Messages in this thread | | | Date | Fri, 1 Jun 2018 10:45:26 -0700 | From | Joel Fernandes <> | Subject | Re: [PATCH v5 05/10] cpufreq/schedutil: get max utilization |
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On Fri, Jun 01, 2018 at 03:53:07PM +0200, Vincent Guittot wrote: > > >> >> The example with a RT task described in the cover letter can be > > >> >> run with a DL task and will give similar results. > > > > > > In the cover letter you says: > > > > > > 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 would say that's a quite specific corner case where your always > > > running CFS task has never accumulated a util_est sample. > > > > > > Do we really have these cases in real systems? > > > > My example is voluntary an extreme one because it's easier to > > highlight the problem > > > > > > > > Otherwise, it seems to me that we are trying to solve quite specific > > > corner cases by adding a not negligible level of "complexity". > > > > By complexity, do you mean : > > > > Taking into account the number cfs running task to choose between > > rq->dl.running_bw and avg_dl.util_avg > > > > I'm preparing a patchset that will provide the cfs waiting time in > > addition to dl/rt util_avg for almost no additional cost. I will try > > to sent the proposal later today > > > The code below adds the tracking of the waiting level of cfs tasks because of > rt/dl preemption. This waiting time can then be used when selecting an OPP > instead of the dl util_avg which could become higher than dl bandwidth with > "long" runtime > > We need only one new call for the 1st cfs task that is enqueued to get these additional metrics > the call to arch_scale_cpu_capacity() can be removed once the later will be > taken into account when computing the load (which scales only with freq > currently) > > For rt task, we must keep to take into account util_avg to have an idea of the > rt level on the cpu which is given by the badnwodth for dl > > --- > kernel/sched/fair.c | 27 +++++++++++++++++++++++++++ > kernel/sched/pelt.c | 8 ++++++-- > kernel/sched/sched.h | 4 +++- > 3 files changed, 36 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index eac1f9a..1682ea7 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -5148,6 +5148,30 @@ static inline void hrtick_update(struct rq *rq) > } > #endif > > +static inline void update_cfs_wait_util_avg(struct rq *rq) > +{ > + /* > + * If cfs is already enqueued, we don't have anything to do because > + * we already updated the non waiting time > + */ > + if (rq->cfs.h_nr_running) > + return; > + > + /* > + * If rt is running, we update the non wait time before increasing > + * cfs.h_nr_running) > + */ > + if (rq->curr->sched_class == &rt_sched_class) > + update_rt_rq_load_avg(rq_clock_task(rq), rq, 1); > + > + /* > + * If dl is running, we update the non time before increasing > + * cfs.h_nr_running) > + */ > + if (rq->curr->sched_class == &dl_sched_class) > + update_dl_rq_load_avg(rq_clock_task(rq), rq, 1); > +} > +
Please correct me if I'm wrong but the CFS preemption-decay happens in set_next_entity -> update_load_avg when the CFS task is scheduled again after the preemption. Then can we not fix this issue by doing our UTIL_EST magic from set_next_entity? But yeah probably we need to be careful with overhead..
IMO I feel its overkill to account dl_avg when we already have DL's running bandwidth we can use. I understand it may be too instanenous, but perhaps we can fix CFS's problems within CFS itself and not have to do this kind of extra external accounting ?
I also feel its better if we don't have to call update_{rt,dl}_rq_load_avg from within CFS class as being done above.
thanks,
- Joel
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