Re: [PATCH 07/25] Check in advance if the zonelist needsadditional filtering

[Pekka Enberg]

Hi Mel,

On Mon, 2009-04-20 at 23:19 +0100, Mel Gorman wrote:
> Zonelist are filtered based on nodemasks for memory policies normally.
> It can be additionally filters on cpusets if they exist as well as
> noting when zones are full. These simple checks are expensive enough to
> be noticed in profiles. This patch checks in advance if zonelist
> filtering will ever be needed. If not, then the bulk of the checks are
> skipped.
> 
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> @@ -1401,6 +1405,7 @@ get_page_from_freelist(gfp_t gfp_mask, nodemask_t *nodemask, unsigned int order,
>  	nodemask_t *allowednodes = NULL;/* zonelist_cache approximation */
>  	int zlc_active = 0;		/* set if using zonelist_cache */
>  	int did_zlc_setup = 0;		/* just call zlc_setup() one time */
> +	int zonelist_filter = 0;
>  
>  	(void)first_zones_zonelist(zonelist, high_zoneidx, nodemask,
>  							&preferred_zone);
> @@ -1411,6 +1416,10 @@ get_page_from_freelist(gfp_t gfp_mask, nodemask_t *nodemask, unsigned int order,
>  
>  	VM_BUG_ON(order >= MAX_ORDER);
>  
> +	/* Determine in advance if the zonelist needs filtering */
> +	if ((alloc_flags & ALLOC_CPUSET) && unlikely(number_of_cpusets > 1))
> +		zonelist_filter = 1;
> +
>  zonelist_scan:
>  	/*
>  	 * Scan zonelist, looking for a zone with enough free.
> @@ -1418,12 +1427,16 @@ zonelist_scan:
>  	 */
>  	for_each_zone_zonelist_nodemask(zone, z, zonelist,
>  						high_zoneidx, nodemask) {
> -		if (NUMA_BUILD && zlc_active &&
> -			!zlc_zone_worth_trying(zonelist, z, allowednodes))
> -				continue;
> -		if ((alloc_flags & ALLOC_CPUSET) &&
> -			!cpuset_zone_allowed_softwall(zone, gfp_mask))
> -				goto try_next_zone;
> +
> +		/* Ignore the additional zonelist filter checks if possible */
> +		if (zonelist_filter) {
> +			if (NUMA_BUILD && zlc_active &&
> +				!zlc_zone_worth_trying(zonelist, z, allowednodes))
> +					continue;
> +			if ((alloc_flags & ALLOC_CPUSET) &&

The above expression is always true here because of the earlier
zonelists_filter check, no?

> +				!cpuset_zone_allowed_softwall(zone, gfp_mask))
> +					goto try_next_zone;
> +		}
>  
>  		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
>  			unsigned long mark;


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