Re: [PATCH v2] mm/madvise: add vmstat statistics for madvise_[cold|pageout]

[Minchan Kim <minchan@kernel.org>]

On Wed, Jan 25, 2023 at 10:37:59PM +0100, Michal Hocko wrote:
> On Wed 25-01-23 10:07:49, Minchan Kim wrote:
> > On Wed, Jan 25, 2023 at 06:07:00PM +0100, Michal Hocko wrote:
> > > On Wed 25-01-23 08:36:02, Minchan Kim wrote:
> > > > On Wed, Jan 25, 2023 at 09:04:16AM +0100, Michal Hocko wrote:
> > > > > On Tue 24-01-23 16:54:57, Minchan Kim wrote:
> > > > > > madvise LRU manipulation APIs need to scan address ranges to find
> > > > > > present pages at page table and provides advice hints for them.
> > > > > > 
> > > > > > Likewise pg[scan/steal] count on vmstat, madvise_pg[scanned/hinted]
> > > > > > shows the proactive reclaim efficiency so this patch adds those
> > > > > > two statistics in vmstat.
> > > > > > 
> > > > > > 	madvise_pgscanned, madvise_pghinted
> > > > > > 
> > > > > > Since proactive reclaim using process_madvise(2) as userland
> > > > > > memory policy is popular(e.g,. Android ActivityManagerService),
> > > > > > those stats are helpful to know how efficiently the policy works
> > > > > > well.
> > > > > 
> > > > > The usecase description is still too vague. What are those values useful
> > > > > for? Is there anything actionable based on those numbers? How do you
> > > > > deal with multiple parties using madvise resp. process_madvise so that
> > > > > their stats are combined?
> > > > 
> > > > The metric helps monitoing system MM health under fleet and experimental
> > > > tuning with diffrent policies from the centralized userland memory daemon.
> > > 
> > > That is just too vague for me to imagine anything more specific then, we
> > > have numbers and we can show them in a report. What does it actually
> > > mean that madvise_pgscanned is high. Or that pghinted / pgscanned is
> > > low (that you tend to manually reclaim sparse mappings)?
> > 
> > If that's low, it means the userspace daemon's current tune/policy are
> > inefficient or too aggressive since it is working on address spacess
> > of processes which don't have enough memory the hint can work(e.g.,
> > shared addresses, cold address ranges or some special address ranges like
> > VM_PFNMAP) so sometime, we can detect regression to find culprit or
> > have a chance to look into better ideas to improve.
> 
> Are you sure this is really meaningful metric? Just consider a large and
> sparsely populated mapping. This can be a perfect candidate for user
> space reclaim target (e.g. consider a mapping covering a large matrix
> or other similar data structure). pghinted/pgscanned would be really
> small while the reclaim efficiency could be quite high in that case,
> wouldn't it?

Why do you think it's efficient? It need to spend quite CPU cycle to
scan a few of pages to evict. I don't see it's efficient if it happens
quite a lot.