linux-kernel - Re: [PATCH] sched: Take thermal pressure into account when determine rt fits capacity

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Date:   Tue, 3 May 2022 15:43:52 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Xuewen Yan <xuewen.yan94@...il.com>
Cc:     Xuewen Yan <xuewen.yan@...soc.com>, dietmar.eggemann@....com,
        lukasz.luba@....com, rafael@...nel.org, viresh.kumar@...aro.org,
        mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        rostedt@...dmis.org, linux-kernel@...r.kernel.org,
        di.shen@...soc.com
Subject: Re: [PATCH] sched: Take thermal pressure into account when determine
 rt fits capacity

Hi Xuewen

On 05/01/22 11:20, Xuewen Yan wrote:
> Hi Qais
> Thanks for the patient explanation.:)
> And I have some other concerns.
> 
> On Wed, Apr 27, 2022 at 6:58 PM Qais Yousef <qais.yousef@....com> wrote:
> >
> > On 04/27/22 09:38, Xuewen Yan wrote:
> > > > > > The best (simplest) way forward IMHO is to introduce a new function
> > > > > >
> > > > > >         bool cpu_in_capacity_inversion(int cpu);
> 
> Maybe the implementation of this function, I have not thought of a
> good solution.
> (1)how to define the inversion, if the cpu has two
> cluster(little/big)，it is easy, but still need mark which is the big
> cpu...

I'd define it as:

	capacity_orig_of(cpu) - thermal_pressure(cpu) < capacity_orig_of(next_level_cpu)

> (2)because the mainline kernel should be common, if the cpu has three
> or more clusters, maybe the mid cpus also would be inversion;

Yes. I pray this is highly unlikely though! We should cater for it still.

> (3)what time update the cpu inversion state, if we judge the cpu
> inversion whenever the thermal pressure changed, could we receive the
> complexity? because may we should traverse all possible cpu.

In my head, it would make sense to detect the inversion in
update_cpu_capacity() OR in topology_update_thermal_pressure(). So at whatever
rate this happens at.

Does this answer your question?

Basically I believe something like this should be enough (completely untested)

--->8---


diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a68482d66535..44c7c2598d87 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8399,16 +8399,37 @@ static unsigned long scale_rt_capacity(int cpu)
 
 static void update_cpu_capacity(struct sched_domain *sd, int cpu)
 {
+	unsigned long capacity_orig = arch_scale_cpu_capacity(cpu);
 	unsigned long capacity = scale_rt_capacity(cpu);
 	struct sched_group *sdg = sd->groups;
+	struct rq *rq = cpu_rq(cpu);
 
-	cpu_rq(cpu)->cpu_capacity_orig = arch_scale_cpu_capacity(cpu);
+	rq->cpu_capacity_orig = capacity_orig;
 
 	if (!capacity)
 		capacity = 1;
 
-	cpu_rq(cpu)->cpu_capacity = capacity;
-	trace_sched_cpu_capacity_tp(cpu_rq(cpu));
+	rq->cpu_capacity = capacity;
+	trace_sched_cpu_capacity_tp(rq);
+
+	if (static_branch_unlikely(&sched_asym_cpucapacity)) {
+		unsigned long inv_cap = capacity_orig - thermal_load_avg(rq);
+
+		rq->cpu_capacity_inverted = 0;
+
+		for_each_possible_cpu(cpu) {
+			unsigned long cap = arch_scale_cpu_capacity(cpu);
+
+			if (capacity_orig <= cap)
+				continue;
+
+			if (cap > inv_cap) {
+				rq->cpu_capacity_inverted = inv_cap;
+				break;
+			}
+		}
+
+	}
 
 	sdg->sgc->capacity = capacity;
 	sdg->sgc->min_capacity = capacity;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 8dccb34eb190..bfe84c870bf9 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -992,6 +992,7 @@ struct rq {
 
 	unsigned long		cpu_capacity;
 	unsigned long		cpu_capacity_orig;
+	unsigned long		cpu_capacity_inverted;
 
 	struct callback_head	*balance_callback;
 
@@ -2807,6 +2808,11 @@ static inline unsigned long capacity_orig_of(int cpu)
 	return cpu_rq(cpu)->cpu_capacity_orig;
 }
 
+static inline unsigned long cpu_in_capacity_inversion(int cpu)
+{
+	return cpu_rq(cpu)->cpu_capacity_inverted;
+}
+
 /**
  * enum cpu_util_type - CPU utilization type
  * @FREQUENCY_UTIL:	Utilization used to select frequency


--->8---

Thanks

--
Qais Yousef