Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods
To: Thomas Gleixner <tglx@linutronix.de>,
        "Li, Aubrey" <aubrey.li@linux.intel.com>
References: <20170713182820.sn3fjitnd3mca27p@hirez.programming.kicks-ass.net>
 <31170ac6-9db1-f0b8-4841-f1661c8ed6e1@linux.intel.com>
 <20170714153818.pjauqxebxyhs6ljp@hirez.programming.kicks-ass.net>
 <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>
 <20170718065926.GP3441@tassilo.jf.intel.com>
 <alpine.DEB.2.20.1707180918080.1945@nanos>
 <348019f4-85ae-ba91-3fce-9886533e8d22@linux.intel.com>
 <alpine.DEB.2.20.1707190928510.2286@nanos>
 <0c346d95-baae-715a-bff5-4738b924ccff@linux.intel.com>
 <alpine.DEB.2.20.1707200936260.3168@nanos>
Cc: 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,
        paulmck@linux.vnet.ibm.com, yang.zhang.wz@gmail.com, x86@kernel.org,
        linux-kernel@vger.kernel.org, daniel.lezcano@linaro.org
From: Arjan van de Ven <arjan@linux.intel.com>
Message-ID: <970eb7df-1a42-9ed7-a69d-58af8c206f7b@linux.intel.com>
Date: Thu, 20 Jul 2017 06:48:41 -0700
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On 7/20/2017 1:11 AM, Thomas Gleixner wrote:
> On Thu, 20 Jul 2017, Li, Aubrey wrote:
>> Don't get me wrong, even if a fast path is acceptable, we still need to
>> figure out if the coming idle is short and when to switch. I'm just worried
>> about if irq timings is not an ideal statistics, we have to skip it too.
>
> There is no ideal solution ever.
>
> Lets sit back and look at that from the big picture first before dismissing
> a particular item upfront.
>
> The current NOHZ implementation does:
>
>     predict = nohz_predict(timers, rcu, arch, irqwork);
>
>     if ((predict - now) > X)
>     	stop_tick()
>
> The C-State machinery does something like:
>
>     predict = cstate_predict(next_timer, scheduler);
>
>     cstate = cstate_select(predict);
>
> That disconnect is part of the problem. What we really want is:
>
>     predict = idle_predict(timers, rcu, arch, irqwork, scheduler, irq timings);

two separate predictors is clearly a recipe for badness.

(likewise, C and P states try to estimate "performance sensitivity" and sometimes estimate in opposite directions)


to be honest, performance sensitivity estimation is probably 10x more critical for C state
selection than idle duration; a lot of modern hardware will do the energy efficiency stuff
in a microcontroller when it coordinates between multiple cores in the system on C and P states.

(both x86 and ARM have such microcontroller nowadays, at least for the higher performance
designs)