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

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Message-ID: <alpine.DEB.2.00.0810300255290.16350@chino.kir.corp.google.com>
Date:	Thu, 30 Oct 2008 03:02:45 -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:

> > > > +/*
> > > > + * 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.
> 
> Since we read these things for every evaluation, getting it wrong isn't
> too harmful.
> 
> So I would suggest just enough locking to ensure we don't reference any
> NULL pointers and such.
> 
> IIRC the cpuset stuff is RCU freed, so some racy read should be
> possible, no?
> 

Ah, that sounds reasonable.  We'll no longer require callback_mutex if we 
accept races when current attaches to another cpuset here.  We'll need 
rcu_read_lock() to safely dereference task_cs(current) unless it's 
top_cpuset, but that's much better than callback_mutex and spinning on 
task_lock(current).

Thanks!
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