linux-kernel - Re: [PATCH v9 10/10] cpuidle: menu: Avoid selecting shallow states with stopped tick

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Message-ID: <20180405124757.GQ4082@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Apr 2018 14:47:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     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>,
        Len Brown <len.brown@...el.com>
Subject: Re: [PATCH v9 10/10] cpuidle: menu: Avoid selecting shallow states
 with stopped tick

On Wed, Apr 04, 2018 at 10:50:36AM +0200, Rafael J. Wysocki wrote:
> +	if (tick_nohz_tick_stopped()) {
> +		/*
> +		 * If the tick is already stopped, the cost of possible short
> +		 * idle duration misprediction is much higher, because the CPU
> +		 * may be stuck in a shallow idle state for a long time as a
> +		 * result of it.  In that case say we might mispredict and try
> +		 * to force the CPU into a state for which we would have stopped
> +		 * the tick, unless the tick timer is going to expire really
> +		 * soon anyway.

Wait what; the tick was stopped, therefore it _cannot_ expire soon.

*confused*

Did you mean s/tick/a/ ?

> +		 */
> +		if (data->predicted_us < TICK_USEC)
> +			data->predicted_us = min_t(unsigned int, TICK_USEC,
> +						   ktime_to_us(delta_next));
> +	} else {
> +		/*
> +		 * Use the performance multiplier and the user-configurable
> +		 * latency_req to determine the maximum exit latency.
> +		 */
> +		interactivity_req = data->predicted_us / performance_multiplier(nr_iowaiters, cpu_load);
> +		if (latency_req > interactivity_req)
> +			latency_req = interactivity_req;
> +	}
>  
>  	expected_interval = data->predicted_us;
>  	/*
>