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

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Date:	Wed, 6 Apr 2016 17:51:45 -0400
From:	Waiman Long <waiman.long@....com>
To:	Tejun Heo <tj@...nel.org>
CC:	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Christoph Lameter <cl@...ux.com>, <linux-ext4@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Toshimitsu Kani <toshi.kani@....com>
Subject: Re: [PATCH 2/3] percpu_stats: Simple per-cpu statistics count helper
 functions

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.

Got that, will incorporate 64-bit counter support for 32-bit architecture.

>> +/*
>> + * Reset the all statistics counts to 0 in the percpu_stats structure
> Proper function description please.

Sure. Will do that for all the functions.

>> + */
>> +static inline void percpu_stats_reset(struct percpu_stats *pcs)
> Why is this function inline?

It doesn't need to be inlined, but I need to add a lib/percpu_stats.c 
file to hold the function which I will do in my v2 patch.

>
>> +{
>> +	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.

I am not sure what you mean here by not acceptable. Please enlighten me 
on that.

>> +}
>> +
>> +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?

Will move this function to lib/percpu_stats.c.

>> +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