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

[Daniel Lezcano]

Hi Qais,

On 04/03/2020 16:01, Qais Yousef wrote:
> Hi Daniel
> 
> Adding Rafael to CC as I think he might be interested in this too.
> 
> On 03/04/20 12:48, Daniel Lezcano 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.
>>
>> In order to be more energy efficient but without impacting the
>> performances, let's use another criteria: the break even deadline.
> 
> What is 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.
> 
> I think that's its definition, but could do with more explanation.
> 
> So the break even deadline is the time window during which we can abort the CPU
> while entering its shallowest idle state?

No, it is the moment in absolute time when the min residency is reached.

>From Documentation/devicetree/bindings/arm/idle-states.yaml

"
min-residency is defined for a given idle state as the minimum expected
residency time for a state (inclusive of preparation and entry) after
which choosing that state become the most energy efficient option. A
good way to visualise this, is by taking the same graph above and
comparing some states energy consumptions plots.
"

> So if we have 2 cpus entering the shallowest idle state, we pick the one that
> has a faster abort? And the energy saving comes from the fact we avoided
> unnecessary sleep-just-to-wakeup-immediately cycle?

No, actually it is about to choose a CPU where it has a better chance to
have reach its min residency. Basically, when the CPU enters an idle
state, that has a cost (cache flush / refill, context saving/restore etc
...), so there is a peak of energy and the CPU has to save energy long
enough to compensate this extra consumption.

If the scheduler is constantly waking up an idle CPU before it slept
long enough, we lose energy and performance.

Example 1, the CPUs are in a state:
 - CPU0 (power down)
 - CPU1 (power down)
 - CPU2 (WFI)
 - CPU3 (power down)

 The routine choose CPU2 because it is the shallowest state.

Example 2, the CPUs are in a state:
 - CPU0 (power down) (bedl = 1234)
 - CPU1 (power down) (bedl = 4321)
 - CPU2 (power down) (bedl = 9876)
 - CPU3 (power down) (bedl = 3421)

* bedl = break even deadline

  The routine choose CPU1 because the bedl is the smallest.


>> 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:
>>
>> sched/perf and messaging does not show a performance regression. Ran
>> 10 times schbench and adrestia.
>>
>> with break-even deadline:
>> -------------------------
>> schbench *99.0th        : 14844.8
>> adrestia / periodic     : 57.95
>> adrestia / single       : 49.3
>>
>> Without break-even deadline:
>> ----------------------------
>> schbench / *99.0th      : 15017.6
>> adrestia / periodic     : 57
>> adrestia / single       : 55.4
> 
> Lower is better or worse? The numbers might be popular and maybe I should have
> known them, but adding some explanation will always help.

 * lesser is better

> Do you have any energy measurement on the impact of this patch?

No, I don't have any platform allowing that. I've an instrumented
big.Little but this patch is not relevant as the EAS takes over.

The only platform I have in my hands now is the synquacer 24 cores which
makes sense for this slow path selection.

>> Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
>> ---
>>  kernel/sched/fair.c  | 18 ++++++++++++++++--
>>  kernel/sched/idle.c  |  9 ++++++++-
>>  kernel/sched/sched.h | 20 ++++++++++++++++++++
>>  3 files changed, 44 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index fcc968669aea..520c5e822fdd 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) {
> 
> Shouldn't you retain the original if condition here? You omitted the 2nd part
> of this check compared to the original condition
> 
> 	(!idle || >>>idle->exit_latency == min_exit_latency<<<)

It is done on purpose because of the condition right before.

>>  				/*
>>  				 * 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..189cd51cd474 100644
>> --- a/kernel/sched/idle.c
>> +++ b/kernel/sched/idle.c
>> @@ -19,7 +19,14 @@ 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();
>> +	ktime_t kt;
>> +
>> +	if (likely(idle_state)) {
>> +		kt = ktime_add_ns(ktime_get(), idle_state->exit_latency_ns);
>> +		idle_set_state(rq, idle_state);
> 
> This changes the behavior of the function.
> 
> There's a call to sched_idle_set_state(NULL), so with this it'd be a NOP.
> 
> Is this intentional?

Nope, thanks for spotting it. It should be:

 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);
+
+       if (likely(idle_state)) {
+               ktime_t kt = ktime_add_ns(ktime_get(),
+                                         idle_state->exit_latency_ns);
+               idle_set_break_even(rq, ktime_to_ns(kt));
+       }
 }


> Don't you need to reset the break_even value if idle_state is NULL too?

If the idle_state is NULL, the routine won't use the break_even.

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