linux-kernel - [RFC PATCH 1/9] sched/fair: Chance LOAD_AVG_MAX

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Date:	Mon, 16 May 2016 02:59:26 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	peterz@...radead.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org
Cc:	bsegall@...gle.com, pjt@...gle.com, morten.rasmussen@....com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	juri.lelli@....com, Yuyang Du <yuyang.du@...el.com>
Subject: [RFC PATCH 1/9] sched/fair: Chance LOAD_AVG_MAX_N from 345 to 347

In commit 5b51f2f80b3b906ce59bd4dce6eca3c7f34cb1b9
Author: Paul Turner <pjt@...gle.com>
Date:   Thu Oct 4 13:18:32 2012 +0200

    sched: Make __update_entity_runnable_avg() fast

Paul has a program to compute LOAD_AVG_MAX_N, which basically means
how many periods (at least) are needed for LOAD_AVG_MAX, and the result
of calc_conv(1024) is 345:

  long mult_inv(long c, int n) {
  	return (c * runnable_avg_yN_inv[n]) >>  WMULT_SHIFT;
  }

  void calc_conv(long n) {
  	long old_n;
  	int i = -1;

  	printf("convergence (LOAD_AVG_MAX, LOAD_AVG_MAX_N)\n");
  	do {
  		old_n = n;
  		n = mult_inv(n, 1) + 1024;
  		i++;
  	} while (n != old_n);
  	printf("%d> %ld\n", i - 1, n);
  	printf("\n");
  }

The initial value of i is -1, which should be 1 as far as I can tell.
Accordingly, the final result of LOAD_AVG_MAX_N should be changed
from 345 to 347.

Signed-off-by: Yuyang Du <yuyang.du@...el.com>
---
 kernel/sched/fair.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 218f8e8..2635561 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -666,7 +666,7 @@ static unsigned long task_h_load(struct task_struct *p);
  */
 #define LOAD_AVG_PERIOD 32
 #define LOAD_AVG_MAX 47742 /* maximum possible load avg */
-#define LOAD_AVG_MAX_N 345 /* number of full periods to produce LOAD_AVG_MAX */
+#define LOAD_AVG_MAX_N 347 /* number of full periods to produce LOAD_AVG_MAX */
 
 /* Give new sched_entity start runnable values to heavy its load in infant time */
 void init_entity_runnable_average(struct sched_entity *se)
-- 
1.7.9.5