linux-kernel - Re: [Update 2x] Re: [PATCH v3]PM/Sleep: Timer quiesce in freeze state

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Date:	Mon, 9 Feb 2015 16:44:08 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"Li, Aubrey" <aubrey.li@...ux.intel.com>,
	"Brown, Len" <len.brown@...el.com>,
	Alan Cox <alan@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [Update 2x] Re: [PATCH v3]PM/Sleep: Timer quiesce in freeze state

On Mon, Feb 09, 2015 at 03:54:22AM +0100, Rafael J. Wysocki wrote:
> Complete patch with that modification is appended.  In the next few days I'm
> going to split it into smaller parts and send along with cpuidle driver
> patches implementing ->enter_freeze.
> 
> Please let me know what you think.

> @@ -104,6 +105,21 @@ static void cpuidle_idle_call(void)
>  	rcu_idle_enter();
>  
>  	/*
> +	 * Suspend-to-idle ("freeze") is a system state in which all user space
> +	 * has been frozen, all I/O devices have been suspended and the only
> +	 * activity happens here and in iterrupts (if any).  In that case bypass
> +	 * the cpuidle governor and go stratight for the deepest idle state
> +	 * available.  Possibly also suspend the local tick and the entire
> +	 * timekeeping to prevent timer interrupts from kicking us out of idle
> +	 * until a proper wakeup interrupt happens.
> +	 */
> +	if (idle_should_freeze()) {
> +		cpuidle_enter_freeze();
> +		local_irq_enable();
> +		goto exit_idle;
> +	}
> +
> +	/*
>  	 * Ask the cpuidle framework to choose a convenient idle state.
>  	 * Fall back to the default arch idle method on errors.
>  	 */

I was hoping to not have to put that into the regular idle path; say
maybe share a single special branch with the play-dead call. People seem
to start complaining about the total amount of time it takes to just
'run' the idle path.

Now I don't think we can do that, because we need the
arch_cpu_idle_enter() nonsense for the one but not the other; also all
this really only makes sense in the cpuidle context, so nothing to be
done about that.

In any case, you could make that:

static inline bool idle_should_freeze(void)
{
	return unlikely(suspend_freeze_state == FREEZE_STATE_ENTER);
}

which should help a bit I suppose.

> +static void enter_freeze_proper(struct cpuidle_driver *drv,
> +                               struct cpuidle_device *dev, int index)
> +{
> +       tick_freeze();
> +       drv->states[index].enter_freeze(dev, drv, index);

This is slightly different from cpuidle_enter() in that it does not
consider the coupled states nonsense, is that on purpose? And if so,
does that want a comment?

> +       /*
> +        * timekeeping_resume() that will be called by tick_unfreeze() for the
> +        * last CPU executing it calls functions containing RCU read-side
> +        * critical sections, so tell RCU about that.
> +        */
> +       RCU_NONIDLE(tick_unfreeze());
> +}


But over all it looks fine to me.
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