards
Ridong