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

[Daniel Lezcano]

On 12/03/2020 13:27, Vincent Guittot wrote:
> On Thu, 12 Mar 2020 at 11:04, Daniel Lezcano <daniel.lezcano@xxxxxxxxxx> wrote:
>>
>> On 12/03/2020 09:36, Vincent Guittot wrote:
>>> Hi Daniel,
>>>
>>> On Wed, 11 Mar 2020 at 21:28, Daniel Lezcano <daniel.lezcano@xxxxxxxxxx> 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@xxxxxxxxxx>
>>>> ---
>>>>  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?

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


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