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SubjectRe: [PATCH V2] sched: fair: Use the earliest break even
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

>
> >> +
> >> + 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

>
> >> + idle_state->exit_latency_ns);
> >> }
>
> [ ... ]
>
>
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