4.18-stable review patch. If anyone has any objections, please let me know.
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From: Rafael J. Wysocki rafael.j.wysocki@intel.com
commit 5ef499cd571c293b74a30d77e7ef512edb6ded6b upstream.
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 use the time till the closest timer event instead of the predicted idle duration if the latter is less than the tick period length and the tick has been stopped already. Also make it extend the search for a matching idle state if the tick is stopped to avoid settling on a shallow state if deep states with target residencies above the tick period length are available.
In addition, make it always indicate that the tick should be stopped if it has been stopped already for consistency.
Fixes: 87c9fe6ee495 (cpuidle: menu: Avoid selecting shallow states with stopped tick) Reported-by: Leo Yan leo.yan@linaro.org Acked-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: 4.17+ stable@vger.kernel.org # 4.17+ Signed-off-by: Rafael J. Wysocki rafael.j.wysocki@intel.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- drivers/cpuidle/governors/menu.c | 36 ++++++++++++++++++++++++------------ 1 file changed, 24 insertions(+), 12 deletions(-)
--- a/drivers/cpuidle/governors/menu.c +++ b/drivers/cpuidle/governors/menu.c @@ -349,14 +349,12 @@ static int menu_select(struct cpuidle_dr * 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. + * 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 (data->predicted_us < TICK_USEC) - data->predicted_us = min_t(unsigned int, TICK_USEC, - ktime_to_us(delta_next)); + data->predicted_us = ktime_to_us(delta_next); } else { /* * Use the performance multiplier and the user-configurable @@ -381,8 +379,22 @@ static int menu_select(struct cpuidle_dr continue; if (idx == -1) idx = i; /* first enabled state */ - if (s->target_residency > data->predicted_us) - break; + if (s->target_residency > data->predicted_us) { + if (!tick_nohz_tick_stopped()) + break; + + /* + * If the state selected so far is shallow and this + * state's target residency matches the time till the + * closest timer event, select this one to avoid getting + * stuck in the shallow one for too long. + */ + if (drv->states[idx].target_residency < TICK_USEC && + s->target_residency <= ktime_to_us(delta_next)) + idx = i; + + goto out; + } if (s->exit_latency > latency_req) { /* * If we break out of the loop for latency reasons, use @@ -403,14 +415,13 @@ 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()) { 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 @@ -429,6 +440,7 @@ static int menu_select(struct cpuidle_dr } }
+out: data->last_state_idx = idx;
return data->last_state_idx;