On Sun, 07 Dec 2008 10:25:45 +0100 Eric Dumazet <dada1@xxxxxxxxxxxxx> wrote:
> In this second version I guarded hotcpu_notifier() call by
> a #ifdef CONFIG_HOTPLUG_CPU
>
> I wonder why hotcpu_notifier() is not a null op if !CONFIG_HOTPLUG_CPU
>
> Thank you
>
> [PATCH] 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.
Yup, anything using NR_CPUS is probably wrong.
> 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;
y:/usr/src/linux-2.6.28-rc7> grep -r FBC_BATCH . | wc -l
7
Can we fix this properly please? It should now become lower case, and
it was a pretty dopey name anyway - now would be a good time to improve
it. `percpu_counter_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..e21ce7c 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 +150,14 @@ 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();
> +#ifdef CONFIG_HOTPLUG_CPU
> hotcpu_notifier(percpu_counter_hotcpu_callback, 0);
> +#endif
> return 0;
> }
> module_init(percpu_counter_startup);
> -#endif
hm, now what's going on in there? We should be able to drop the #ifdef
CONFIG_HOTPLUG_CPU from lib/percpu_counter.c altogether.
hotcpu_notifier() will do the right thing, and the compiler should
generate no code for percpu_counter_hotcpu_callback() if
CONFIG_HOTPLUG_CPU=n.
Do
$EDITOR $(grep -l hotcpu_notifier */*.c)
and you'll see lots of code gets it right, and lots of code gets it wrong.
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