Re: [PATCH] sched/fair: Revert boost in cpu_util()

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 18.05.26 04:40, hongyan.xia(夏弘彦) wrote:
> From: Hongyan Xia <hongyan.xia@transsion.com>
> 
> We have seen a massive power consumption regression (20% SoC power
> increase in many apps) after updating our kernel. After bisection we
> pinpointed the regression to the cpu_util(boost) feature. After
> reverting the boost feature the massive energy regression is gone.
> Detailed trace analysis down below. The regression is found across quite
> many apps but Youtube is one of the worst offenders, shown in the
> 1080p60fps video benchmark:
> 
>  Setup      FPS   SoC Power (mW)  diff
> w/  boost  59.94      913.6
> w/o boost  59.93      720.4     -21.15%
> 
> Signed-off-by: Hongyan Xia <hongyan.xia@transsion.com>

Looks like I missed running one of the low-power test cases back then.

That said, boosting tasks under contention does not seem like a
particularly good idea.

Jankbench, being primarily a UI rendering benchmark, was probably
overemphasizing stress on the Android Graphics Pipeline (AGP), where the
runnable_avg boosting happened to help. Support for Jankbench also
appears to have stopped with Android 12 — I am not sure why.

At the same time, low-power benchmarks, and even games, do not seem to
have equally strict requirements for consistently meeting UI rendering
deadlines.

It is also possible that the AGP itself has evolved since then. I
vaguely remember a Google Bootcamp presentation discussing the injection
of performance hints at the beginning of a frame-rendering cycle to
mitigate early jank, but I can no longer find it. I also do not know how
that approach relates to the removal of all vendor hooks.

Given all of this, I would also lean toward removing the runnable_avg
boosting functionality entirely, rather than keeping it behind a sched
feature flag that defaults to false.

> @@ -8229,16 +8222,10 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>   * Return: (Boosted) (estimated) utilization for the specified CPU.

s/(Boosted) (e/(E/

>   */
>  static unsigned long
> -cpu_util(int cpu, struct task_struct *p, int dst_cpu, int boost)
> +cpu_util(int cpu, struct task_struct *p, int dst_cpu)
>  {
>  	struct cfs_rq *cfs_rq = &cpu_rq(cpu)->cfs;
>  	unsigned long util = READ_ONCE(cfs_rq->avg.util_avg);
> -	unsigned long runnable;
> -
> -	if (boost) {
> -		runnable = READ_ONCE(cfs_rq->avg.runnable_avg);
> -		util = max(util, runnable);
> -	}
>  
>  	/*
>  	 * If @dst_cpu is -1 or @p migrates from @cpu to @dst_cpu remove its