Re: [PATCH 2/2] mm: vmstat: Use a single setter function and callback for adjusting percpu thresholds

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 28 Oct 2010 16:13:36 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> reduce_pgdat_percpu_threshold() and restore_pgdat_percpu_threshold()
> exist to adjust the per-cpu vmstat thresholds while kswapd is awake to
> avoid errors due to counter drift. The functions duplicate some code so
> this patch replaces them with a single set_pgdat_percpu_threshold() that
> takes a callback function to calculate the desired threshold as a
> parameter.

hm.  Could have passed in some silly flag rather than a function
pointer but whatever.

>
> ...
>
> -void reduce_pgdat_percpu_threshold(pg_data_t *pgdat)
> +void set_pgdat_percpu_threshold(pg_data_t *pgdat,
> +				int (*calculate_pressure)(struct zone *))
>  {
>  	struct zone *zone;
>  	int cpu;
> @@ -196,28 +197,7 @@ void reduce_pgdat_percpu_threshold(pg_data_t *pgdat)
>  		if (!zone->percpu_drift_mark)
>  			continue;
>  
> -		threshold = calculate_pressure_threshold(zone);
> -		for_each_online_cpu(cpu)
> -			per_cpu_ptr(zone->pageset, cpu)->stat_threshold
> -							= threshold;
> -	}
> -	put_online_cpus();
> -}
> -
> -void restore_pgdat_percpu_threshold(pg_data_t *pgdat)
> -{
> -	struct zone *zone;
> -	int cpu;
> -	int threshold;
> -	int i;
> -
> -	get_online_cpus();
> -	for (i = 0; i < pgdat->nr_zones; i++) {
> -		zone = &pgdat->node_zones[i];
> -		if (!zone->percpu_drift_mark)
> -			continue;
> -
> -		threshold = calculate_threshold(zone);
> +		threshold = calculate_pressure(zone);

Readability nit: it's better to use the

		threshold = (*calculate_pressure)(zone);

syntax here.  So the code reader doesn't go running around trying to
find the function "calculate_pressure".  I've been fooled that way
plenty of times.


>  		for_each_online_cpu(cpu)
>  			per_cpu_ptr(zone->pageset, cpu)->stat_threshold
>  							= threshold;