Re: [PATCH 2/2] sched/fair: util_est: add running_sum tracking

Joel Fernandes
2018-06-04T17:46:18Z

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