Re: [PATCH 1/9] mm/mglru: add MM_WALK_EMPTY stats and tracepoint

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

On 08/28/26 at 01:37pm, Barry Song wrote:
> On Mon, Aug 24, 2026 at 3:38 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_ELIGIBLE    - folios belonging to this lruvec
> >   MM_WALK_TOTAL       - page-table walks completed
> >   MM_WALK_EMPTY       - walks that found no eligible folio
> >   MM_LEAF_TOTAL_EMPTY - leaf entries scanned by empty walks
> 
> Hi Baoquan,
> 
> I'm having a hard time understanding what MM_LEAF_ELIGIBLE means.
> In particular, I'm not sure why "eligible" refers to folios here.
> 
> I'm also finding MM_LEAF_TOTAL_EMPTY quite difficult to understand
> without looking at the implementation.
> 
> Could we come up with clearer and more descriptive names for these
> counters?  might be?
> 
> MM_LEAF_ASSOCIATED,          /* folios associated with this lruvec */
> MM_WALK_TOTAL,               /* completed page-table walks */
> MM_WALK_WITHOUT_ASSOCIATED,  /* walks with no associated folio */
> MM_LEAF_WITHOUT_ASSOCIATED,  /* leaf entries in such walks */

Sorry about the confusion, let me think about this, and your naming
suggestions sound good, I will update here if I have different
thinking. Thanks for careful reviewing.

> 
> >
> > 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>
> > ---
> >  include/linux/mmzone.h        |  4 ++++
> >  include/trace/events/vmscan.h | 28 ++++++++++++++++++++++++++++
> >  mm/vmscan.c                   | 30 ++++++++++++++++++++++++++----
> >  3 files changed, 58 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index 94f9c3ff5416..229d27fbfb54 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -595,6 +595,10 @@ enum {
> >         MM_LEAF_YOUNG,          /* young leaf entries */
> >         MM_NONLEAF_FOUND,       /* non-leaf entries found in Bloom filters */
> >         MM_NONLEAF_ADDED,       /* non-leaf entries added to Bloom filters */
> > +       MM_LEAF_ELIGIBLE,       /* folios belonging to this lruvec (node+memcg) */
> > +       MM_WALK_TOTAL,          /* page-table walks completed */
> > +       MM_WALK_EMPTY,          /* walks that found no eligible folio */
> > +       MM_LEAF_TOTAL_EMPTY,    /* leaf entries scanned by empty walks */
> >         NR_MM_STATS
> >  };
> >
> 
> Best Regards
> Barry
>