Re: [v3 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout

[Yang Shi]

On 5/22/19 9:23 AM, Huang, Ying wrote:
> Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> writes:
>
> > Since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after
> > swapped out"), THP can be swapped out in a whole.  But, nr_reclaimed
> > and some other vm counters still get inc'ed by one even though a whole
> > THP (512 pages) gets swapped out.
> >
> > This doesn't make too much sense to memory reclaim.  For example, direct
> > reclaim may just need reclaim SWAP_CLUSTER_MAX pages, reclaiming one THP
> > could fulfill it.  But, if nr_reclaimed is not increased correctly,
> > direct reclaim may just waste time to reclaim more pages,
> > SWAP_CLUSTER_MAX * 512 pages in worst case.
> >
> > And, it may cause pgsteal_{kswapd|direct} is greater than
> > pgscan_{kswapd|direct}, like the below:
> >
> > pgsteal_kswapd 122933
> > pgsteal_direct 26600225
> > pgscan_kswapd 174153
> > pgscan_direct 14678312
> >
> > nr_reclaimed and nr_scanned must be fixed in parallel otherwise it would
> > break some page reclaim logic, e.g.
> >
> > vmpressure: this looks at the scanned/reclaimed ratio so it won't
> > change semantics as long as scanned & reclaimed are fixed in parallel.
> >
> > compaction/reclaim: compaction wants a certain number of physical pages
> > freed up before going back to compacting.
> >
> > kswapd priority raising: kswapd raises priority if we scan fewer pages
> > than the reclaim target (which itself is obviously expressed in order-0
> > pages). As a result, kswapd can falsely raise its aggressiveness even
> > when it's making great progress.
> >
> > Other than nr_scanned and nr_reclaimed, some other counters, e.g.
> > pgactivate, nr_skipped, nr_ref_keep and nr_unmap_fail need to be fixed
> > too since they are user visible via cgroup, /proc/vmstat or trace
> > points, otherwise they would be underreported.
> >
> > When isolating pages from LRUs, nr_taken has been accounted in base
> > page, but nr_scanned and nr_skipped are still accounted in THP.  It
> > doesn't make too much sense too since this may cause trace point
> > underreport the numbers as well.
> >
> > So accounting those counters in base page instead of accounting THP as
> > one page.
> >
> > This change may result in lower steal/scan ratio in some cases since
> > THP may get split during page reclaim, then a part of tail pages get
> > reclaimed instead of the whole 512 pages, but nr_scanned is accounted
> > by 512, particularly for direct reclaim.  But, this should be not a
> > significant issue.
> >
> > Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> > Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > ---
> > v3: Removed Shakeel's Reviewed-by since the patch has been changed significantly
> >      Switched back to use compound_order per Matthew
> >      Fixed more counters per Johannes
> > v2: Added Shakeel's Reviewed-by
> >      Use hpage_nr_pages instead of compound_order per Huang Ying and William Kucharski
> >
> >   mm/vmscan.c | 40 ++++++++++++++++++++++++++++------------
> >   1 file changed, 28 insertions(+), 12 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index b65bc50..1044834 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1250,7 +1250,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >   		case PAGEREF_ACTIVATE:
> >   			goto activate_locked;
> >   		case PAGEREF_KEEP:
> > -			stat->nr_ref_keep++;
> > +			stat->nr_ref_keep += (1 << compound_order(page));
> >   			goto keep_locked;
> >   		case PAGEREF_RECLAIM:
> >   		case PAGEREF_RECLAIM_CLEAN:
> > @@ -1294,6 +1294,17 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >   						goto activate_locked;
> >   				}
> >   
> > +				/*
> > +				 * Account all tail pages when THP is added
> > +				 * into swap cache successfully.
> > +				 * The head page has been accounted at the
> > +				 * first place.
> > +				 */
> > +				if (PageTransHuge(page))
> > +					sc->nr_scanned +=
> > +						((1 << compound_order(page)) -
> > +							1);
> > +
> The "if" here could be changed to "else if" because if add_to_swap()
> fails we don't need to call PageTransHuge() here.  But this isn't a big
> deal.

This could be moved to the beginning according to Johannes.

> You have analyzed the code and found that nr_dirty, nr_unqueued_dirty,
> nr_congested and nr_writeback are file cache related and not impacted by
> THP swap out.  How about add your findings in the patch description?

Yes, sure. Will add in v4.

> Best Regards,
> Huang, Ying