Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods
To: paulmck@linux.vnet.ibm.com
Cc: Thomas Gleixner <tglx@linutronix.de>, Andi Kleen <ak@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Christoph Lameter <cl@linux.com>, Aubrey Li <aubrey.li@intel.com>,
        len.brown@intel.com, rjw@rjwysocki.net, tim.c.chen@linux.intel.com,
        arjan@linux.intel.com, yang.zhang.wz@gmail.com, x86@kernel.org,
        linux-kernel@vger.kernel.org, daniel.lezcano@linaro.org
References: <20170714155356.GH3441@tassilo.jf.intel.com>
 <20170714160648.tg2u6eo2id6gmnjz@hirez.programming.kicks-ass.net>
 <20170714162619.GJ3441@tassilo.jf.intel.com>
 <20170717192309.ubn5muvc3u7htuaw@hirez.programming.kicks-ass.net>
 <34371ef8-b8bc-d2bf-93de-3fccd6beb032@linux.intel.com>
 <20170718044521.GO3441@tassilo.jf.intel.com>
 <alpine.DEB.2.20.1707180841290.1945@nanos>
 <f0ca590c-ea67-c17a-b0b9-13940150f1ae@linux.intel.com>
 <20170718152014.GB3981@linux.vnet.ibm.com>
 <df9cd535-3739-ebd2-f27d-e73c1806f0a1@linux.intel.com>
 <20170719144827.GB3730@linux.vnet.ibm.com>
From: "Li, Aubrey" <aubrey.li@linux.intel.com>
Message-ID: <dc7401df-c574-c77a-199c-0cdf5ba07aaa@linux.intel.com>
Date: Thu, 20 Jul 2017 09:40:49 +0800
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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