linux-kernel - Re: [PATCH 6/8] sched: task_sched_runtime introduce micro optimization

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Message-ID: <CAHGf_=rNPM+6S1j_9p-RrmgTYiukMVpC0U_2_L1Gy3-xreXGFQ@mail.gmail.com>
Date:	Tue, 18 Jun 2013 11:28:26 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Olivier Langlois <olivier@...llion01.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 6/8] sched: task_sched_runtime introduce micro optimization

On Tue, Jun 18, 2013 at 11:17 AM, KOSAKI Motohiro
<kosaki.motohiro@...il.com> wrote:
>>> +#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
>
>>Stronger:
>>
>>+#ifdef CONFIG_64BIT
>>+       if (!p->on_cpu)
>>+               return p->se.sum_exec_runtime;
>>+#endif
>>
>>[ 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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