linux-kernel - Re: [patch 6/7] cpusets: per cpuset dirty ratios

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Date:	Thu, 30 Oct 2008 02:03:14 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>, Paul Menage <menage@...gle.com>,
	Derek Fults <dfults@....com>, linux-kernel@...r.kernel.org
Subject: Re: [patch 6/7] cpusets: per cpuset dirty ratios

On Thu, 30 Oct 2008, Peter Zijlstra wrote:

> On Tue, 2008-10-28 at 09:08 -0700, David Rientjes wrote:
> 
> > +/*
> > + * Determine the dirty ratios for the currently active cpuset
> > + */
> > +void cpuset_get_current_dirty_ratios(int *background, int *throttle)
> > +{
> > +	mutex_lock(&callback_mutex);
> > +	task_lock(current);
> > +	*background = task_cs(current)->dirty_background_ratio;
> > +	*throttle = task_cs(current)->cpuset_dirty_ratio;
> > +	task_unlock(current);
> > +	mutex_unlock(&callback_mutex);
> > +
> > +	if (*background == -1)
> > +		*background = dirty_background_ratio;
> > +	if (*throttle == -1)
> > +		*throttle = vm_dirty_ratio;
> > +}
> 
> That's rather an awful lot of locking to read just two integers.
> 

As far as I know, task_lock(current) is required to dereference 
task_cs(current) and callback_mutex is required to ensure its the same 
cpuset.
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