linux-kernel - Re: [RFT][PATCH v4 3/7] sched: idle: Do not stop the tick before cpuidle_idle

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Date:   Thu, 15 Mar 2018 19:19:00 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>
Subject: Re: [RFT][PATCH v4 3/7] sched: idle: Do not stop the tick before
 cpuidle_idle_call()

On Mon, Mar 12, 2018 at 10:53:25AM +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> Make cpuidle_idle_call() decide whether or not to stop the tick.
> 
> First, the cpuidle_enter_s2idle() path deals with the tick (and with
> the entire timekeeping for that matter) by itself and it doesn't need
> the tick to be stopped beforehand.

Not sure you meant timekeeping either :)

>  	if (idle_should_enter_s2idle() || dev->use_deepest_state) {
>  		if (idle_should_enter_s2idle()) {
> +			rcu_idle_enter();
> +
>  			entered_state = cpuidle_enter_s2idle(drv, dev);
>  			if (entered_state > 0) {
>  				local_irq_enable();
>  				goto exit_idle;
>  			}
> +
> +			rcu_idle_exit();
>  		}

I'm not sure how the tick is stopped on suspend to idle. Perhaps through
hrtimer (tick_cancel_sched_timer()) or clockevents code.

But we may have a similar problem than with idle_poll() called right after
call_cpuidle(). Ie: we arrive in cpuidle_enter_s2idle() with a tick that
should be reprogrammed while it is not. No idea if that can hurt somehow.

I guess it depends what happens to the tick on s2idle, I'm not clear with that.