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

Peter Zijlstra
2018-04-05T12:48:14Z

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;
> /*
>