Messages in this thread | | | Date | Mon, 4 Jun 2018 10:46:18 -0700 | From | Joel Fernandes <> | Subject | Re: [PATCH 2/2] sched/fair: util_est: add running_sum tracking |
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Hi Patrick,
On Mon, Jun 04, 2018 at 05:06:00PM +0100, Patrick Bellasi wrote: > The estimated utilization of a task is affected by the task being > preempted, either by another FAIR task of by a task of an higher > priority class (i.e. RT or DL). Indeed, when a preemption happens, the > PELT utilization of the preempted task is going to be decayed a bit. > That's actually correct for utilization, which goal is to measure the > actual CPU bandwidth consumed by a task. > > However, the above behavior does not allow to know exactly what is the > utilization a task "would have used" if it was running without > being preempted. Thus, this reduces the effectiveness of util_est for a > task because it does not always allow to predict how much CPU a task is > likely to require. > > Let's improve the estimated utilization by adding a new "sort-of" PELT > signal, explicitly only for SE which has the following behavior: > a) at each enqueue time of a task, its value is the (already decayed) > util_avg of the task being enqueued > b) it's updated at each update_load_avg > c) it can just increase, whenever the task is actually RUNNING on a > CPU, while it's kept stable while the task is RUNNANBLE but not > actively consuming CPU bandwidth > > Such a defined signal is exactly equivalent to the util_avg for a task > running alone on a CPU while, in case the task is preempted, it allows > to know at dequeue time how much would have been the task utilization if > it was running alone on that CPU. > > This new signal is named "running_avg", since it tracks the actual > RUNNING time of a task by ignoring any form of preemption. > > From an implementation standpoint, since the sched_avg should fit into a > single cache line, we save space by tracking only a new runnable sum: > p->se.avg.running_sum > while the conversion into a running_avg is done on demand whenever we > need it, which is at task dequeue time when a new util_est sample has to > be collected. > > The conversion from "running_sum" to "running_avg" is done by performing > a single division by LOAD_AVG_MAX, which introduces a small error since > in the division we do not consider the (sa->period_contrib - 1024) > compensation factor used in ___update_load_avg(). However: > a) this error is expected to be limited (~2-3%) > b) it can be safely ignored since the estimated utilization is the only > consumer which is already subject to small estimation errors > > The additional corresponding benefit is that, at run-time, we pay the > cost for a additional sum and multiply, while the more expensive > division is required only at dequeue time. > > Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com> > Cc: Ingo Molnar <mingo@redhat.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Vincent Guittot <vincent.guittot@linaro.org> > Cc: Juri Lelli <juri.lelli@redhat.com> > Cc: Todd Kjos <tkjos@google.com> > Cc: Joel Fernandes <joelaf@google.com> > Cc: Steve Muckle <smuckle@google.com> > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com> > Cc: Morten Rasmussen <morten.rasmussen@arm.com> > Cc: linux-kernel@vger.kernel.org > Cc: linux-pm@vger.kernel.org > --- > include/linux/sched.h | 1 + > kernel/sched/fair.c | 16 ++++++++++++++-- > 2 files changed, 15 insertions(+), 2 deletions(-) > > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 9d8732dab264..2bd5f1c68da9 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -399,6 +399,7 @@ struct sched_avg { > u64 load_sum; > u64 runnable_load_sum; > u32 util_sum; > + u32 running_sum; > u32 period_contrib; > unsigned long load_avg; > unsigned long runnable_load_avg;
Should update the documentation comments above the struct too?
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index f74441be3f44..5d54d6a4c31f 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -3161,6 +3161,8 @@ accumulate_sum(u64 delta, int cpu, struct sched_avg *sa, > sa->runnable_load_sum = > decay_load(sa->runnable_load_sum, periods); > sa->util_sum = decay_load((u64)(sa->util_sum), periods); > + if (running) > + sa->running_sum = decay_load(sa->running_sum, periods); > > /* > * Step 2 > @@ -3176,8 +3178,10 @@ accumulate_sum(u64 delta, int cpu, struct sched_avg *sa, > sa->load_sum += load * contrib; > if (runnable) > sa->runnable_load_sum += runnable * contrib; > - if (running) > + if (running) { > sa->util_sum += contrib * scale_cpu; > + sa->running_sum += contrib * scale_cpu; > + } > > return periods; > } > @@ -3963,6 +3967,12 @@ static inline void util_est_enqueue(struct cfs_rq *cfs_rq, > WRITE_ONCE(cfs_rq->avg.util_est.enqueued, enqueued); > }
PELT changes look nice and makes sense :)
> +static inline void util_est_enqueue_running(struct task_struct *p) > +{ > + /* Initilize the (non-preempted) utilization */ > + p->se.avg.running_sum = p->se.avg.util_sum; > +} > + > /* > * Check if a (signed) value is within a specified (unsigned) margin, > * based on the observation that: > @@ -4018,7 +4028,7 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep) > * Skip update of task's estimated utilization when its EWMA is > * already ~1% close to its last activation value. > */ > - ue.enqueued = (task_util(p) | UTIL_AVG_UNCHANGED); > + ue.enqueued = p->se.avg.running_sum / LOAD_AVG_MAX;
I guess we are doing extra division here which adds some cost. Does performance look Ok with the change?
thanks,
- Joel
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