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

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Message-ID: <20180305124217.GZ25181@hirez.programming.kicks-ass.net>
Date:   Mon, 5 Mar 2018 13:42:17 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
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>,
        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 6/7] sched: idle: Predict idle duration before
 stopping the tick

On Mon, Mar 05, 2018 at 01:07:07PM +0100, Rafael J. Wysocki wrote:
> On Mon, Mar 5, 2018 at 12:50 PM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> > On Mon, Mar 5, 2018 at 12:45 PM, Peter Zijlstra <peterz@...radead.org> wrote:

> >> So I think this is entirely wrong, I would much rather see something
> >> like:
> >>
> >>         tick_nohz_idle_go_idle(next_state->nohz);
> >>
> >> Where the selected state itself has the nohz property or not.
> >
> > Can you elaborate here, I'm not following?
> >
> >> We can always insert an extra state at whatever the right boundary point
> >> is for nohz if it doesn't line up with an existing point.
> 
> OK, I guess I know what you mean: to add a state flag meaning "stop
> the tick if this state is selected".

Yes, that.

> That could work, but I see problems, like having to go through all of
> the already defined states and deciding what to do with them.

Shouldn't be too hard, upon registering a cpuidle driver to the cpuidle
core, the core could go through the provided states and flag all those <
TICK_USEC as not stopping, all those > TICK_USEC as stopping and
splitting the state we'd select for TICK_NSEC sleeps, stopping it for <
and disabling it for >.