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Date:   Tue, 5 Jun 2018 14:52:15 +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: <20180605135215.GD12193@e108498-lin.cambridge.arm.com>
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On Tuesday 05 Jun 2018 at 15:18:38 (+0200), Vincent Guittot wrote:
> On 5 June 2018 at 15:12, Quentin Perret <quentin.perret@arm.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@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 ?
> 
> 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 :-)

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