Re: [PATCH v5 00/10] track CPU utilization

Vincent Guittot
2018-06-05T12:00:19Z

On 5 June 2018 at 12:57, Quentin Perret <quentin.perret@arm.com> wrote:
> Hi Vincent,
>
> On Tuesday 05 Jun 2018 at 10:36:26 (+0200), Vincent Guittot wrote:
>> Hi Quentin,
>>
>> On 25 May 2018 at 15:12, Vincent Guittot <vincent.guittot@linaro.org> wrote:
>> > This patchset initially tracked only the utilization of RT rq. During
>> > OSPM summit, it has been discussed the opportunity to extend it in order
>> > to get an estimate of the utilization of the CPU.
>> >
>> > - Patches 1-3 correspond to the content of patchset v4 and add utilization
>> > tracking for rt_rq.
>> >
>> > When both cfs and rt tasks compete to run on a CPU, we can see some frequency
>> > drops with schedutil governor. In such case, the cfs_rq's utilization doesn't
>> > reflect anymore the utilization of cfs tasks but only the remaining part that
>> > is not used by rt tasks. We should monitor the stolen utilization and take
>> > it into account when selecting OPP. This patchset doesn't change the OPP
>> > selection policy for RT tasks but only for CFS tasks
>> >
>> > A rt-app use case which creates an always running cfs thread and a rt threads
>> > that wakes up periodically with both threads pinned on same CPU, show lot of
>> > frequency switches of the CPU whereas the CPU never goes idles during the
>> > test. I can share the json file that I used for the test if someone is
>> > interested in.
>> >
>> > For a 15 seconds long test on a hikey 6220 (octo core cortex A53 platfrom),
>> > the cpufreq statistics outputs (stats are reset just before the test) :
>> > $ cat /sys/devices/system/cpu/cpufreq/policy0/stats/total_trans
>> > without patchset : 1230
>> > with patchset : 14
>>
>> I have attached the rt-app json file that I use for this test
>
> Thank you very much ! I did a quick test with a much simpler fix to this
> RT-steals-time-from-CFS issue using just the existing scale_rt_capacity().
> I get the following results on Hikey960:
>
> Without patch:
> cat /sys/devices/system/cpu/cpufreq/policy0/stats/total_trans
> 12
> cat /sys/devices/system/cpu/cpufreq/policy4/stats/total_trans
> 640
> With patch
> cat /sys/devices/system/cpu/cpufreq/policy0/stats/total_trans
> 8
> cat /sys/devices/system/cpu/cpufreq/policy4/stats/total_trans
> 12
>
> Yes the rt_avg stuff is out of sync with the PELT signal, but do you think
> this is an actual issue for realistic use-cases ?

yes I think that it's worth syncing and consolidating things on the
same metric. The result will be saner and more robust as we will have
the same behavior

>
> What about the diff below (just a quick hack to show the idea) applied
> on tip/sched/core ?
>
> ---8<---
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index a8ba6d1f262a..23a4fb1c2c25 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -180,9 +180,12 @@ static void sugov_get_util(struct sugov_cpu *sg_cpu)
> sg_cpu->util_dl = cpu_util_dl(rq);
> }
>
> +unsigned long scale_rt_capacity(int cpu);
> static unsigned long sugov_aggregate_util(struct sugov_cpu *sg_cpu)
> {
> struct rq *rq = cpu_rq(sg_cpu->cpu);
> + int cpu = sg_cpu->cpu;
> + unsigned long util, dl_bw;
>
> if (rq->rt.rt_nr_running)
> return sg_cpu->max;
> @@ -197,7 +200,14 @@ static unsigned long sugov_aggregate_util(struct sugov_cpu *sg_cpu)
> * util_cfs + util_dl as requested freq. However, cpufreq is not yet
> * ready for such an interface. So, we only do the latter for now.
> */
> - return min(sg_cpu->max, (sg_cpu->util_dl + sg_cpu->util_cfs));
> + util = arch_scale_cpu_capacity(NULL, cpu) * scale_rt_capacity(cpu);
> + util >>= SCHED_CAPACITY_SHIFT;
> + util = arch_scale_cpu_capacity(NULL, cpu) - util;
> + util += sg_cpu->util_cfs;
> + dl_bw = (rq->dl.this_bw * SCHED_CAPACITY_SCALE) >> BW_SHIFT;
> +
> + /* Make sure to always provide the reserved freq to DL. */
> + return max(util, dl_bw);
> }
>
> static void sugov_set_iowait_boost(struct sugov_cpu *sg_cpu, u64 time, unsigned int flags)
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f01f0f395f9a..0e87cbe47c8b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7868,7 +7868,7 @@ static inline int get_sd_load_idx(struct sched_domain *sd,
> return load_idx;
> }
>
> -static unsigned long scale_rt_capacity(int cpu)
> +unsigned long scale_rt_capacity(int cpu)
> {
> struct rq *rq = cpu_rq(cpu);
> u64 total, used, age_stamp, avg;
> --->8---
>
>
>
>>
>> >
>> > If we replace the cfs thread of rt-app by a sysbench cpu test, we can see
>> > performance improvements:
>> >
>> > - Without patchset :
>> > Test execution summary:
>> > total time: 15.0009s
>> > total number of events: 4903
>> > total time taken by event execution: 14.9972
>> > per-request statistics:
>> > min: 1.23ms
>> > avg: 3.06ms
>> > max: 13.16ms
>> > approx. 95 percentile: 12.73ms
>> >
>> > Threads fairness:
>> > events (avg/stddev): 4903.0000/0.00
>> > execution time (avg/stddev): 14.9972/0.00
>> >
>> > - With patchset:
>> > Test execution summary:
>> > total time: 15.0014s
>> > total number of events: 7694
>> > total time taken by event execution: 14.9979
>> > per-request statistics:
>> > min: 1.23ms
>> > avg: 1.95ms
>> > max: 10.49ms
>> > approx. 95 percentile: 10.39ms
>> >
>> > Threads fairness:
>> > events (avg/stddev): 7694.0000/0.00
>> > execution time (avg/stddev): 14.9979/0.00
>> >
>> > The performance improvement is 56% for this use case.
>> >
>> > - Patches 4-5 add utilization tracking for dl_rq in order to solve similar
>> > problem as with rt_rq
>> >
>> > - Patches 6 uses dl and rt utilization in the scale_rt_capacity() and remove
>> > dl and rt from sched_rt_avg_update
>> >
>> > - Patches 7-8 add utilization tracking for interrupt and use it select OPP
>> > A test with iperf on hikey 6220 gives:
>> > w/o patchset w/ patchset
>> > Tx 276 Mbits/sec 304 Mbits/sec +10%
>> > Rx 299 Mbits/sec 328 Mbits/sec +09%
>> >
>> > 8 iterations of iperf -c server_address -r -t 5
>> > stdev is lower than 1%
>> > Only WFI idle state is enable (shallowest arm idle state)
>> >
>> > - Patches 9 removes the unused sched_avg_update code
>> >
>> > - Patch 10 removes the unused sched_time_avg_ms
>> >
>> > Change since v3:
>> > - add support of periodic update of blocked utilization
>> > - rebase on lastest tip/sched/core
>> >
>> > Change since v2:
>> > - move pelt code into a dedicated pelt.c file
>> > - rebase on load tracking changes
>> >
>> > Change since v1:
>> > - Only a rebase. I have addressed the comments on previous version in
>> > patch 1/2
>> >
>> > Vincent Guittot (10):
>> > sched/pelt: Move pelt related code in a dedicated file
>> > sched/rt: add rt_rq utilization tracking
>> > cpufreq/schedutil: add rt utilization tracking
>> > sched/dl: add dl_rq utilization tracking
>> > cpufreq/schedutil: get max utilization
>> > sched: remove rt and dl from sched_avg
>> > sched/irq: add irq utilization tracking
>> > cpufreq/schedutil: take into account interrupt
>> > sched: remove rt_avg code
>> > proc/sched: remove unused sched_time_avg_ms
>> >
>> > include/linux/sched/sysctl.h | 1 -
>> > kernel/sched/Makefile | 2 +-
>> > kernel/sched/core.c | 38 +---
>> > kernel/sched/cpufreq_schedutil.c | 24 ++-
>> > kernel/sched/deadline.c | 7 +-
>> > kernel/sched/fair.c | 381 +++----------------------------------
>> > kernel/sched/pelt.c | 395 +++++++++++++++++++++++++++++++++++++++
>> > kernel/sched/pelt.h | 63 +++++++
>> > kernel/sched/rt.c | 10 +-
>> > kernel/sched/sched.h | 57 ++++--
>> > kernel/sysctl.c | 8 -
>> > 11 files changed, 563 insertions(+), 423 deletions(-)
>> > create mode 100644 kernel/sched/pelt.c
>> > create mode 100644 kernel/sched/pelt.h
>> >
>> > --
>> > 2.7.4
>> >
>
>