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 12:50:03 +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 12:45 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.