[PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages

[Nishanth Aravamudan <nacc@linux.vnet.ibm.com>]

On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote:
> On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote:
> > @@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
> >  
> >         for (i = 0; i <= ZONE_NORMAL; i++) {
> >                 zone = &pgdat->node_zones[i];
> > -               if (!populated_zone(zone))
> > +               if (!populated_zone(zone) || !zone_reclaimable(zone))
> >                         continue;
> >  
> >                 pfmemalloc_reserve += min_wmark_pages(zone);
> 
> Do you really want zone_reclaimable()?  Or do you want something more
> direct like "zone_reclaimable_pages(zone) == 0"?

Yeah, I guess in my testing this worked out to be the same, since
zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will
always be false. Thanks!

Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc
reserves if node has no ZONE_NORMAL") from Mel.

We have a system with the following topology:

# numactl -H
available: 3 nodes (0,2-3)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
23 24 25 26 27 28 29 30 31
node 0 size: 28273 MB
node 0 free: 27323 MB
node 2 cpus:
node 2 size: 16384 MB
node 2 free: 0 MB
node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
node 3 size: 30533 MB
node 3 free: 13273 MB
node distances:
node   0   2   3
  0:  10  20  20
  2:  20  10  20
  3:  20  20  10

Node 2 has no free memory, because:
# cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages
1

This leads to the following zoneinfo:

Node 2, zone      DMA
  pages free     0
        min      1840
        low      2300
        high     2760
        scanned  0
        spanned  262144
        present  262144
        managed  262144
...
  all_unreclaimable: 1

If one then attempts to allocate some normal 16M hugepages via

echo 37 > /proc/sys/vm/nr_hugepages

The echo never returns and kswapd2 consumes CPU cycles.

This is because throttle_direct_reclaim ends up calling
wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...).
pfmemalloc_watermark_ok() in turn checks all zones on the node if there
are any reserves, and if so, then indicates the watermarks are ok, by
seeing if there are sufficient free pages.

675becce15 added a condition already for memoryless nodes. In this case,
though, the node has memory, it is just all consumed (and not
reclaimable). Effectively, though, the result is the same on this call
to pfmemalloc_watermark_ok() and thus seems like a reasonable additional
condition.

With this change, the afore-mentioned 16M hugepage allocation attempt
succeeds and correctly round-robins between Nodes 1 and 3.

Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>

---
v1 -> v2:
  Check against zone_reclaimable_pages, rather zone_reclaimable, based
  upon feedback from Dave Hansen.

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5e8eadd71bac..c627fa4c991f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2646,7 +2646,8 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
 
 	for (i = 0; i <= ZONE_NORMAL; i++) {
 		zone = &pgdat->node_zones[i];
-		if (!populated_zone(zone))
+		if (!populated_zone(zone) ||
+		    zone_reclaimable_pages(zone) == 0)
 			continue;
 
 		pfmemalloc_reserve += min_wmark_pages(zone);