Re: [PATCH v2 1/4] mm/mglru: add MM_WALK_EMPTY stats and tracepoint

[Baoquan He <baoquan.he@linux.dev>]

On 09/01/26 at 03:11pm, Barry Song wrote:
> On Tue, Sep 1, 2026 at 2:38 PM Baoquan He <hebaoquan@kylinos.cn> wrote:
> >
> > Add per-walk counters to measure empty aging walks which traverse
> > an mm's page tables but find no folio for the current lruvec (node+memcg).
> > These are common on multi-NUMA node systems because lru_gen_use_mm() marks
> > an mm for all nodes at every context switch.
> >
> > New counters (accumulated in mm_state->stats[]):
> >
> >   MM_LEAF_ASSOCIATED   - leaf entries whose folio is in this lruvec
> >   MM_WALK_TOTAL        - page-table walks completed
> >   MM_WALK_EMPTY        - walks that found no folio in this lruvec
> >   MM_LEAF_EMPTY_WALKS  - leaf entries scanned during empty walks
> >
> > A new tracepoint, mm_vmscan_lru_gen_walk(), fires after each walk, and the
> > debugfs lru_gen output ("TYFALWEE") exposes the new counters.
> >
> > Signed-off-by: Baoquan He <hebaoquan@kylinos.cn>
> > ---
> [...]
> >
> > @@ -4109,8 +4113,28 @@ static bool try_to_inc_max_seq(struct lruvec *lruvec, unsigned long seq,
> >
> >         do {
> >                 success = iterate_mm_list(walk, &mm);
> > -               if (mm)
> > +               if (mm) {
> > +                       bool empty = false;
> > +
> >                         walk_mm(mm, walk);
> > +                       /*
> > +                        * A walk that traversed page tables but found no folio
> > +                        * belonging to this lruvec (node+memcg) is pure waste.
> > +                        */
> > +                       if (walk->mm_stats[MM_LEAF_TOTAL]) {
> > +                               walk->mm_stats[MM_WALK_TOTAL]++;
> > +                               if (walk->mm_stats[MM_LEAF_ASSOCIATED] == 0) {
> > +                                       walk->mm_stats[MM_WALK_EMPTY]++;
> > +                                       walk->mm_stats[MM_LEAF_EMPTY_WALKS] +=
> > +                                               walk->mm_stats[MM_LEAF_TOTAL];
> > +                                       empty = true;
> > +                               }
> > +                       }
> 
> Hi Baoquan,
> 
> Where are we clearing these counters between different
> mms?
> 
> It seems the previous mm_stats will affect the next mm
> if they aren't cleared.

No, they won't. The counters are cleared between mms. iterate_mm_list()
calls reset_mm_stats() from its "done:" path for every mm. reset_mm_stats()
accumulates walk->mm_stats into the per-hist mm_state->stats[hist] and
then zeroes walk->mm_stats[i].

> 
> 
> > +                       trace_mm_vmscan_lru_gen_walk(
> > +                                       lruvec_pgdat(lruvec)->node_id, walk->seq,
> > +                                       walk->mm_stats[MM_LEAF_TOTAL],
> > +                                       walk->mm_stats[MM_LEAF_ASSOCIATED], empty);
> > +               }
> >         } while (mm);
> >  done:
> 
> Best Regards
> Barry
>