linux-kernel - Re: [RFC/RFT][PATCH 6/7] sched: idle: Predict idle duration before stopping the tick

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Date:   Mon, 5 Mar 2018 14:46:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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>,
        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 6/7] sched: idle: Predict idle duration before
 stopping the tick

On Mon, Mar 05, 2018 at 02:37:25PM +0100, Peter Zijlstra wrote:
> On Mon, Mar 05, 2018 at 08:19:15AM -0500, Rik van Riel wrote:

> > > Also, I think that at this point you've introduced a problem; by not
> > > disabling the tick unconditionally, we'll have extra wakeups due to
> > > the (now still running) tick, which will bias the estimation, as per
> > > reflect(), downwards.
> > > 
> > > We should effectively discard tick wakeups when we could have
> > > entered nohz but didn't, accumulating the idle period in reflect and
> > > only commit once we get a !tick wakeup.
> > 
> > How much of a problem would that actually be?
> > 
> > Don't all but the very deepest C-states have
> > target residencies that are orders of magnitude
> > smaller than the tick period?
> > 
> > In other words, if our sleeps end up getting
> > "cut short" to 600us, we will still select C6,
> > and it will not result in picking C3 by mistake.
> > 
> > This only seems to affect C7 states and deeper.
> 
> On modern Intel, what about other platforms? This is something that
> should work across the board.

Look at this for example:

arch/arm64/boot/dts/hisilicon/hi3660.dtsi:                              min-residency-us = <20000>;

That's 20ms right there..

But on average, considering ARM64 defaults to HZ=250, most of them are
<TICK_USEC.