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

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Date:   Thu, 20 Jul 2017 09:40:49 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     paulmck@...ux.vnet.ibm.com
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Christoph Lameter <cl@...ux.com>,
        Aubrey Li <aubrey.li@...el.com>, len.brown@...el.com,
        rjw@...ysocki.net, tim.c.chen@...ux.intel.com,
        arjan@...ux.intel.com, yang.zhang.wz@...il.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, daniel.lezcano@...aro.org
Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

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