Messages in this thread | | | Date | Fri, 26 Sep 2014 20:57:43 +0100 | From | Dietmar Eggemann <> | Subject | Re: [PATCH v6 4/6] sched: get CPU's usage statistic |
| |
On 26/09/14 13:17, 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:
[...]
>>> >>> +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.
> >> >> 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!
> >> >> 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. > >> >> 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?
> >> 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@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).
> >> >> 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).
[...]
| |