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Subject: Re: [PATCH] percpu_counter : add percpu_counter_add_fast()
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Christoph Lameter <cl@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Nick Piggin <npiggin@kernel.dk>,
        Dave Chinner <david@fromorbit.com>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
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Date: Mon, 18 Oct 2010 17:39:40 +0200
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Le lundi 18 octobre 2010 à 10:24 -0500, Christoph Lameter a écrit :
> On Sat, 16 Oct 2010, Eric Dumazet wrote:
> 
> > I based following patch against linux-2.6, I dont know if previous
> > Christoph patch is in a git tree. I'll respin it eventually.
> 
> The prior patch was accepted by Andrew.
> 
> > + * - It is preempt safe, but not IRQ safe (on UP)
> 
> The IRQ safeness depends on the arch. this_cpu_add() in general only
> guarantees safety against preemption. It so happens that the x86
> implementation is irq safe as well.
> 
> The IRQ safety for UP is therefore not an issue if you use this_cpu_add().
> 

Nope, on UP, we dont use a per_cpu field, just a "s64 count".

struct percpu_counter {
        s64 count;
};



> If you want to guarantee irqsafeness then use irqsafe_cpu_add() instead.
> It generates the same code on x86 for SMP but takes care of the UP issues.
> 
> > +static inline void percpu_counter_add_fast(struct percpu_counter *fbc, long amount)
> > +{
> > +	this_cpu_add(*fbc->counters, amount);
> > +}
> 
> What happens in case of counter overflow?
> 

Nothing special, as I stated the usuable width of such counters would be
restricted to a long, not an s64.

It should be enough to count "number of inodes, of sockets, ..."