On 26/09/14 13:17, Vincent Guittot wrote:

On 25 September 2014 21:05, Dietmar Eggemann [off-list ref] wrote:

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On 23/09/14 17:08, Vincent Guittot wrote:

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quoted

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+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.

Ok, I got it now, thanks!


When running 'hackbench -p -T -s 10 -l 1' on TC2, the usage for a cpu
goes occasionally also much higher than SCHED_LOAD_SCALE. After all,
p->se.avg.running_avg_sum is initialized to slice in
init_task_runnable_average.

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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

Makes sense, we don't want to do anything in this case on a sched level
(e.g. DIE), the appropriate level below (e.g. MC) should balance this
out first. Got it!

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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).

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+
+     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.

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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.

Understood so your current patch-set is doing uArch scaling for capacity
and since you're not doing uArch scaling for utilization, you do this '*
capacity) >> SCHED_LOAD_SHIFT' thing. Correct?

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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

With mainline dts file for ARM TC2, the rq->cpu_capacity-orig is 1024
for all 5 cpus (A15's and A7's). The arm topology shim layer barfs a

  /cpus/cpu at x missing clock-frequency property

per cpu in this case and doesn't scale the capacity. Only when I add

 clock-frequency = <xxxxxxxxx>;

per cpuX node into the dts file, I get a system with asymmetric
rq->cpu_capacity_orig values (606 for an A7 and 1441 for an A15).

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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).

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