Re: [rfc 2/3] mm: vmscan: treat inactive cycling as neutral

[Johannes Weiner]

On Wed, Nov 02, 2011 at 11:04:30AM -0700, KOSAKI Motohiro wrote:
> (11/2/2011 9:32 AM), Johannes Weiner wrote:
> > Each page that is scanned but put back to the inactive list is counted
> > as a successful reclaim, which tips the balance between file and anon
> > lists more towards the cycling list.
> > 
> > This does - in my opinion - not make too much sense, but at the same
> > time it was not much of a problem, as the conditions that lead to an
> > inactive list cycle were mostly temporary - locked page, concurrent
> > page table changes, backing device congested - or at least limited to
> > a single reclaimer that was not allowed to unmap or meddle with IO.
> > More important than being moderately rare, those conditions should
> > apply to both anon and mapped file pages equally and balance out in
> > the end.
> > 
> > Recently, we started cycling file pages in particular on the inactive
> > list much more aggressively, for used-once detection of mapped pages,
> > and when avoiding writeback from direct reclaim.
> > 
> > Those rotated pages do not exactly speak for the reclaimability of the
> > list they sit on and we risk putting immense pressure on file list for
> > no good reason.
> > 
> > Instead, count each page not reclaimed and put back to any list,
> > active or inactive, as rotated, so they are neutral with respect to
> > the scan/rotate ratio of the list class, as they should be.
> > 
> > Signed-off-by: Johannes Weiner <jweiner@xxxxxxxxxx>
> > ---
> >  mm/vmscan.c |    9 ++++-----
> >  1 files changed, 4 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 39d3da3..6da66a7 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1360,7 +1360,9 @@ putback_lru_pages(struct zone *zone, struct scan_control *sc,
> >  	 */
> >  	spin_lock(&zone->lru_lock);
> >  	while (!list_empty(page_list)) {
> > +		int file;
> >  		int lru;
> > +
> >  		page = lru_to_page(page_list);
> >  		VM_BUG_ON(PageLRU(page));
> >  		list_del(&page->lru);
> > @@ -1373,11 +1375,8 @@ putback_lru_pages(struct zone *zone, struct scan_control *sc,
> >  		SetPageLRU(page);
> >  		lru = page_lru(page);
> >  		add_page_to_lru_list(zone, page, lru);
> > -		if (is_active_lru(lru)) {
> > -			int file = is_file_lru(lru);
> > -			int numpages = hpage_nr_pages(page);
> > -			reclaim_stat->recent_rotated[file] += numpages;
> > -		}
> > +		file = is_file_lru(lru);
> > +		reclaim_stat->recent_rotated[file] += hpage_nr_pages(page);
> >  		if (!pagevec_add(&pvec, page)) {
> >  			spin_unlock_irq(&zone->lru_lock);
> >  			__pagevec_release(&pvec);
> 
> When avoiding writeback from direct reclaim case, I think we shouldn't increase
> recent_rotated because VM decided "the page should be eviceted, but also it
> should be delayed". i'm not sure it's minor factor or not.

But we DO increase recent_scanned another time when the page is
reclaimed on the next round.

If we don't increase recent_rotated for deferred reclaims, they are
counted as success twice and so considered more valuable than
immediate reclaims.  I don't think that makes sense.
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