Re: [PATCH 2/3] percpu_stats: Simple per-cpu statistics count helper functions

[Waiman Long]

On 04/04/2016 12:02 PM, Tejun Heo wrote:
> Hello,
>
> On Fri, Apr 01, 2016 at 11:09:37PM -0400, Waiman Long wrote:
> ...
> > +struct percpu_stats {
> > +	unsigned long __percpu *stats;
> I'm not sure ulong is the best choice here.  Atomic reads on 32bit are
> nice but people often need 64bit counters for stats.  It probably is a
> better idea to use u64_stats_sync.
>
> > +/*
> > + * Reset the all statistics counts to 0 in the percpu_stats structure
> Proper function description please.
>
> > + */
> > +static inline void percpu_stats_reset(struct percpu_stats *pcs)
> Why is this function inline?
>
> > +{
> > +	int cpu;
> > +
> > +	for_each_possible_cpu(cpu) {
> > +		unsigned long *pstats =  per_cpu_ptr(pcs->stats, cpu);
>                                         ^^
> > +		int stat;
> > +
> > +		for (stat = 0; stat<  pcs->nstats; stat++, pstats++)
> > +			*pstats = 0;
> > +	}
> > +
> > +	/*
> > +	 * If a statistics count is in the middle of being updated, it
> > +	 * is possible that the above clearing may not work. So we need
> > +	 * to double check again to make sure that the counters are really
> > +	 * cleared. Still there is a still a very small chance that the
> > +	 * second clearing does not work.
> > +	 */
> > +	for_each_possible_cpu(cpu) {
> > +		unsigned long *pstats =  per_cpu_ptr(pcs->stats, cpu);
> > +		int stat;
> > +
> > +		for (stat = 0; stat<  pcs->nstats; stat++, pstats++)
> > +			if (*pstats)
> > +				*pstats = 0;
> > +	}
> I don't think this is acceptable.
>
> > +}
> > +
> > +static inline int percpu_stats_init(struct percpu_stats *pcs, int num)
> > +{
> > +	pcs->nstats = num;
> > +	pcs->stats  = __alloc_percpu(sizeof(unsigned long) * num,
> > +				     __alignof__(unsigned long));
> > +	if (!pcs->stats)
> > +		return -ENOMEM;
> > +
> > +	percpu_stats_reset(pcs);
> > +	return 0;
> > +}
> > +
> > +static inline void percpu_stats_destroy(struct percpu_stats *pcs)
> > +{
> > +	free_percpu(pcs->stats);
> > +	pcs->stats  = NULL;
> > +	pcs->nstats = 0;
> > +}
> Why inline the above functions?
>
> > +static inline void
> > +__percpu_stats_add(struct percpu_stats *pcs, int stat, int cnt)
> > +{
> > +	unsigned long *pstat;
> > +
> > +	if ((unsigned int)stat>= pcs->nstats)
> > +		return;
> This is a critical bug.  Please don't fail silently.  BUG_ON(),
> please.

Sure.

> > +	preempt_disable();
> > +	pstat = this_cpu_ptr(&pcs->stats[stat]);
> > +	*pstat += cnt;
> > +	preempt_enable();
> > +}
> this_cpu_add() is atomic w.r.t. local operations.

Will use this_cpu_add().

> > +static inline unsigned long
> > +percpu_stats_sum(struct percpu_stats *pcs, int stat)
> > +{
> > +	int cpu;
> > +	unsigned long sum = 0;
> > +
> > +	if ((unsigned int)stat>= pcs->nstats)
> > +		return sum;
> Ditto.
>
> > +	for_each_possible_cpu(cpu)
> > +		sum += per_cpu(pcs->stats[stat], cpu);
> > +	return sum;
> > +}
> Thanks.

Cheers,
Longman