linux-kernel - Re: [RFC/RFT][PATCH 4/7] cpuidle: menu: Split idle duration prediction from state selection

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Message-ID: <20180305113803.GO25201@hirez.programming.kicks-ass.net>
Date:   Mon, 5 Mar 2018 12:38:03 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Ilsche <thomas.ilsche@...dresden.de>,
        Doug Smythies <dsmythies@...us.net>,
        Rik van Riel <riel@...riel.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Mike Galbraith <mgalbraith@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RFC/RFT][PATCH 4/7] cpuidle: menu: Split idle duration
 prediction from state selection

On Sun, Mar 04, 2018 at 11:26:24PM +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> In order to address the issue with short idle duration predictions
> by the idle governor after the tick has been stopped, prepare the
> menu governor code for reordering with respect to the timekeeping
> code that stops the tick.
> 
> Use the observation that menu_select() can be split into two
> functions, one predicting the idle duration and one selecting the
> idle state, and rework it accordingly.

I actually think this is the wrong way around.

We really should be predicting state not duration. Yes the duration
thing is an intermediate value, but I don't think it makes any sense
what so ever to preserve that in the predictor. The end result is the
idle state, we should aim for that.

As per:

  https://lkml.org/lkml/2017/7/18/615

there are definite advantages to _not_ preserving duration information
beyond the state boundaries.