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Message-ID: <CAKfTPtDUmqYB1i7UcYXxcNjnQOoGufsB9do-9NxTMeWdJAfP2w@mail.gmail.com>
Date: Fri, 13 Mar 2020 14:15:05 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Daniel Lezcano <daniel.lezcano@...aro.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>,
linux-kernel <linux-kernel@...r.kernel.org>,
Qais Yousef <qais.yousef@....com>,
Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH V2] sched: fair: Use the earliest break even
On Fri, 13 Mar 2020 at 13:15, Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
>
> On 12/03/2020 13:27, Vincent Guittot wrote:
> > On Thu, 12 Mar 2020 at 11:04, Daniel Lezcano <daniel.lezcano@...aro.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@...aro.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@...aro.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
> >>
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> >>
>
>
> --
> <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs
>
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