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Date: Fri, 13 Mar 2020 13:15:41 +0100
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: Vincent Guittot <vincent.guittot@...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 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?
>>>> + idle_state->exit_latency_ns);
>>>> }
>>
>> [ ... ]
>>
>>
>> --
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>>
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