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

[Dennis Zhou]

On Thu, Apr 18, 2024 at 10:20:07PM +0800, Peng Zhang wrote:
> From: ZhangPeng <zhangpeng362@xxxxxxxxxx>
> 
> 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.
> 
> Suggested-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: ZhangPeng <zhangpeng362@xxxxxxxxxx>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> ---
>  include/linux/percpu_counter.h | 43 +++++++++++++++++++++++++++++++---
>  lib/percpu_counter.c           | 31 ++++++++++++++++++++++--
>  2 files changed, 69 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
> index 3a44dd1e33d2..160f9734c0bb 100644
> --- a/include/linux/percpu_counter.h
> +++ b/include/linux/percpu_counter.h
> @@ -21,7 +21,13 @@
>  
>  struct percpu_counter {
>  	raw_spinlock_t lock;
> -	s64 count;
> +	/* Depending on whether counters is NULL, we can support two modes,
> +	 * atomic mode using count_atomic and perpcu mode using count.
> +	 */
> +	union {
> +		s64 count;
> +		atomic64_t count_atomic;
> +	};
>  #ifdef CONFIG_HOTPLUG_CPU
>  	struct list_head list;	/* All percpu_counters are on a list */
>  #endif
> @@ -32,14 +38,14 @@ extern int percpu_counter_batch;
>  
>  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);

Nit: the lock_class_key at the end.
>  
>  #define percpu_counter_init_many(fbc, value, gfp, nr_counters)		\
>  	({								\
>  		static struct lock_class_key __key;			\
>  									\
>  		__percpu_counter_init_many(fbc, value, gfp, nr_counters,\
> -					   &__key);			\
> +					   &__key, false);		\
>  	})
>  
>  
> @@ -130,6 +136,20 @@ static inline bool percpu_counter_initialized(struct percpu_counter *fbc)
>  	return (fbc->counters != NULL);
>  }
>  
> +static inline s64 percpu_counter_atomic_read(struct percpu_counter *fbc)
> +{
> +	return atomic64_read(&fbc->count_atomic);
> +}
> +
> +static inline void percpu_counter_atomic_add(struct percpu_counter *fbc,
> +					     s64 amount)
> +{
> +	atomic64_add(amount, &fbc->count_atomic);
> +}
> +
> +int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
> +				       u32 nr_counters);
> +
>  #else /* !CONFIG_SMP */
>  
>  struct percpu_counter {
> @@ -260,6 +280,23 @@ static inline bool percpu_counter_initialized(struct percpu_counter *fbc)
>  static inline void percpu_counter_sync(struct percpu_counter *fbc)
>  {
>  }
> +
> +static inline s64 percpu_counter_atomic_read(struct percpu_counter *fbc)
> +{
> +	return fbc->count;
> +}
> +
> +static inline void percpu_counter_atomic_add(struct percpu_counter *fbc,
> +					     s64 amount)
> +{
> +	percpu_counter_add(fbc, amount);
> +}
> +
> +static inline int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
> +						     u32 nr_counters)
> +{
> +	return 0;
> +}
>  #endif	/* CONFIG_SMP */
>  
>  static inline void percpu_counter_inc(struct percpu_counter *fbc)
> diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
> index 44dd133594d4..95c4e038051a 100644
> --- 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.
> + */
> +int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
> +				       u32 nr_counters)
> +{
> +	static struct lock_class_key __key;

This is an improper use of lockdep. Now all of the percpu_counters
initialized on this path will key off of this lock_class_key.

> +	unsigned long flags;
> +	bool ret = 0;
> +
> +	if (percpu_counter_initialized(fbc))
> +		return 0;
> +
> +	preempt_disable();
> +	local_irq_save(flags);
> +	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;
> +}

I'm staring at this API and I'm not in love with it. I think it hinges 
on that there's one user of mm_stats prior hence it's safe. Generically
though, I can't see why this is safe.

I need to give this a little more thought, but my gut reaction is I'd
rather this look like percpu_refcount where we can init dead minus the
percpu allocation. Maybe that's not quite right, but I'd feel better
about it than requiring external synchronization on a percpu_counter to
ensure that it's correct.

> +
>  static int __init percpu_counter_startup(void)
>  {
>  	int ret;
> -- 
> 2.25.1
> 

Thanks,
Dennis