Re: [PATCH] sched/fair: update scale invariance of pelt

[Yuyang Du <yuyang.du@intel.com>]

Hi Vincent,

I don't quite catch what this is doing, maybe I need more time
to ramp up to the gory detail difficult like this.

Do you scale or not scale? You seem removed the scaling, but added it
after "Remainder of delta accrued against u_0"..

Thanks,
Yuyang

On Tue, Nov 24, 2015 at 02:49:30PM +0100, Vincent Guittot wrote:
> The current implementation of load tracking invariance scales the load
> tracking value with current frequency and uarch performance (only for
> utilization) of the CPU.
> 
> One main result of the current formula is that the figures are capped by
> the current capacity of the CPU. This limitation is the main reason of not
> including the uarch invariance (arch_scale_cpu_capacity) in the calculation
> of load_avg because capping the load can generate erroneous system load
> statistic as described with this example [1]
> 
> Instead of scaling the complete value of PELT algo, we should only scale
> the running time by the current capacity of the CPU. It seems more correct
> to only scale the running time because the non running time of a task
> (sleeping or waiting for a runqueue) is the same whatever the current freq
> and the compute capacity of the CPU.
> 
> Then, one main advantage of this change is that the load of a task can
> reach max value whatever the current freq and the uarch of the CPU on which
> it run. It will just take more time at a lower freq than a max freq or on a
> "little" CPU compared to a "big" one. The load and the utilization stay
> invariant across system so we can still compared them between CPU but with
> a wider range of values.
> 
> With this change, we don't have to test if a CPU is overloaded or not in
> order to use one metric (util) or another (load) as all metrics are always
> valid.
> 
> I have put below some examples of duration to reach some typical load value
> according to the capacity of the CPU with current implementation
> and with this patch. 
> 
> Util (%)     max capacity  half capacity(mainline)  half capacity(w/ patch)
> 972 (95%)    138ms	   not reachable	    276ms
> 486 (47.5%)  30ms	   138ms		     60ms
> 256 (25%)    13ms	    32ms		     26ms
> 
> We can see that at half capacity, we need twice the duration of max
> capacity with this patch whereas we have a non linear increase of the
> duration with current implementation.
> 
> [1] https://lkml.org/lkml/2014/12/18/128
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  kernel/sched/fair.c | 28 +++++++++++++---------------
>  1 file changed, 13 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 824aa9f..f2a18e1 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2560,10 +2560,9 @@ static __always_inline int
>  __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
>  		  unsigned long weight, int running, struct cfs_rq *cfs_rq)
>  {
> -	u64 delta, scaled_delta, periods;
> +	u64 delta, periods;
>  	u32 contrib;
> -	unsigned int delta_w, scaled_delta_w, decayed = 0;
> -	unsigned long scale_freq, scale_cpu;
> +	unsigned int delta_w, decayed = 0;
>  
>  	delta = now - sa->last_update_time;
>  	/*
> @@ -2584,8 +2583,10 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
>  		return 0;
>  	sa->last_update_time = now;
>  
> -	scale_freq = arch_scale_freq_capacity(NULL, cpu);
> -	scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
> +	if (running) {
> +		delta = cap_scale(delta, arch_scale_freq_capacity(NULL, cpu));
> +		delta = cap_scale(delta, arch_scale_cpu_capacity(NULL, cpu));
> +	}
>  
>  	/* delta_w is the amount already accumulated against our next period */
>  	delta_w = sa->period_contrib;
> @@ -2601,16 +2602,15 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
>  		 * period and accrue it.
>  		 */
>  		delta_w = 1024 - delta_w;
> -		scaled_delta_w = cap_scale(delta_w, scale_freq);
>  		if (weight) {
> -			sa->load_sum += weight * scaled_delta_w;
> +			sa->load_sum += weight * delta_w;
>  			if (cfs_rq) {
>  				cfs_rq->runnable_load_sum +=
> -						weight * scaled_delta_w;
> +						weight * delta_w;
>  			}
>  		}
>  		if (running)
> -			sa->util_sum += scaled_delta_w * scale_cpu;
> +			sa->util_sum += delta_w << SCHED_CAPACITY_SHIFT;
>  
>  		delta -= delta_w;
>  
> @@ -2627,25 +2627,23 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
>  
>  		/* Efficiently calculate \sum (1..n_period) 1024*y^i */
>  		contrib = __compute_runnable_contrib(periods);
> -		contrib = cap_scale(contrib, scale_freq);
>  		if (weight) {
>  			sa->load_sum += weight * contrib;
>  			if (cfs_rq)
>  				cfs_rq->runnable_load_sum += weight * contrib;
>  		}
>  		if (running)
> -			sa->util_sum += contrib * scale_cpu;
> +			sa->util_sum += contrib << SCHED_CAPACITY_SHIFT;
>  	}
>  
>  	/* Remainder of delta accrued against u_0` */
> -	scaled_delta = cap_scale(delta, scale_freq);
>  	if (weight) {
> -		sa->load_sum += weight * scaled_delta;
> +		sa->load_sum += weight * delta;
>  		if (cfs_rq)
> -			cfs_rq->runnable_load_sum += weight * scaled_delta;
> +			cfs_rq->runnable_load_sum += weight * delta;
>  	}
>  	if (running)
> -		sa->util_sum += scaled_delta * scale_cpu;
> +		sa->util_sum += delta << SCHED_CAPACITY_SHIFT;
>  
>  	sa->period_contrib += delta;
>  
> -- 
> 1.9.1