Subject: Re: [PATCH 1/4] sched/fair: Remove SIS_AVG_CPU

From: Mel Gorman <[email-masked]>
Date: Tue, 8 Dec 2020 15:12:26 +0000

On Tue, Dec 08, 2020 at 03:47:40PM +0100, Vincent Guittot wrote:
> > I considered it but made the choice to exclude the cost of cpumask_and()
> > from the avg_scan_cost instead. It's minor but when doing the original
> 
> At the cost of a less readable code
> 

Slightly less readable, yes.

> > prototype, I didn't think it was appropriate to count the cpumask
> > clearing as part of the scan cost as it's not directly related.
> 
> hmm... I think it is because the number of loop is directly related to
> the allowed cpus
> 

While that is true, the cost of initialising the map is constant and
what is most important is tracking the scan cost which is variable.
Without SIS_AVG_CPU, the cpumask init can go before SIS_PROP without any
penalty so is this version preferable?

--8<--
sched/fair: Move avg_scan_cost calculations under SIS_PROP

As noted by Vincent Guittot, avg_scan_costs are calculated for SIS_PROP
even if SIS_PROP is disabled. Move the time calculations under a SIS_PROP
check and while we are at it, exclude the cost of initialising the CPU
mask from the average scan cost.

Signed-off-by: Mel Gorman <[email-masked]>
---
 kernel/sched/fair.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ac7b34e7372b..5c41875aec23 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6153,6 +6153,8 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 	if (!this_sd)
 		return -1;
 
+	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+
 	if (sched_feat(SIS_PROP)) {
 		u64 avg_cost, avg_idle, span_avg;
 
@@ -6168,11 +6170,9 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 			nr = div_u64(span_avg, avg_cost);
 		else
 			nr = 4;
-	}
-
-	time = cpu_clock(this);
 
-	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+		time = cpu_clock(this);
+	}
 
 	for_each_cpu_wrap(cpu, cpus, target) {
 		if (!--nr)
@@ -6181,8 +6181,10 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 			break;
 	}
 
-	time = cpu_clock(this) - time;
-	update_avg(&this_sd->avg_scan_cost, time);
+	if (sched_feat(SIS_PROP)) {
+		time = cpu_clock(this) - time;
+		update_avg(&this_sd->avg_scan_cost, time);
+	}
 
 	return cpu;
 }

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