linux-kernel - Re: [PATCH 04/10] lib: percpu_counter

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Message-Id: <20070421025517.d9f9bc14.akpm@linux-foundation.org>
Date:	Sat, 21 Apr 2007 02:55:17 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	miklos@...redi.hu, neilb@...e.de, dgc@....com,
	tomoki.sekiyama.qu@...achi.com, nikita@...sterfs.com,
	trond.myklebust@....uio.no, yingchao.zhou@...il.com
Subject: Re: [PATCH 04/10] lib: percpu_counter_mod64

On Fri, 20 Apr 2007 17:51:58 +0200 Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> Add percpu_counter_mod64() to allow large modifications.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
>  include/linux/percpu_counter.h |    9 +++++++++
>  lib/percpu_counter.c           |   28 ++++++++++++++++++++++++++++
>  2 files changed, 37 insertions(+)
> 
> Index: linux-2.6/include/linux/percpu_counter.h
> ===================================================================
> --- linux-2.6.orig/include/linux/percpu_counter.h	2007-04-12 13:54:55.000000000 +0200
> +++ linux-2.6/include/linux/percpu_counter.h	2007-04-12 14:00:21.000000000 +0200
> @@ -36,6 +36,7 @@ static inline void percpu_counter_destro
>  }
>  
>  void percpu_counter_mod(struct percpu_counter *fbc, s32 amount);
> +void percpu_counter_mod64(struct percpu_counter *fbc, s64 amount);
>  s64 percpu_counter_sum(struct percpu_counter *fbc);
>  
>  static inline s64 percpu_counter_read(struct percpu_counter *fbc)
> @@ -81,6 +82,14 @@ percpu_counter_mod(struct percpu_counter
>  	preempt_enable();
>  }
>  
> +static inline void
> +percpu_counter_mod64(struct percpu_counter *fbc, s64 amount)
> +{
> +	preempt_disable();
> +	fbc->count += amount;
> +	preempt_enable();
> +}
> +
>  static inline s64 percpu_counter_read(struct percpu_counter *fbc)
>  {
>  	return fbc->count;
> Index: linux-2.6/lib/percpu_counter.c
> ===================================================================
> --- linux-2.6.orig/lib/percpu_counter.c	2006-07-31 13:07:38.000000000 +0200
> +++ linux-2.6/lib/percpu_counter.c	2007-04-12 14:17:12.000000000 +0200
> @@ -25,6 +25,34 @@ void percpu_counter_mod(struct percpu_co
>  }
>  EXPORT_SYMBOL(percpu_counter_mod);
>  
> +void percpu_counter_mod64(struct percpu_counter *fbc, s64 amount)
> +{
> +	long count;
> +	s32 *pcount;
> +	int cpu;
> +
> +	if (amount >= FBC_BATCH || amount <= -FBC_BATCH) {
> +		spin_lock(&fbc->lock);
> +		fbc->count += amount;
> +		spin_unlock(&fbc->lock);
> +		return;
> +	}

This is wrong, a little.

If the counter was at -FBC_BATCH/2 and the caller passed in FBC_BATCH, we
could just set the cpu-local counter to FBC_BATCH/2 instead of going for
the lock.

Probably doesn't matter though.

> +	cpu = get_cpu();
> +	pcount = per_cpu_ptr(fbc->counters, cpu);
> +	count = *pcount + amount;
> +	if (count >= FBC_BATCH || count <= -FBC_BATCH) {
> +		spin_lock(&fbc->lock);
> +		fbc->count += count;
> +		*pcount = 0;
> +		spin_unlock(&fbc->lock);
> +	} else {
> +		*pcount = count;
> +	}
> +	put_cpu();
> +}
> +EXPORT_SYMBOL(percpu_counter_mod64);

Bloaty.  Surely we won't be needing this on 32-bit kernels?  Even monster
PAE has only 64,000,000 pages and won't be using deltas of more than 4
gigapages?

<Does even 64-bit need to handle 4 gigapages in a single hit?  /me suspects
another changelog bug>
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