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

Quentin Perret
2018-06-05T13:12:29Z

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
> >> >
> >
> >