Re: [PATCH v2] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime count

[KOSAKI Motohiro]

>> For archs which define VIRT_CPU_ACCOUNTING, every tick would result
>> in >1000 units of cputime updates and since this is much much greater
>> than percpu_batch_counter, we end up taking spinlock on every tick.
>>
>> This patch change batch rule. now, any cpu can store "percpu_counter_bach * jiffies"
>> cputime in per-cpu cache.
>> it mean this patch don't have behavior change if VIRT_CPU_ACCOUNTING=n.
>
> Does this actually matter?
>
> If we're calling cpuacct_update_stats() with large values of `cputime'
> then presumably we're also calling cpuacct_update_stats() at a low
> frequency, so the common lock-taking won't cause performance problems?

VIRT_CPU_ACCOUNTING change cputime_t meaning. but don't change calling
update time frequency.

example,
ia64, HZ=1000, VIRT_CPU_ACCOUNTING=y (1 cputime == 1ns, ie 1 jiffies
== 1000000 cputime)

every tick updating makes 1000000 cputime. (see jiffies_to_cputime)

-----------------------------------------------------------------------------------------
void account_process_tick(struct task_struct *p, int user_tick)
{
        cputime_t one_jiffy = jiffies_to_cputime(1);
        cputime_t one_jiffy_scaled = cputime_to_scaled(one_jiffy);
        struct rq *rq = this_rq();

        if (user_tick)
                account_user_time(p, one_jiffy, one_jiffy_scaled);
        else if (p != rq->idle)
                account_system_time(p, HARDIRQ_OFFSET, one_jiffy,
                                    one_jiffy_scaled);
        else
                account_idle_time(one_jiffy);
}
-----------------------------------------------------------------------------------------

but tick updating frequency don't changed.
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