Re: [PATCH 03/10] Use percpu stats

Nitin Gupta
2010-08-11T16:39:08Z

On 08/10/2010 10:04 AM, Andrew Morton wrote:
> On Mon, 9 Aug 2010 22:56:49 +0530 Nitin Gupta <ngupta@vflare.org> wrote:
>
>> +/*
>> + * Individual percpu values can go negative but the sum across all CPUs
>> + * must always be positive (we store various counts). So, return sum as
>> + * unsigned value.
>> + */
>> +static u64 zram_get_stat(struct zram *zram, enum zram_stats_index idx)
>> {
>> - u64 val;
>> -
>> - spin_lock(&zram->stat64_lock);
>> - val = *v;
>> - spin_unlock(&zram->stat64_lock);
>> + int cpu;
>> + s64 val = 0;
>> +
>> + for_each_possible_cpu(cpu) {
>> + s64 temp;
>> + unsigned int start;
>> + struct zram_stats_cpu *stats;
>> +
>> + stats = per_cpu_ptr(zram->stats, cpu);
>> + do {
>> + start = u64_stats_fetch_begin(&stats->syncp);
>> + temp = stats->count[idx];
>> + } while (u64_stats_fetch_retry(&stats->syncp, start));
>> + val += temp;
>> + }
>>
>> + WARN_ON(val < 0);
>> return val;
>> }
>
> That reimplements include/linux/percpu_counter.h, poorly.
>
> Please see the June discussion "[PATCH v2 1/2] tmpfs: Quick token
> library to allow scalable retrieval of tokens from token jar" for some
> discussion.
>
>

I read the discussion you pointed out but still fail to see how percpu_counters,
with all their overhead, are better than simple pcpu variable used in current
version. What is the advantage?

Thanks,
Nitin