[RFC PATCH] mm/vmscan: fix high cpu usage of kswapd if there

[Jia He <hejianet@gmail.com>]

When I try to dynamically allocate the hugepages more than system total
free memory:
e.g. echo 4000 >/proc/sys/vm/nr_hugepages

Then the kswapd will take 100% cpu for a long time(more than 3 hours, and
will not be about to end)
top result:
top - 13:42:59 up  3:37,  1 user,  load average: 1.09, 1.03, 1.01
Tasks:   1 total,   1 running,   0 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.0 us, 12.5 sy,  0.0 ni, 85.5 id,  2.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:  31371520 total, 30915136 used,   456384 free,      320 buffers
KiB Swap:  6284224 total,   115712 used,  6168512 free.    48192 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND    
   76 root      20   0       0      0      0 R 100.0 0.000 217:17.29 kswapd3 

The root cause is kswapd3 is trying to do relaim again and again but it 
makes no progress
# numactl -H
available: 3 nodes (0,2-3)
node 0 cpus:
node 0 size: 0 MB
node 0 free: 0 MB
node 2 cpus: 0 1 2 3 4 5 6 7
node 2 size: 15299 MB
node 2 free: 289 MB
node 3 cpus:
node 3 size: 15336 MB
node 3 free: 184 MB        <--- kswapd works
node distances:
node   0   2   3 
  0:  10  40  40 
  2:  40  10  20 
  3:  40  20  10 
At that time, there are no relaimable pages in that node:
Node 3, zone      DMA
  per-node stats
      nr_inactive_anon 0
      nr_active_anon 0
      nr_inactive_file 0
      nr_active_file 0
      nr_unevictable 0
      nr_isolated_anon 0
      nr_isolated_file 0
      nr_pages_scanned 0
      workingset_refault 0
      workingset_activate 0
      workingset_nodereclaim 0
      nr_anon_pages 0
      nr_mapped    0
      nr_file_pages 0
      nr_dirty     0
      nr_writeback 0
      nr_writeback_temp 0
      nr_shmem     0
      nr_shmem_hugepages 0
      nr_shmem_pmdmapped 0
      nr_anon_transparent_hugepages 0
      nr_unstable  0
      nr_vmscan_write 0
      nr_vmscan_immediate_reclaim 0
      nr_dirtied   0
      nr_written   0
  pages free     2951
        min      2821
        low      3526
        high     4231
   node_scanned  0
        spanned  245760
        present  245760
        managed  245388
      nr_free_pages 2951
      nr_zone_inactive_anon 0
      nr_zone_active_anon 0
      nr_zone_inactive_file 0
      nr_zone_active_file 0
      nr_zone_unevictable 0
      nr_zone_write_pending 0
      nr_mlock     0
      nr_slab_reclaimable 46
      nr_slab_unreclaimable 90
      nr_page_table_pages 0
      nr_kernel_stack 0
      nr_bounce    0
      nr_zspages   0
      numa_hit     2257
      numa_miss    0
      numa_foreign 0
      numa_interleave 982
      numa_local   0
      numa_other   2257
      nr_free_cma  0
        protection: (0, 0, 0, 0) 
  
This patch resolves the issue from 2 aspects:
1. In prepare_kswapd_sleep, only when zone is not balanced and there is
  reclaimable pages in this zone, kswapd will go to do relaim without sleeping
2. Don't wake up kswapd if there are no reclaimable pages in that node

After this patch:
top - 07:13:40 up 28 min,  1 user,  load average: 0.00, 0.00, 0.00
Tasks:   1 total,   0 running,   1 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.0 us,  0.0 sy,  0.0 ni, 99.9 id,  0.1 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:  31371520 total, 30908096 used,   463424 free,      384 buffers
KiB Swap:  6284224 total,    77504 used,  6206720 free.   131328 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND    
   77 root      20   0       0      0      0 S 0.000 0.000   0:00.00 kswapd3

Signed-off-by: Jia He <hejianet@gmail.com>
---
 mm/vmscan.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 532a2a7..a05e3ab 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3139,7 +3139,8 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, int classzone_idx)
 		if (!managed_zone(zone))
 			continue;
 
-		if (!zone_balanced(zone, order, classzone_idx))
+		if (!zone_balanced(zone, order, classzone_idx)
+			&& zone_reclaimable_pages(zone))
 			return false;
 	}
 
@@ -3502,6 +3503,7 @@ void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
 {
 	pg_data_t *pgdat;
 	int z;
+	int node_has_relaimable_pages = 0;
 
 	if (!managed_zone(zone))
 		return;
@@ -3522,8 +3524,15 @@ void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
 
 		if (zone_balanced(zone, order, classzone_idx))
 			return;
+
+		if (!zone_reclaimable_pages(zone))
+			node_has_relaimable_pages = 1;
 	}
 
+	/* Dont wake kswapd if no reclaimable pages */
+	if (!node_has_relaimable_pages)
+		return;
+
 	trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);
 	wake_up_interruptible(&pgdat->kswapd_wait);
 }
-- 
1.8.5.6