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

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Date:	Fri, 29 Oct 2010 11:17:19 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Shaohua Li <shaohua.li@...el.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux.com>,
	David Rientjes <rientjes@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 2/2] mm: vmstat: Use a single setter function and
	callback for adjusting percpu thresholds

On Thu, Oct 28, 2010 at 03:09:04PM -0700, Andrew Morton wrote:
> On Thu, 28 Oct 2010 16:13:36 +0100
> Mel Gorman <mel@....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.
> 

Fair point, I'll know for future reference. Thanks

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

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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