Re: [PATCH 12/27] mm, vmscan: Make shrink_node decisions more node-centric

[Vlastimil Babka]

On 06/09/2016 08:04 PM, Mel Gorman wrote:
Earlier patches focused on having direct reclaim and kswapd use data that
is node-centric for reclaiming but shrink_node() itself still uses too much
zone information. This patch removes unnecessary zone-based information
with the most important decision being whether to continue reclaim or
not. Some memcg APIs are adjusted as a result even though memcg itself
still uses some zone information.
Signed-off-by: Mel Gorman <mgor...@techsingularity.net>

[...]

@@ -2372,21 +2374,27 @@ static inline bool should_continue_reclaim(struct zone 
*zone,
         * inactive lists are large enough, continue reclaiming
         */
        pages_for_compaction = (2UL << sc->order);
-       inactive_lru_pages = node_page_state(zone->zone_pgdat, 
NR_INACTIVE_FILE);
+       inactive_lru_pages = node_page_state(pgdat, NR_INACTIVE_FILE);
        if (get_nr_swap_pages() > 0)
-               inactive_lru_pages += node_page_state(zone->zone_pgdat, 
NR_INACTIVE_ANON);
+               inactive_lru_pages += node_page_state(pgdat, NR_INACTIVE_ANON);
        if (sc->nr_reclaimed < pages_for_compaction &&
                        inactive_lru_pages > pages_for_compaction)
                return true;

        /* If compaction would go ahead or the allocation would succeed, stop */
-       switch (compaction_suitable(zone, sc->order, 0, 0)) {
-       case COMPACT_PARTIAL:
-       case COMPACT_CONTINUE:
-               return false;
-       default:
-               return true;
+       for (z = 0; z <= sc->reclaim_idx; z++) {
+               struct zone *zone = &pgdat->node_zones[z];
+
+               switch (compaction_suitable(zone, sc->order, 0, 0)) {

Using 0 for classzone_idx here was sort of OK when each zone was 
reclaimed separately, as a Normal allocation not passing appropriate 
classzone_idx (and thus subtracting lowmem reserve from free pages) 
means that a false COMPACT_PARTIAL (or COMPACT_CONTINUE) could be 
returned for e.g. DMA zone. It means a premature end of reclaim for this 
single zone, which is relatively small anyway, so no big deal (and we 
might avoid useless over-reclaim, when even reclaiming everything 
wouldn't get us above the lowmem_reserve).

But in node-centric reclaim, such premature "return false" from a DMA 
zone stops reclaiming the whole node. So I think we should involve the 
real classzone_idx here.

+               case COMPACT_PARTIAL:
+               case COMPACT_CONTINUE:
+                       return false;
+               default:
+                       /* check next zone */
+                       ;
+               }
        }
+       return true;
 }