Messages in this thread | | | Subject | Re: [PATCH V2] sched: fair: Use the earliest break even | From | Daniel Lezcano <> | Date | Thu, 12 Mar 2020 11:04:19 +0100 |
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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?
>> + >> + 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.
>> + idle_state->exit_latency_ns); >> }
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