percpu_counter: FBC_BATCH might be too big

For NR_CPUS >= 16 values, FBC_BATCH is 2*NR_CPUS

Considering more and more distros are using high NR_CPUS values,
it makes sense to use a more sensible value for FBC_BATCH.

A sensible value is 2*num_online_cpus(), with a minimum value of 32
(This minimum value helps branch prediction in __percpu_counter_add())

We already have a hotcpu notifier, so we can adjust FBC_BATCH dynamically.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
---
 include/linux/percpu_counter.h |    6 +-----
 lib/percpu_counter.c           |   14 +++++++++++++-
 2 files changed, 14 insertions(+), 6 deletions(-)

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On Fri, 2008-12-05 at 17:05 +0100, Eric Dumazet wrote:
> For NR_CPUS >= 16 values, FBC_BATCH is 2*NR_CPUS
> 
> Considering more and more distros are using high NR_CPUS values,
> it makes sense to use a more sensible value for FBC_BATCH.
> 
> A sensible value is 2*num_online_cpus(), with a minimum value of 32
> (This minimum value helps branch prediction in __percpu_counter_add())
> 
> We already have a hotcpu notifier, so we can adjust FBC_BATCH dynamically.

Agreed, that is one of the reasons the proportion code doesn't use the
FBC_BATCH stuff.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

> Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
> ---
>  include/linux/percpu_counter.h |    6 +-----
>  lib/percpu_counter.c           |   14 +++++++++++++-
>  2 files changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
> index 9007ccd..c42a184 100644
> --- a/include/linux/percpu_counter.h
> +++ b/include/linux/percpu_counter.h
> @@ -24,11 +24,7 @@ struct percpu_counter {
>  	s32 *counters;
>  };
>  
> -#if NR_CPUS >= 16
> -#define FBC_BATCH	(NR_CPUS*2)
> -#else
> -#define FBC_BATCH	(NR_CPUS*4)
> -#endif
> +extern int FBC_BATCH;
>  
>  int percpu_counter_init(struct percpu_counter *fbc, s64 amount);
>  int percpu_counter_init_irq(struct percpu_counter *fbc, s64 amount);
> diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
> index a866389..2fcf943 100644
> --- a/lib/percpu_counter.c
> +++ b/lib/percpu_counter.c
> @@ -114,6 +114,16 @@ void percpu_counter_destroy(struct percpu_counter *fbc)
>  }
>  EXPORT_SYMBOL(percpu_counter_destroy);
>  
> +int FBC_BATCH __read_mostly = 32;
> +EXPORT_SYMBOL(FBC_BATCH);
> +
> +static void compute_batch_value(void)
> +{
> +	int nr = num_online_cpus();
> +
> +	FBC_BATCH = max(32, nr*2);
> +}
> +
>  #ifdef CONFIG_HOTPLUG_CPU
>  static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
>  					unsigned long action, void *hcpu)
> @@ -121,6 +131,7 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
>  	unsigned int cpu;
>  	struct percpu_counter *fbc;
>  
> +	compute_batch_value();
>  	if (action != CPU_DEAD)
>  		return NOTIFY_OK;
>  
> @@ -139,11 +150,12 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
>  	mutex_unlock(&percpu_counters_lock);
>  	return NOTIFY_OK;
>  }
> +#endif
>  
>  static int __init percpu_counter_startup(void)
>  {
> +	compute_batch_value();
>  	hotcpu_notifier(percpu_counter_hotcpu_callback, 0);
>  	return 0;
>  }
>  module_init(percpu_counter_startup);
> -#endif

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From: Eric Dumazet <dada1@cosmosbay.com>
Date: Fri, 05 Dec 2008 17:05:16 +0100

> For NR_CPUS >= 16 values, FBC_BATCH is 2*NR_CPUS
> 
> Considering more and more distros are using high NR_CPUS values,
> it makes sense to use a more sensible value for FBC_BATCH.
> 
> A sensible value is 2*num_online_cpus(), with a minimum value of 32
> (This minimum value helps branch prediction in __percpu_counter_add())
> 
> We already have a hotcpu notifier, so we can adjust FBC_BATCH dynamically.
> 
> Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>

The downside is now we must load this value in these common
routines.  But I think the gain outweights the loss so:

Acked-by: David S. Miller <davem@davemloft.net>
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