Re: [PATCH v2 2/4] percpu_stats: Enable 64-bit counts in 32-bit architectures

Waiman Long
2016-04-08T17:33:11Z

On 04/08/2016 12:47 PM, Tejun Heo wrote:
> Hello, Waiman.
>
> On Fri, Apr 08, 2016 at 12:16:20PM -0400, Waiman Long wrote:
>> +/**
>> + * __percpu_stats_add - add given count to percpu value
>> + * @pcs : Pointer to percpu_stats structure
>> + * @stat: The statistics count that needs to be updated
>> + * @cnt: The value to be added to the statistics count
>> + */
>> +void __percpu_stats_add(struct percpu_stats *pcs, int stat, int cnt)
>> +{
>> + /*
>> + * u64_stats_update_begin/u64_stats_update_end alone are not safe
>> + * against recursive add on the same CPU caused by interrupt.
>> + * So we need to set the PCPU_STAT_INTSAFE flag if this is required.
>> + */
>> + if (IS_STATS64(pcs)) {
>> + uint64_t *pstats64;
>> + unsigned long flags;
>> +
>> + pstats64 = get_cpu_ptr(pcs->stats64);
>> + if (pcs->flags& PCPU_STAT_INTSAFE)
>> + local_irq_save(flags);
>> +
>> + u64_stats_update_begin(&pcs->sync);
>> + pstats64[stat] += cnt;
>> + u64_stats_update_end(&pcs->sync);
>> +
>> + if (pcs->flags& PCPU_STAT_INTSAFE)
>> + local_irq_restore(flags);
>> +
>> + put_cpu_ptr(pcs->stats64);
>> + }
>> +}
> Heh, that's a handful, and, right, u64_stats needs separate irq
> protection. I'm not sure. If we have to do the above, it's likely
> that it'll perform worse than percpu_counter on 32bits. On 64bits,
> percpu_counter would incur extra preempt_disable/enable() operations
> but that comes from it not using this_cpu_add_return(). I wonder
> whether it'd be better to either use percpu_counter instead or if
> necessary extend it to handle multiple counters. What do you think?
>
> Thanks.
>

Yes, I think it will be more efficient to use percpu_counter in this
case. The preempt_disable/enable() calls are pretty cheap. Once in a
while, you need to take the lock and update the global count. How about
I change the 2nd patch to use percpu_counter internally when 64-bit
counts are needed in 32-bit archs, but use the regular percpu counts on
64-bit archs? If you are OK with that, I can update the patch accordingly.

Cheers,
Longman