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

[Joel Fernandes]

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@xxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Cc: Todd Kjos <tkjos@xxxxxxxxxx>
> Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
> Cc: Steve Muckle <smuckle@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Morten Rasmussen <morten.rasmussen@xxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-pm@xxxxxxxxxxxxxxx
> ---
>  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