Messages in this thread | | | Date | Fri, 10 Aug 2018 20:31:43 +0800 | From | leo.yan@linaro ... | Subject | Re: [PATCH v2] cpuidle: menu: Handle stopped tick more aggressively |
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On Fri, Aug 10, 2018 at 01:04:22PM +0200, Rafael J. Wysocki wrote: > 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".
You could see after move code from blow to this position, the typical pattern interval will not be accounted; so if in the middle of idles there have a bunch of interrupts with fix pattern, the upper code cannot detect this pattern anymore.
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> > > - 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?
Please consider below situation, not sure if this case is existed or not:
step1: first time: enter one idle state with stopping tick; step2: second time: select POLLING state and tick_nohz_tick_stopped() is true;
So in step2, it cannot set stop_tick to false with below sentence.
> > > unsigned int delta_next_us = ktime_to_us(delta_next); > > > > > > *stop_tick = false;
[...]
Thanks, Leo Yan
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