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 13:07:07 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Peter Zijlstra <peterz@...radead.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 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:
>> On Sun, Mar 04, 2018 at 11:28:56PM +0100, Rafael J. Wysocki wrote:
>>> Index: linux-pm/kernel/sched/idle.c
>>> ===================================================================
>>> --- linux-pm.orig/kernel/sched/idle.c
>>> +++ linux-pm/kernel/sched/idle.c
>>> @@ -188,13 +188,14 @@ static void cpuidle_idle_call(void)
>>>       } else {
>>>               unsigned int duration_us;
>>>
>>> -             tick_nohz_idle_go_idle(true);
>>> -             rcu_idle_enter();
>>> -
>>>               /*
>>>                * Ask the cpuidle framework to choose a convenient idle state.
>>>                */
>>>               next_state = cpuidle_select(drv, dev, &duration_us);
>>> +
>>> +             tick_nohz_idle_go_idle(duration_us > USEC_PER_SEC / HZ);
>>> +             rcu_idle_enter();
>>> +
>>>               entered_state = call_cpuidle(drv, dev, next_state);
>>>               /*
>>>                * Give the governor an opportunity to reflect on the outcome
>>
>> 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".

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.