Re: [PATCH -next] mm: vmscan: correct nr_requested tracing in scan_folios

[David Hildenbrand (Red Hat)]

On 12/4/25 01:46, Chen Ridong wrote:
> On 2025/12/3 19:33, David Hildenbrand (Red Hat) wrote:
> > On 12/3/25 10:40, Chen Ridong wrote:
> > > From: Chen Ridong <chenridong@xxxxxxxxxx>
> > >
> > > When enabling vmscan tracing, it is observed that nr_requested is always
> > > 4096, which is confusing.
> > >
> > >           mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
> > >           mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
> > >           mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
> > >           mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
> > >           mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
> > >           mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
> > >           mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
> > >
> > > This is because it prints MAX_LRU_BATCH, which is meaningless as it's a
> > > constant. To fix this, modify it to print nr_to_scan as isolate_lru_folios
> > > does.
> > >
> > > Fixes: 8c2214fc9a47 ("mm: multi-gen LRU: reuse some legacy trace events")
> > > Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
> > > ---
> > >    mm/vmscan.c | 2 +-
> > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index fddd168a9737..8cfafd50a7a8 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -4601,7 +4601,7 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
> > >        count_memcg_events(memcg, item, isolated);
> > >        count_memcg_events(memcg, PGREFILL, sorted);
> > >        __count_vm_events(PGSCAN_ANON + type, isolated);
> > > -    trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, MAX_LRU_BATCH,
> > > +    trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan,
> > >                    scanned, skipped, isolated,
> >
> > We do that in isolate_lru_folios().
> >
> > Given that we do
> >
> >      int remaining = min(nr_to_scan, MAX_LRU_BATCH);
> >
> > and effectively cap it, I wonder if we would want to trace that capped valued instead of MAX_LRU_BATCH.
>
> I prefer tracing nr_to_scan, as it reflects the original target number of pages we intended to scan.

But it's misleading, because we're also tracing "scanned, skipped, 
isolated", and one might wonder how it relates to nr_to_scan?

> Even if nr_to_scan exceeds MAX_LRU_BATCH, we can still deduce that it was effectively capped by
> examining the actual scanned, skipped, or isolated counts. However, if we trace min(nr_to_scan,
> MAX_LRU_BATCH) instead, we would lose visibility into what the original nr_to_scan value was.

Is that really required for the purpose we are tracing here?

--
Cheers

David