Re: [PATCH v6 4/6] sched: get CPU's usage statistic

Wanpeng Li
2014-11-21T05:37:14Z

Hi Vincent,
On 9/26/14, 8:17 PM, Vincent Guittot wrote:
> On 25 September 2014 21:05, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>> On 23/09/14 17:08, Vincent Guittot wrote:
>>> Monitor the usage level of each group of each sched_domain level. The usage is
>>> the amount of cpu_capacity that is currently used on a CPU or group of CPUs.
>>> We use the utilization_load_avg to evaluate the usage level of each group.
>>>
>>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
>>> ---
>>> kernel/sched/fair.c | 13 +++++++++++++
>>> 1 file changed, 13 insertions(+)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index 2cf153d..4097e3f 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -4523,6 +4523,17 @@ static int select_idle_sibling(struct task_struct *p, int target)
>>> return target;
>>> }
>>>
>>> +static int get_cpu_usage(int cpu)
>>> +{
>>> + unsigned long usage = cpu_rq(cpu)->cfs.utilization_load_avg;
>>> + unsigned long capacity = capacity_orig_of(cpu);
>>> +
>>> + if (usage >= SCHED_LOAD_SCALE)
>>> + return capacity + 1;
>> Why you are returning rq->cpu_capacity_orig + 1 (1025) in case
>> utilization_load_avg is greater or equal than 1024 and not usage or
>> (usage * capacity) >> SCHED_LOAD_SHIFT too?
> The usage can't be higher than the full capacity of the CPU because
> it's about the running time on this CPU. Nevertheless, usage can be
> higher than SCHED_LOAD_SCALE because of unfortunate rounding in
> avg_period and running_load_avg or just after migrating tasks until
> the average stabilizes with the new running time.
>
>> In case the weight of a sched group is greater than 1, you might loose
>> the information that the whole sched group is over-utilized too.
> that's exactly for sched_group with more than 1 CPU that we need to
> cap the usage of a CPU to 100%. Otherwise, the group could be seen as
> overloaded (CPU0 usage at 121% + CPU1 usage at 80%) whereas CPU1 has
> 20% of available capacity
>
>> You add up the individual cpu usage values for a group by
>> sgs->group_usage += get_cpu_usage(i) in update_sg_lb_stats and later use
>> sgs->group_usage in group_is_overloaded to compare it against
>> sgs->group_capacity (taking imbalance_pct into consideration).
>>
>>> +
>>> + return (usage * capacity) >> SCHED_LOAD_SHIFT;
>> Nit-pick: Since you're multiplying by a capacity value
>> (rq->cpu_capacity_orig) you should shift by SCHED_CAPACITY_SHIFT.
> we want to compare the output of the function with some capacity
> figures so i think that >> SCHED_LOAD_SHIFT is the right operation.

Could you explain more why '>> SCHED_LOAD_SHIFT' instead of '>>
SCHED_CAPACITY_SHIFT'?

Regards,
Wanpeng Li

>
>> Just to make sure: You do this scaling of usage by cpu_capacity_orig
>> here only to cater for the fact that cpu_capacity_orig might be uarch
>> scaled (by arch_scale_cpu_capacity, !SMT) in update_cpu_capacity while
> I do this for any system with CPUs that have an original capacity that
> is different from SCHED_CAPACITY_SCALE so it's for both uArch and SMT.
>
>> utilization_load_avg is currently not.
>> We don't even uArch scale on ARM TC2 big.LITTLE platform in mainline
>> today due to the missing clock-frequency property in the device tree.
> sorry i don't catch your point
>
>> I think it's hard for people to grasp that your patch-set takes uArch
>> scaling of capacity into consideration but not frequency scaling of
>> capacity (via arch_scale_freq_capacity, not used at the moment).
>>
>>> +}
>>> +
>>> /*
>>> * select_task_rq_fair: Select target runqueue for the waking task in domains
>>> * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE,
>>> @@ -5663,6 +5674,7 @@ struct sg_lb_stats {
>>> unsigned long sum_weighted_load; /* Weighted load of group's tasks */
>>> unsigned long load_per_task;
>>> unsigned long group_capacity;
>>> + unsigned long group_usage; /* Total usage of the group */
>>> unsigned int sum_nr_running; /* Nr tasks running in the group */
>>> unsigned int group_capacity_factor;
>>> unsigned int idle_cpus;
>>> @@ -6037,6 +6049,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>>> load = source_load(i, load_idx);
>>>
>>> sgs->group_load += load;
>>> + sgs->group_usage += get_cpu_usage(i);
>>> sgs->sum_nr_running += rq->cfs.h_nr_running;
>>>
>>> if (rq->nr_running > 1)
>>>
>>
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