Re: [RFC PATCH v2 1/2] percpu_counter: introduce atomic mode for percpu_counter

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 18 Apr 2024 22:20:07 +0800 Peng Zhang <zhangpeng362@huawei.com> wrote:

> From: ZhangPeng <zhangpeng362@huawei.com>
> 
> Depending on whether counters is NULL, we can support two modes:
> atomic mode and perpcu mode. We implement both modes by grouping
> the s64 count and atomic64_t count_atomic in a union. At the same time,
> we create the interface for adding and reading in atomic mode and for
> switching atomic mode to percpu mode.
> 

I think it would be better if we had a detailed code comment in an
appropriate place which fully describes the tradeoffs here.  Tell
people when they would benefit from using one mode versus the other.


> --- a/lib/percpu_counter.c
> +++ b/lib/percpu_counter.c
> @@ -153,7 +153,7 @@ EXPORT_SYMBOL(__percpu_counter_sum);
>  
>  int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
>  			       gfp_t gfp, u32 nr_counters,
> -			       struct lock_class_key *key)
> +			       struct lock_class_key *key, bool switch_mode)
>  {
>  	unsigned long flags __maybe_unused;
>  	size_t counter_size;
> @@ -174,7 +174,8 @@ int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
>  #ifdef CONFIG_HOTPLUG_CPU
>  		INIT_LIST_HEAD(&fbc[i].list);
>  #endif
> -		fbc[i].count = amount;
> +		if (likely(!switch_mode))
> +			fbc[i].count = amount;
>  		fbc[i].counters = (void *)counters + (i * counter_size);
>  
>  		debug_percpu_counter_activate(&fbc[i]);
> @@ -357,6 +358,32 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
>  	return good;
>  }
>  
> +/*
> + * percpu_counter_switch_to_pcpu_many: Converts struct percpu_counters from
> + * atomic mode to percpu mode.

Describe what happens if that GFP_ATOMIC allocation fails.  We remain
in atomic mode, yes?

> + */
> +int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
> +				       u32 nr_counters)
> +{
> +	static struct lock_class_key __key;
> +	unsigned long flags;
> +	bool ret = 0;
> +
> +	if (percpu_counter_initialized(fbc))
> +		return 0;
> +
> +	preempt_disable();
> +	local_irq_save(flags);

Do we need both?  Does local_irq_save() always disable preemption? 
This might not be the case for RT kernels, I always forget.

> +	if (likely(!percpu_counter_initialized(fbc)))
> +		ret = __percpu_counter_init_many(fbc, 0,
> +					GFP_ATOMIC|__GFP_NOWARN|__GFP_ZERO,
> +					nr_counters, &__key, true);
> +	local_irq_restore(flags);
> +	preempt_enable();
> +
> +	return ret;
> +}
> +

Why is there no API for switching back to atomic mode?