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

Quentin Perret
2018-06-05T13:52:20Z

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