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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Tue, 5 Jun 2018 13:59:56 +0200
Message-ID: <CAKfTPtBduambCpfeNHuaqnayUoB8J224LDHcdG9EjH8kpJApnQ@mail.gmail.com>
Subject: Re: [PATCH v5 00/10] track CPU utilization
To:     Quentin Perret <quentin.perret@arm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <Morten.Rasmussen@arm.com>,
        viresh kumar <viresh.kumar@linaro.org>,
        Valentin Schneider <valentin.schneider@arm.com>
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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
>> >
>
>