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Date: Thu, 9 Aug 2018 17:42:30 +0200 From: "Rafael J. Wysocki" <rafael@...nel.org> To: Leo Yan <leo.yan@...aro.org> Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>, Daniel Lezcano <daniel.lezcano@...aro.org>, Vincent Guittot <vincent.guittot@...aro.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Linux PM <linux-pm@...r.kernel.org> Subject: Re: [PATCH] sched: idle: Reenable sched tick for cpuidle request On Thu, Aug 9, 2018 at 2:05 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote: > On Thursday, August 9, 2018 7:47:27 AM CEST Leo Yan wrote: >> The idle loop stops tick by respecting the decision from cpuidle >> framework, if the condition 'need_resched()' is false without any task >> scheduling, the CPU keeps running in the loop in do_idle() and it has no >> chance call tick_nohz_idle_exit() to enable the tick. This results in >> the idle loop cannot reenable sched tick afterwards, if the idle >> governor selects a shallow state, thus the powernightmares issue can >> occur again. > > Well, there is code in the menu governor to avoid that. So the governor is not expected to select a shallow state then, unless it knows that there will be a timer interrupt (which is not a tick, obviously), that will wake up the CPU early enough. The menu governor works like that, but I agree that it should not request the tick to be running then (which it does). >> This issue can be easily reproduce with the case on Arm Hikey board: use >> CPU0 to send IPI to CPU7, CPU7 receives the IPI and in the callback >> function it start a hrtimer with 4ms, so the 4ms timer delta value can >> let 'menu' governor to choose deepest state in the next entering idle >> time. From then on, CPU7 restarts hrtimer with 1ms interval for total >> 10 times, so this can utilize the typical pattern in 'menu' governor to >> have prediction for 1ms duration, finally idle governor is easily to >> select a shallow state, on Hikey board it usually is to select CPU off >> state. From then on, CPU7 stays in this shallow state for long time >> until there have other interrupts on it. > > And which means that the above-mentioned code misses this case. And I don't really understand how this happens. :-/ If menu sees that the tick has been stopped, it sets data->predicted_us to the minimum of TICK_USEC and ktime_to_us(delta_next) and the latency requirements comes from PM QoS (no interactivity boost). Thus the only case when it will say "do not stop the tick" is when delta_next is below the tick period length, but that's OK, because it means that there is a timer pending that much time away, so it doesn't make sense to select a deeper idle state then. If there is a short-interval timer pending every time we go idle, it doesn't matter that the tick is stopped really, because the other timer will wake the CPU up anyway. Have I missed anything? >> >> C2: cluster off; C1: CPU off >> >> Idle state: C2 C2 C2 C2 C2 C2 C2 C1 >> ---------------------------------------------------------> >> Interrupt: ^ ^ ^ ^ ^ ^ ^ ^ ^ >> IPI Timer Timer Timer Timer Timer Timer Timer Timer >> 4ms 1ms 1ms 1ms 1ms 1ms 1ms 1ms >> >> To fix this issue, the idle loop needs to support reenabling sched tick. >> This patch checks the conditions 'stop_tick' is false when the tick is >> stopped, this condition indicates the cpuidle governor asks to reenable >> the tick and we can use tick_nohz_idle_restart_tick() for this purpose. >> >> A synthetic case is used to to verify this patch, we use CPU0 to send >> IPI to wake up CPU7 with 50ms interval, CPU7 generate a series hrtimer >> events (the first interval is 4ms, then the sequential 10 timer events >> are 1ms interval, same as described above). We do statistics for idle >> states duration, the unit is second (s), the testing result shows the >> C2 state (deepest state) staying time can be improved significantly for >> CPU7 (+7.942s for 10s execution time on CPU7) and all CPUs wide >> (+13.360s for ~80s of all CPUs execution time). >> >> Without patches With patches Difference >> -------------------- -------------------- ----------------------- >> CPU C0 C1 C2 C0 C1 C2 C0 C1 C2 >> 0 0.000 0.027 9.941 0.055 0.038 9.700 +0.055 +0.010 -0.240 >> 1 0.045 0.000 9.964 0.019 0.000 9.943 -0.026 +0.000 -0.020 >> 2 0.002 0.003 10.007 0.035 0.053 9.916 +0.033 +0.049 -0.090 >> 3 0.000 0.023 9.994 0.024 0.246 9.732 +0.024 +0.222 -0.261 >> 4 0.032 0.000 9.985 0.015 0.007 9.993 -0.016 +0.007 +0.008 >> 5 0.001 0.000 9.226 0.039 0.000 9.971 +0.038 +0.000 +0.744 >> 6 0.000 0.000 0.000 0.036 0.000 5.278 +0.036 +0.000 +5.278 >> 7 1.894 8.013 0.059 1.509 0.026 8.002 -0.384 -7.987 +7.942 >> All 1.976 8.068 59.179 1.737 0.372 72.539 -0.239 -7.695 +13.360 >> >> Cc: Daniel Lezcano <daniel.lezcano@...aro.org> >> Cc: Vincent Guittot <vincent.guittot@...aro.org> >> Signed-off-by: Leo Yan <leo.yan@...aro.org> >> --- >> kernel/sched/idle.c | 12 ++++++++++-- >> 1 file changed, 10 insertions(+), 2 deletions(-) >> >> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c >> index 1a3e9bd..802286e 100644 >> --- a/kernel/sched/idle.c >> +++ b/kernel/sched/idle.c >> @@ -190,10 +190,18 @@ static void cpuidle_idle_call(void) >> */ >> next_state = cpuidle_select(drv, dev, &stop_tick); >> >> - if (stop_tick) >> + if (stop_tick) { >> tick_nohz_idle_stop_tick(); >> - else >> + } else { >> + /* >> + * The cpuidle framework says to not stop tick but >> + * the tick has been stopped yet, so restart it. >> + */ >> + if (tick_nohz_tick_stopped()) >> + tick_nohz_idle_restart_tick(); > > You need an "else" here IMO as Peter said. And I really would prefer to avoid restarting the tick here, because it is overhead and quite likely unnecessary. >> + >> tick_nohz_idle_retain_tick(); >> + } >> >> rcu_idle_enter(); >> >> > > Please CC cpuidle patches to linux-pm@...r.kernel.org, that helps a lot. Thanks, Rafael
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