Re: [PATCH 6/8] sched: task_sched_runtime introduce micro optimization
From: KOSAKI Motohiro
Date: Tue Jun 18 2013 - 11:28:50 EST
On Tue, Jun 18, 2013 at 11:17 AM, KOSAKI Motohiro
>>> +#ifdef CONFIG_64BIT
>>> + /*
>>> + * 64-bit doesn't need locks to atomically read a 64bit value. So we
>>> + * have two optimization chances, 1) when caller doesn't need
>>> + * delta_exec and 2) when the task's delta_exec is 0. The former is
>>> + * obvious. The latter is complicated. reading ->on_cpu is racy, but
>>> + * this is ok. If we race with it leaving cpu, we'll take a lock. So
>>> + * we're correct. If we race with it entering cpu, unaccounted time
>>> + * is 0. This is indistinguishable from the read occurring a few
>>> + * cycles earlier.
>>> + */
>>> + if (!add_delta || !p->on_cpu)
>>> + return p->se.sum_exec_runtime;
>> I'm not sure this is correct from an smp ordering POV. p->on_cpu may appear
>> to be 0 whereas the task is actually running for a while and p->se.sum_exec_runtime
>> can then be past the actual value on the remote CPU.
> Quate form Paul's last e-mail
>>+ if (!p->on_cpu)
>>+ return p->se.sum_exec_runtime;
>>[ Or !p->on_cpu || !add_delta ].
>>We can take the racy read versus p->on_cpu since:
>> If we race with it leaving cpu: we take lock, we're correct
>> If we race with it entering cpu: unaccounted time ---> 0, this is
>>indistinguishable from the read occurring a few cycles earlier.
> That said, even though we got slightly inaccurate current time, we
> have no way to
> know this is inaccurate. E.g. cpu clock saving feature bring us more
> inaccuracy, but
> we already live in such world.
Ah, RT folks may want to call preempt_disable() in thread_group_cputime()
because preemptive kernel can be preemptible while for-each-threads loop
for getting accurate time.
But it is another story, it's not new issue and it's not introduced by me. :-)
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