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SubjectRe: [PATCH v2] cpuidle: menu: Handle stopped tick more aggressively
On Fri, Aug 10, 2018 at 11:20 AM <leo.yan@linaro.org> wrote:
>
> On Fri, Aug 10, 2018 at 09:57:18AM +0200, Rafael J . Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > Subject: [PATCH] cpuidle: menu: Handle stopped tick more aggressively
> >
> > Commit 87c9fe6ee495 (cpuidle: menu: Avoid selecting shallow states
> > with stopped tick) missed the case when the target residencies of
> > deep idle states of CPUs are above the tick boundary which may cause
> > the CPU to get stuck in a shallow idle state for a long time.
> >
> > Say there are two CPU idle states available: one shallow, with the
> > target residency much below the tick boundary and one deep, with
> > the target residency significantly above the tick boundary. In
> > that case, if the tick has been stopped already and the expected
> > next timer event is relatively far in the future, the governor will
> > assume the idle duration to be equal to TICK_USEC and it will select
> > the idle state for the CPU accordingly. However, that will cause the
> > shallow state to be selected even though it would have been more
> > energy-efficient to select the deep one.
> >
> > To address this issue, modify the governor to always assume idle
> > duration to be equal to the time till the closest timer event if
> > the tick is not running which will cause the selected idle states
> > to always match the known CPU wakeup time.
> >
> > Also make it always indicate that the tick should be stopped in
> > that case for consistency.
> >
> > Fixes: 87c9fe6ee495 (cpuidle: menu: Avoid selecting shallow states with stopped tick)
> > Reported-by: Leo Yan <leo.yan@linaro.org>
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > ---
> >
> > -> v2: Initialize first_idx properly in the stopped tick case.
> >
> > ---
> > drivers/cpuidle/governors/menu.c | 55 +++++++++++++++++----------------------
> > 1 file changed, 25 insertions(+), 30 deletions(-)
> >
> > Index: linux-pm/drivers/cpuidle/governors/menu.c
> > ===================================================================
> > --- linux-pm.orig/drivers/cpuidle/governors/menu.c
> > +++ linux-pm/drivers/cpuidle/governors/menu.c
> > @@ -285,9 +285,8 @@ static int menu_select(struct cpuidle_dr
> > {
> > struct menu_device *data = this_cpu_ptr(&menu_devices);
> > int latency_req = cpuidle_governor_latency_req(dev->cpu);
> > - int i;
> > - int first_idx;
> > - int idx;
> > + int first_idx = 0;
> > + int idx, i;
> > unsigned int interactivity_req;
> > unsigned int expected_interval;
> > unsigned long nr_iowaiters, cpu_load;
> > @@ -307,6 +306,18 @@ static int menu_select(struct cpuidle_dr
> > /* determine the expected residency time, round up */
> > data->next_timer_us = ktime_to_us(tick_nohz_get_sleep_length(&delta_next));
> >
> > + /*
> > + * 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 use the known time till the closest
> > + * timer event for the idle state selection.
> > + */
> > + if (tick_nohz_tick_stopped()) {
> > + data->predicted_us = ktime_to_us(delta_next);
> > + goto select;
> > + }
> > +
>
> This introduce two potential issues:
>
> - This will totally ignore the typical pattern in idle loop; I
> observed on the mmc driver can trigger multiple times (> 10 times)
> with consistent interval;

I'm not sure what you mean by "ignore".

> but I have no strong opinion to not use next timer event for this case.

OK

> - Will this break correction factors when the CPU exit from idle?
> data->bucket is stale value ....

Good point.

I'll send a v3 with this addressed.

>
> > get_iowait_load(&nr_iowaiters, &cpu_load);
> > data->bucket = which_bucket(data->next_timer_us, nr_iowaiters);
> >
> > @@ -322,7 +333,6 @@ static int menu_select(struct cpuidle_dr
> > expected_interval = get_typical_interval(data);
> > expected_interval = min(expected_interval, data->next_timer_us);
> >
> > - first_idx = 0;
> > if (drv->states[0].flags & CPUIDLE_FLAG_POLLING) {
> > struct cpuidle_state *s = &drv->states[1];
> > unsigned int polling_threshold;
> > @@ -344,29 +354,15 @@ static int menu_select(struct cpuidle_dr
> > */
> > data->predicted_us = min(data->predicted_us, expected_interval);
> >
> > - 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 a timer is going to expire really soon
> > - * anyway.
> > - */
> > - 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;
> > - }
> > + /*
> > + * 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;
> >
> > +select:
> > expected_interval = data->predicted_us;
> > /*
> > * Find the idle state with the lowest power while satisfying
> > @@ -403,14 +399,13 @@ static int menu_select(struct cpuidle_dr
> > * Don't stop the tick if the selected state is a polling one or if the
> > * expected idle duration is shorter than the tick period length.
> > */
> > - if ((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) ||
> > - expected_interval < TICK_USEC) {
> > + if (((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) ||
> > + expected_interval < TICK_USEC) && !tick_nohz_tick_stopped()) {
>
> I am not sure this logic is right... Why not use below checking, so
> for POLLING state we will never ask to stop the tick?
>
> if (drv->states[idx].flags & CPUIDLE_FLAG_POLLING ||
> (expected_interval < TICK_USEC && !tick_nohz_tick_stopped())) {
>

The only effect of it would be setting stop_tick to false, but why
would that matter?

> > unsigned int delta_next_us = ktime_to_us(delta_next);
> >
> > *stop_tick = false;
> >
> > - if (!tick_nohz_tick_stopped() && idx > 0 &&
> > - drv->states[idx].target_residency > delta_next_us) {
> > + if (idx > 0 && drv->states[idx].target_residency > delta_next_us) {
> > /*
> > * The tick is not going to be stopped and the target
> > * residency of the state to be returned is not within
> >

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