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 14:00:53 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "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 5, 2018 at 1:42 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 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 >.
>

Well, on Intel everything below C8 has target residencies below 1 ms. :-)

I think that we want C6 to be "nohz" too, though, at least in some cases.

And what about C3 if C6 is disabled?