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

From: hejianet
Date: Wed Feb 22 2017 - 21:38:05 EST

Hi Michal

On 22/02/2017 11:48 PM, Michal Hocko wrote:
On Wed 22-02-17 22:31:50, hejianet wrote:
Hi Michal

On 22/02/2017 7:41 PM, Michal Hocko wrote:
On Wed 22-02-17 17:04:48, Jia He wrote:
When I try to dynamically allocate the hugepages more than system total
free memory:
e.g. echo 4000 >/proc/sys/vm/nr_hugepages

I assume that the command has terminated with less huge pages allocated
than requested but

Yes, at last the allocated hugepages are less than 4000
HugePages_Total: 1864
HugePages_Free: 1864
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 16384 kB

In the bad case, although kswapd takes 100% cpu, the number of
HugePages_Total is not increase at all.

Node 3, zone DMA
pages free 2951
min 2821
low 3526
high 4231

it left the zone below high watermark with

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

no pages reclaimable, so kswapd will not go to sleep. It would be quite
easy and comfortable to call it a misconfiguration but it seems that
it might be quite easy to hit with NUMA machines which have large
differences in the node sizes. I guess it makes sense to back off
the kswapd rather than burning CPU without any way to make forward


please make sure that this information is in the changelog

@@ -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))
@@ -3522,8 +3524,15 @@ void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)

if (zone_balanced(zone, order, classzone_idx))
+ if (!zone_reclaimable_pages(zone))
+ node_has_relaimable_pages = 1;

What, this doesn't make any sense? Did you mean if (zone_reclaimable_pages)?

I mean, if any one zone has reclaimable pages, then this zone's *node* has
reclaimable pages. Thus, the kswapN for this node should be waken up.
e.g. node 1 has 2 zones.
zone A has no reclaimable pages but zone B has.
Thus node 1 has reclaimable pages, and kswapd1 will be waken up.
I use node_has_relaimable_pages in the loop to check all the zones' reclaimable
pages number. So I prefer the name node_has_relaimable_pages instead of

I still do not understand. This code starts with
node_has_relaimable_pages = 0. If you see a zone with no reclaimable
pages then you make it node_has_relaimable_pages = 1 which means that

+ /* Dont wake kswapd if no reclaimable pages */
+ if (!node_has_relaimable_pages)
+ return;

this will not hold and we will wake up the kswapd. I believe what
you want instead, is to skip the wake up if _all_ zones have
!zone_reclaimable_pages() Or I am missing your point. This means that
you want
if (zone_reclaimable_pages(zone)
node_has_relaimable_pages = 1;
You are right, will send v2 soon after testing it

trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);