Re: [PATCH 2/6] mm: kswapd: Keep kswapd awake for high-order allocations until a percentage of the node is balanced

[Minchan Kim <minchan.kim@gmail.com>]

On Thu, Dec 09, 2010 at 11:18:16AM +0000, Mel Gorman wrote:
> When reclaiming for high-orders, kswapd is responsible for balancing a
> node but it should not reclaim excessively. It avoids excessive reclaim by
> considering if any zone in a node is balanced then the node is balanced. In
> the cases where there are imbalanced zone sizes (e.g. ZONE_DMA with both
> ZONE_DMA32 and ZONE_NORMAL), kswapd can go to sleep prematurely as just
> one small zone was balanced.
> 
> This alters the sleep logic of kswapd slightly. It counts the number of pages
> that make up the balanced zones. If the total number of balanced pages is
> more than a quarter of the zone, kswapd will go back to sleep. This should
> keep a node balanced without reclaiming an excessive number of pages.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

Just below trivials.

> ---
>  mm/vmscan.c |   43 ++++++++++++++++++++++++++++++++++---------
>  1 files changed, 34 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 25cb373..b4472a1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2117,10 +2117,26 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
>  }
>  #endif
>  
> +/*
> + * pgdat_balanced is used when checking if a node is balanced for high-order
> + * allocations. Only zones that meet watermarks make up "balanced".
> + * The total of balanced pages must be at least 25% of the node for the

Could you write down why you select 25% in description?
It could help someone who try to change that watermark in future.

> + * node to be considered balanced. Forcing all zones to be balanced for high
> + * orders can cause excessive reclaim when there are imbalanced zones.
> + * Similarly, we do not want kswapd to go to sleep because ZONE_DMA happens
> + * to be balanced when ZONE_DMA32 is huge in comparison and unbalanced
> + */
> +static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced)

I think balanced_pages is more clear.
But it's a preference.

> +{
> +	return balanced > pgdat->node_present_pages / 4;
> +}
> +
>  /* is kswapd sleeping prematurely? */
>  static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
>  {
>  	int i;
> +	unsigned long balanced = 0;
> +	bool all_zones_ok = true;
>  
>  	/* If a direct reclaimer woke kswapd within HZ/10, it's premature */
>  	if (remaining)
> @@ -2138,10 +2154,19 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
>  
>  		if (!zone_watermark_ok(zone, order, high_wmark_pages(zone),
>  								0, 0))
> -			return 1;
> +			all_zones_ok = false;
> +		else
> +			balanced += zone->present_pages;
>  	}
>  
> -	return 0;
> +	/*
> +	 * For high-order requests, any zone meeting the watermark allows
> +	 * kswapd to sleep. For order-0, all zones must be balanced

The comment isn't exact.
could allow?

> +	 */
> +	if (order)
> +		return pgdat_balanced(pgdat, balanced);
> +	else
> +		return !all_zones_ok;
>  }
>  
>  /*
> @@ -2169,7 +2194,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  							int classzone_idx)
>  {
>  	int all_zones_ok;
> -	int any_zone_ok;
> +	unsigned long balanced;
>  	int priority;
>  	int i;
>  	int end_zone = 0;	/* Inclusive.  0 = ZONE_DMA */
> @@ -2203,7 +2228,7 @@ loop_again:
>  			disable_swap_token();
>  
>  		all_zones_ok = 1;
> -		any_zone_ok = 0;
> +		balanced = 0;
>  
>  		/*
>  		 * Scan in the highmem->dma direction for the highest
> @@ -2314,11 +2339,11 @@ loop_again:
>  				 */
>  				zone_clear_flag(zone, ZONE_CONGESTED);
>  				if (i <= classzone_idx)
> -					any_zone_ok = 1;
> +					balanced += zone->present_pages;
>  			}
>  
>  		}
> -		if (all_zones_ok || (order && any_zone_ok))
> +		if (all_zones_ok || (order && pgdat_balanced(pgdat, balanced)))
>  			break;		/* kswapd: all done */
>  		/*
>  		 * OK, kswapd is getting into trouble.  Take a nap, then take
> @@ -2344,10 +2369,10 @@ out:
>  
>  	/*
>  	 * order-0: All zones must meet high watermark for a balanced node
> -	 * high-order: Any zone below pgdats classzone_idx must meet the high
> -	 *             watermark for a balanced node
> +	 * high-order: Balanced zones must make up at least 25% of the node
> +	 *             for the node to be balanced
>  	 */
> -	if (!(all_zones_ok || (order && any_zone_ok))) {
> +	if (!(all_zones_ok || (order && pgdat_balanced(pgdat, balanced)))) {
>  		cond_resched();
>  
>  		try_to_freeze();
> -- 
> 1.7.1
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

-- 
Kind regards,
Minchan Kim

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>