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
> 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
progress.
[...]
> 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;
OK, this makes some sense, although zone_reclaimable_pages doesn't count
SLAB reclaimable pages. So we might go to sleep with a reclaimable slab
still around. This is not really easy to address because the reclaimable
slab doesn't really imply that those pages will be reclaimed...
> }
>
> @@ -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;
What, this doesn't make any sense? Did you mean if (zone_reclaimable_pages)?
> }
>
> + /* 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
>
> --
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--
Michal Hocko
SUSE Labs
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