Date:   Tue, 5 Jun 2018 15:55:43 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Quentin Perret <quentin.perret@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        viresh kumar <viresh.kumar@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH v5 00/10] track CPU utilization

On 5 June 2018 at 15:52, Quentin Perret <quentin.perret@....com> wrote:
> On Tuesday 05 Jun 2018 at 15:18:38 (+0200), Vincent Guittot wrote:
>> On 5 June 2018 at 15:12, Quentin Perret <quentin.perret@....com> wrote:
>> > On Tuesday 05 Jun 2018 at 13:59:56 (+0200), Vincent Guittot wrote:
>> >> On 5 June 2018 at 12:57, Quentin Perret <quentin.perret@....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@...aro.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
>> >
>> > TBH I'm not disagreeing with that, the PELT-everywhere approach feels
>> > cleaner in a way, but do you have a use-case in mind where this will
>> > definitely help ?
>> >
>> > I mean, yes the rt_avg is a slow response to the RT pressure, but is
>> > this always a problem ? Ramping down slower might actually help in some
>> > cases no ?
>>
>> I would say no because when one will decrease the other one will not
>> increase at the same pace and we will have some wrong behavior or
>> decision
>
> I think I get your point. Yes, sometimes, the slow-moving rt_avg can be
> off a little bit (which can be good or bad, depending in the case) if your
> RT task runs a lot with very changing behaviour. And again, I'm not
> fundamentally against the idea of having extra complexity for RT/IRQ PELT
> signals _if_ we have a use-case. But is there a real use-case where we
> really need all of that ? That's a true question, I honestly don't have
> the answer :-)

The iperf test result is another example of the benefit

>
>>
>> >
>> >>
>> >> >
>> >> > 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
>> >> >> >
>> >> >
>> >> >

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