Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

Li, Aubrey
2017-07-20T01:41:20Z

On 2017/7/19 22:48, Paul E. McKenney wrote:
> On Wed, Jul 19, 2017 at 01:44:06PM +0800, Li, Aubrey wrote:
>> On 2017/7/18 23:20, Paul E. McKenney wrote:
>>
>>>> 2) for rcu idle enter/exit, I measured the details which Paul provided, and
>>>> the result matches with what I have measured before, nothing notable found.
>>>> But it still makes more sense if we can make rcu idle enter/exit hooked with
>>>> tick off. (it's possible other workloads behave differently)
>>>
>>> Again, assuming that RCU is informed of CPUs in the kernel, regardless
>>> of whether or not the tick is on that that point in time.
>>>
>> Yeah, I see, no problem for a normal idle.
>>
>> But for a short idle, we want to return to the task ASAP. Even though RCU cost
>> is not notable, it would still be better for me if we can save some cycles in
>> idle entry and idle exit.
>>
>> Do we have any problem if we skip RCU idle enter/exit under a fast idle scenario?
>> My understanding is, if tick is not stopped, then we don't need inform RCU in
>> idle path, it can be informed in irq exit.
>
> Indeed, the problem arises when the tick is stopped.

My question is, does problem arise when the tick is *not* stopped (skipping nohz idle)?

instead of

static void cpuidle_idle_call()
{
rcu_idle_enter()
......
rcu_idle_exit()
}

I want

static void cpuidle_idle_call()
{
if (tick stopped)
rcu_idle_enter()
......
if (tick stopped)
rcu_idle_exit()
}

Or checking tick stop can be put into rcu_idle_enter/exit

Thanks,
-Aubrey