Re: [PATCH 04/31] mm, vmscan: begin reclaiming pages on a per-node basis

From: Vlastimil Babka
Date: Thu Jul 14 2016 - 03:49:00 EST

On 07/14/2016 08:28 AM, Joonsoo Kim wrote:
On Fri, Jul 08, 2016 at 11:05:32AM +0100, Mel Gorman wrote:
On Fri, Jul 08, 2016 at 11:28:52AM +0900, Joonsoo Kim wrote:
On Thu, Jul 07, 2016 at 10:48:08AM +0100, Mel Gorman wrote:
On Thu, Jul 07, 2016 at 10:12:12AM +0900, Joonsoo Kim wrote:
@@ -1402,6 +1406,11 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,

VM_BUG_ON_PAGE(!PageLRU(page), page);

+ if (page_zonenum(page) > sc->reclaim_idx) {
+ list_move(&page->lru, &pages_skipped);
+ continue;
+ }

I think that we don't need to skip LRU pages in active list. What we'd
like to do is just skipping actual reclaim since it doesn't make
freepage that we need. It's unrelated to skip the page in active list.


The active aging is sometimes about simply aging the LRU list. Aging the
active list based on the timing of when a zone-constrained allocation arrives
potentially introduces the same zone-balancing problems we currently have
and applying them to node-lru.

Could you explain more? I don't understand why aging the active list
based on the timing of when a zone-constrained allocation arrives
introduces the zone-balancing problem again.

I mispoke. Avoid rotation of the active list based on the timing of a
zone-constrained allocation is what I think potentially introduces problems.
If there are zone-constrained allocations aging the active list then I worry
that pages would be artificially preserved on the active list. No matter
what we do, there is distortion of the aging for zone-constrained allocation
because right now, it may deactivate high zone pages sooner than expected.

I think that if above logic is applied to both the active/inactive
list, it could cause zone-balancing problem. LRU pages on lower zone
can be resident on memory with more chance.

If anything, with node-based LRU, it's high zone pages that can be resident
on memory for longer but only if there are zone-constrained allocations.
If we always reclaim based on age regardless of allocation requirements
then there is a risk that high zones are reclaimed far earlier than expected.

Basically, whether we skip pages in the active list or not there are
distortions with page aging and the impact is workload dependent. Right now,
I see no clear advantage to special casing active aging.

If we suspect this is a problem in the future, it would be a simple matter
of adding an additional bool parameter to isolate_lru_pages.

Okay. I agree that it would be a simple matter.

And, I have a concern that if inactive LRU is full with higher zone's
LRU pages, reclaim with low reclaim_idx could be stuck.

That is an outside possibility but unlikely given that it would require
that all outstanding allocation requests are zone-contrained. If it happens

I'm not sure that it is outside possibility. It can also happens if there
is zone-contrained allocation requestor and parallel memory hogger. In
this case, memory would be reclaimed by memory hogger but memory hogger would
consume them again so inactive LRU is continually full with higher
zone's LRU pages and zone-contrained allocation requestor cannot

The same memory hogger will also be reclaiming the highmem pages and
reallocating highmem pages.

It would be preferred to have an actual test case for this so the
altered ratio can be tested instead of introducing code that may be
useless or dead.

Yes, actual test case would be preferred. I will try to implement
an artificial test case by myself but I'm not sure when I can do it.

That would be appreciated.

I make an artificial test case and test this series by using next tree
(next-20160713) and found a regression.


active_anon:18779 inactive_anon:18 isolated_anon:0
active_file:91577 inactive_file:320615 isolated_file:0
unevictable:0 dirty:0 writeback:0 unstable:0
slab_reclaimable:6741 slab_unreclaimable:18124
mapped:389774 shmem:95 pagetables:18332 bounce:0
free:8194 free_pcp:140 free_cma:0
Node 0 active_anon:75116kB inactive_anon:72kB active_file:366308kB inactive_file:1282460kB unevictable:0kB isolated(anon):0kB isolated(file):0kB mapped:1559096kB dirty:0kB writeback:0kB shmem:0kB shmem_thp: 0kB shmem_pmdmapped: 0kB anon_thp: 380kB writeback_tmp:0kB unstable:0kB all_unreclaimable? yes
Node 0 DMA free:2172kB min:204kB low:252kB high:300kB present:15992kB managed:15908kB mlocked:0kB slab_reclaimable:0kB slab_unreclaimable:2380kB kernel_stack:1632kB pagetables:3632kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB node_pages_scanned:13673372
lowmem_reserve[]: 0 493 493 1955
Node 0 DMA32 free:6444kB min:6492kB low:8112kB high:9732kB present:2080632kB managed:508600kB mlocked:0kB slab_reclaimable:26964kB slab_unreclaimable:70116kB kernel_stack:30496kB pagetables:69696kB bounce:0kB free_pcp:316kB local_pcp:100kB free_cma:0kB node_pages_scanned:13673372
lowmem_reserve[]: 0 0 0 1462
Node 0 Normal free:0kB min:0kB low:0kB high:0kB present:18446744073708015752kB managed:0kB mlocked:0kB slab_reclaimable:0kB slab_unreclaimable:0kB kernel_stack:0kB pagetables:0kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB node_pages_scanned:13673832


Although unlikely related to your report, that itself doesn't look right. Any idea if that's due to your configuration and would be printed also in the mainline kernel in case of OOM (or if /proc/zoneinfo has similarly bogus value), or is something caused by a patch in mmotm?