Re: [PATCH 2/7] mm, vmscan: add active list aging tracepoint

[Minchan Kim]

On Thu, Dec 29, 2016 at 08:52:46AM +0100, Michal Hocko wrote:
> On Thu 29-12-16 14:33:59, Minchan Kim wrote:
> > On Wed, Dec 28, 2016 at 04:30:27PM +0100, Michal Hocko wrote:
> > > From: Michal Hocko <mhocko@xxxxxxxx>
> > > 
> > > Our reclaim process has several tracepoints to tell us more about how
> > > things are progressing. We are, however, missing a tracepoint to track
> > > active list aging. Introduce mm_vmscan_lru_shrink_active which reports
> > > the number of scanned, rotated, deactivated and freed pages from the
> > > particular node's active list.
> > > 
> > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> > > ---
> > >  include/linux/gfp.h           |  2 +-
> > >  include/trace/events/vmscan.h | 38 ++++++++++++++++++++++++++++++++++++++
> > >  mm/page_alloc.c               |  6 +++++-
> > >  mm/vmscan.c                   | 22 +++++++++++++++++-----
> > >  4 files changed, 61 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> > > index 4175dca4ac39..61aa9b49e86d 100644
> > > --- a/include/linux/gfp.h
> > > +++ b/include/linux/gfp.h
> > > @@ -503,7 +503,7 @@ void * __meminit alloc_pages_exact_nid(int nid, size_t size, gfp_t gfp_mask);
> > >  extern void __free_pages(struct page *page, unsigned int order);
> > >  extern void free_pages(unsigned long addr, unsigned int order);
> > >  extern void free_hot_cold_page(struct page *page, bool cold);
> > > -extern void free_hot_cold_page_list(struct list_head *list, bool cold);
> > > +extern int free_hot_cold_page_list(struct list_head *list, bool cold);
> > >  
> > >  struct page_frag_cache;
> > >  extern void __page_frag_drain(struct page *page, unsigned int order,
> > > diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
> > > index 39bad8921ca1..d34cc0ced2be 100644
> > > --- a/include/trace/events/vmscan.h
> > > +++ b/include/trace/events/vmscan.h
> > > @@ -363,6 +363,44 @@ TRACE_EVENT(mm_vmscan_lru_shrink_inactive,
> > >  		show_reclaim_flags(__entry->reclaim_flags))
> > >  );
> > >  
> > > +TRACE_EVENT(mm_vmscan_lru_shrink_active,
> > > +
> > > +	TP_PROTO(int nid, unsigned long nr_scanned, unsigned long nr_freed,
> > > +		unsigned long nr_unevictable, unsigned long nr_deactivated,
> > > +		unsigned long nr_rotated, int priority, int file),
> > > +
> > > +	TP_ARGS(nid, nr_scanned, nr_freed, nr_unevictable, nr_deactivated, nr_rotated, priority, file),
> > 
> > I agree it is helpful. And it was when I investigated aging problem of 32bit
> > when node-lru was introduced. However, the question is we really need all those
> > kinds of information? just enough with nr_taken, nr_deactivated, priority, file?
> 
> Dunno. Is it harmful to add this information? I like it more when the
> numbers just add up and you have a clear picture. You never know what
> might be useful when debugging a weird behavior. 

Michal, I'm not huge fan of "might be useful" although it's a small piece of code.
It adds just all of kinds overheads (memory footprint, runtime performance,
maintainance) without any proved benefit.

If we allow such things, people would start adding more things with just "why not,
it might be useful. you never know the future" and it ends up making linux fiction
novel mess.

If it's necessary, someday, someone will catch up and will send or ask patch with
detailed description "why the stat is important and how it is good for us to solve
some problem". From that, we can learn workload, way to solve the problem and git
history has the valuable description so new comers can keep the community up easily.
So, finally, overheads are justified and get merged.

Please add must-have for your goal described.