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

[Michal Hocko]

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