Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 13 Mar 2020 14:15:05 +0100 | Subject | Re: [PATCH V2] sched: fair: Use the earliest break even |
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On Fri, 13 Mar 2020 at 13:15, Daniel Lezcano <daniel.lezcano@linaro.org> wrote: > > On 12/03/2020 13:27, Vincent Guittot wrote: > > On Thu, 12 Mar 2020 at 11:04, Daniel Lezcano <daniel.lezcano@linaro.org> wrote: > >> > >> On 12/03/2020 09:36, Vincent Guittot wrote: > >>> Hi Daniel, > >>> > >>> On Wed, 11 Mar 2020 at 21:28, Daniel Lezcano <daniel.lezcano@linaro.org> wrote: > >>>> > >>>> In the idle CPU selection process occuring in the slow path via the > >>>> find_idlest_group_cpu() function, we pick up in priority an idle CPU > >>>> with the shallowest idle state otherwise we fall back to the least > >>>> loaded CPU. > >>> > >>> The idea makes sense but this path is only used by fork and exec so > >>> I'm not sure about the real impact > >> > >> I agree the fork / exec path is called much less often than the wake > >> path but it makes more sense for the decision. > >> > >>>> In order to be more energy efficient but without impacting the > >>>> performances, let's use another criteria: the break even deadline. > >>>> > >>>> At idle time, when we store the idle state the CPU is entering in, we > >>>> compute the next deadline where the CPU could be woken up without > >>>> spending more energy to sleep. > >>>> > >>>> At the selection process, we use the shallowest CPU but in addition we > >>>> choose the one with the minimal break even deadline instead of relying > >>>> on the idle_timestamp. When the CPU is idle, the timestamp has less > >>>> meaning because the CPU could have wake up and sleep again several times > >>>> without exiting the idle loop. In this case the break even deadline is > >>>> more relevant as it increases the probability of choosing a CPU which > >>>> reached its break even. > >>>> > >>>> Tested on: > >>>> - a synquacer 24 cores, 6 sched domains > >>>> - a hikey960 HMP 8 cores, 2 sched domains, with the EAS and energy probe > >>>> > >>>> sched/perf and messaging does not show a performance regression. Ran > >>>> 50 times schbench, adrestia and forkbench. > >>>> > >>>> The tools described at https://lwn.net/Articles/724935/ > >>>> > >>>> -------------------------------------------------------------- > >>>> | Synquacer | With break even | Without break even | > >>>> -------------------------------------------------------------- > >>>> | schbench *99.0th | 14844.8 | 15017.6 | > >>>> | adrestia / periodic | 57.95 | 57 | > >>>> | adrestia / single | 49.3 | 55.4 | > >>>> -------------------------------------------------------------- > >>> > >>> Have you got some figures or cpuidle statistics for the syncquacer ? > >> > >> No, and we just noticed the syncquacer has a bug in the firmware and > >> does not actually go to the idle states. > >> > >> > >>>> | Hikey960 | With break even | Without break even | > >>>> -------------------------------------------------------------- > >>>> | schbench *99.0th | 56140.8 | 56256 | > >>>> | schbench energy | 153.575 | 152.676 | > >>>> | adrestia / periodic | 4.98 | 5.2 | > >>>> | adrestia / single | 9.02 | 9.12 | > >>>> | adrestia energy | 1.18 | 1.233 | > >>>> | forkbench | 7.971 | 8.05 | > >>>> | forkbench energy | 9.37 | 9.42 | > >>>> -------------------------------------------------------------- > >>>> > >>>> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org> > >>>> --- > >>>> kernel/sched/fair.c | 18 ++++++++++++++++-- > >>>> kernel/sched/idle.c | 8 +++++++- > >>>> kernel/sched/sched.h | 20 ++++++++++++++++++++ > >>>> 3 files changed, 43 insertions(+), 3 deletions(-) > >>>> > >>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > >>>> index 4b5d5e5e701e..8bd6ea148db7 100644 > >>>> --- a/kernel/sched/fair.c > >>>> +++ b/kernel/sched/fair.c > >>>> @@ -5793,6 +5793,7 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this > >>>> { > >>>> unsigned long load, min_load = ULONG_MAX; > >>>> unsigned int min_exit_latency = UINT_MAX; > >>>> + s64 min_break_even = S64_MAX; > >>>> u64 latest_idle_timestamp = 0; > >>>> int least_loaded_cpu = this_cpu; > >>>> int shallowest_idle_cpu = -1; > >>>> @@ -5810,6 +5811,8 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this > >>>> if (available_idle_cpu(i)) { > >>>> struct rq *rq = cpu_rq(i); > >>>> struct cpuidle_state *idle = idle_get_state(rq); > >>>> + s64 break_even = idle_get_break_even(rq); > >>>> + > >>>> if (idle && idle->exit_latency < min_exit_latency) { > >>>> /* > >>>> * We give priority to a CPU whose idle state > >>>> @@ -5817,10 +5820,21 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this > >>>> * of any idle timestamp. > >>>> */ > >>>> min_exit_latency = idle->exit_latency; > >>>> + min_break_even = break_even; > >>>> latest_idle_timestamp = rq->idle_stamp; > >>>> shallowest_idle_cpu = i; > >>>> - } else if ((!idle || idle->exit_latency == min_exit_latency) && > >>>> - rq->idle_stamp > latest_idle_timestamp) { > >>>> + } else if ((idle && idle->exit_latency == min_exit_latency) && > >>>> + break_even < min_break_even) { > >>>> + /* > >>>> + * We give priority to the shallowest > >>>> + * idle states with the minimal break > >>>> + * even deadline to decrease the > >>>> + * probability to choose a CPU which > >>>> + * did not reach its break even yet > >>>> + */ > >>>> + min_break_even = break_even; > >>>> + shallowest_idle_cpu = i; > >>>> + } else if (!idle && rq->idle_stamp > latest_idle_timestamp) { > >>>> /* > >>>> * If equal or no active idle state, then > >>>> * the most recently idled CPU might have > >>>> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c > >>>> index b743bf38f08f..3342e7bae072 100644 > >>>> --- a/kernel/sched/idle.c > >>>> +++ b/kernel/sched/idle.c > >>>> @@ -19,7 +19,13 @@ extern char __cpuidle_text_start[], __cpuidle_text_end[]; > >>>> */ > >>>> void sched_idle_set_state(struct cpuidle_state *idle_state) > >>>> { > >>>> - idle_set_state(this_rq(), idle_state); > >>>> + struct rq *rq = this_rq(); > >>>> + > >>>> + idle_set_state(rq, idle_state); > >>> > >>> Shouldn't the state be set after setting break even otherwise you will > >>> have a time window with an idle_state != null but the break_even still > >>> set to the previous value > >> > >> IIUC we are protected in this section. Otherwise the routine above would > >> be also wrong [if (idle && idle->exit_latency)], no? > > > > no there are not the same because it uses the idle pointer to read > > exit_latency so we are sure to use exit_latency related to the idle > > pointer. > > > > In your case it checks idle is not null but then it uses rq to read > > break_even but it might not have been already updated > > Ok I will invert the lines. > > >>>> + > >>>> + if (idle_state) > >>>> + idle_set_break_even(rq, ktime_get_ns() + > >>> > >>> What worries me a bit is that it adds one ktime_get call each time a > >>> cpu enters idle > >> > >> Right, we can improve this in the future by folding the local_clock() in > >> cpuidle when entering idle with this ktime_get. > > > > Using local_clock() would be more latency friendly > > Unfortunately we are comparing the deadline across CPUs, so the > local_clock() can not be used here. > > But if we have one ktime_get() instead of a local_clock() + ktime_get(), > that should be fine, no?
Can't this computation of break_even be done in cpuidle framework while computing other statistics for selecting the idle state instead ? cpuidle already uses ktime_get for next hrtimer as an example. So cpuidle compute break_even and make it available to scheduler like exit_latency. And I can imagine that system wide time value will also be needed when looking at next wakeup event of cluster/group of CPUs
> > >>>> + idle_state->exit_latency_ns); > >>>> } > >> > >> [ ... ] > >> > >> > >> -- > >> <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs > >> > >> Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook | > >> <http://twitter.com/#!/linaroorg> Twitter | > >> <http://www.linaro.org/linaro-blog/> Blog > >> > > > -- > <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs > > Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook | > <http://twitter.com/#!/linaroorg> Twitter | > <http://www.linaro.org/linaro-blog/> Blog >
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