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Date:   Tue, 5 Jun 2018 14:12:24 +0100
From:   Quentin Perret <quentin.perret@arm.com>
To:     Vincent Guittot <vincent.guittot@linaro.org>
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>
Subject: Re: [PATCH v5 00/10] track CPU utilization
Message-ID: <20180605131224.GC12193@e108498-lin.cambridge.arm.com>
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On Tuesday 05 Jun 2018 at 13:59:56 (+0200), Vincent Guittot wrote:
> 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

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 ?

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