linux-kernel - Re: [RFC][PATCH] lib: generic percpu counter array

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Message-Id: <20091106094903.5ede138d.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 6 Nov 2009 09:49:03 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"hugh.dickins@...cali.co.uk" <hugh.dickins@...cali.co.uk>,
	akpm@...ux-foundation.org,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>
Subject: Re: [RFC][PATCH] lib: generic percpu counter array


Thank you for review.

On Thu, 5 Nov 2009 10:15:36 -0500 (EST)
Christoph Lameter <cl@...ux-foundation.org> wrote:

> On Thu, 5 Nov 2009, KAMEZAWA Hiroyuki wrote:
> 
> > +static inline void
> > +counter_array_add(struct counter_array *ca, int idx, int val)
> > +{
> > +	ca->counters[idx] += val;
> > +}
> 
> This is not a per cpu operation and therefore expensive. The new percpu
> this_cpu_inc f.e. generates a single x86 instruction for an increment.
> 
This code is for !SMP.


> > +void __counter_array_add(struct counter_array *ca, int idx, int val, int batch)
> > +{
> > +	long count, *pcount;
> > +
> > +	preempt_disable();
> > +
> > +	pcount = this_cpu_ptr(ca->v.array);
> > +	count = pcount[idx] + val;
> > +	if (!ca->v.nosync && ((count > batch) || (count < -batch))) {
> > +		atomic_long_add(count, &ca->counters[idx]);
> > +		pcount[idx] = 0;
> > +	} else
> > +		pcount[idx] = count;
> > +	preempt_enable();
> > +}
> 
> Too expensive to use in critical VM paths. The percpu operations generate
> a single instruction instead of the code above. No need for preempt etc.
> 
Hmm, ok. I'll have to see your patch, more.
I wonder how to use indexed-array and ops like add_return..


Thanks,
-Kame



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