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

>
> 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 ?


> | 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

> +
> + 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

> + idle_state->exit_latency_ns);
> }
>
> static int __read_mostly cpu_idle_force_poll;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 2a0caf394dd4..eef1e535e2c2 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1015,6 +1015,7 @@ struct rq {
> #ifdef CONFIG_CPU_IDLE
> /* Must be inspected within a rcu lock section */
> struct cpuidle_state *idle_state;
> + s64 break_even;
> #endif
> };
>
> @@ -1850,6 +1851,16 @@ static inline struct cpuidle_state *idle_get_state(struct rq *rq)
>
> return rq->idle_state;
> }
> +
> +static inline void idle_set_break_even(struct rq *rq, s64 break_even)
> +{
> + WRITE_ONCE(rq->break_even, break_even);
> +}
> +
> +static inline s64 idle_get_break_even(struct rq *rq)
> +{
> + return READ_ONCE(rq->break_even);
> +}
> #else
> static inline void idle_set_state(struct rq *rq,
> struct cpuidle_state *idle_state)
> @@ -1860,6 +1871,15 @@ static inline struct cpuidle_state *idle_get_state(struct rq *rq)
> {
> return NULL;
> }
> +
> +static inline void idle_set_break_even(struct rq *rq, s64 break_even)
> +{
> +}
> +
> +static inline s64 idle_get_break_even(struct rq *rq)
> +{
> + return 0;
> +}
> #endif
>
> extern void schedule_idle(void);
> --
> 2.17.1
>

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