linux-kernel - [PATCH v2 03/12] sched/fair: Change the variable to hold the number of periods to 32bit integer

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Message-Id: <1462226078-31904-4-git-send-email-yuyang.du@intel.com>
Date:	Tue,  3 May 2016 05:54:29 +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: [PATCH v2 03/12] sched/fair: Change the variable to hold the number of periods to 32bit integer

In sched average update, a period is about 1ms, so a 32-bit unsigned
integer can approximately hold a maximum of 49 (=2^32/1000/3600/24)
days, which means it is big enough and 64-bit is needless.

Signed-off-by: Yuyang Du <yuyang.du@...el.com>
---
 kernel/sched/fair.c |   27 +++++++++++++--------------
 1 file changed, 13 insertions(+), 14 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 74eaeab..17bc721 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2619,18 +2619,13 @@ static const u32 __accumulated_sum_N32[] = {
  * n is the number of periods past; a period is ~1ms
  * m is called half-life in exponential decay; here it is SCHED_AVG_HALFLIFE=32.
  */
-static __always_inline u64 __decay_sum(u64 val, u64 n)
+static __always_inline u64 __decay_sum(u64 val, u32 n)
 {
-	unsigned int local_n;
-
 	if (!n)
 		return val;
 	else if (unlikely(n > SCHED_AVG_HALFLIFE * 63))
 		return 0;
 
-	/* after bounds checking we can collapse to 32-bit */
-	local_n = n;
-
 	/*
 	 * As y^PERIOD = 1/2, we can combine
 	 *    y^n = 1/2^(n/PERIOD) * y^(n%PERIOD)
@@ -2638,12 +2633,12 @@ static __always_inline u64 __decay_sum(u64 val, u64 n)
 	 *
 	 * To achieve constant time decay_load.
 	 */
-	if (unlikely(local_n >= SCHED_AVG_HALFLIFE)) {
-		val >>= local_n / SCHED_AVG_HALFLIFE;
-		local_n %= SCHED_AVG_HALFLIFE;
+	if (unlikely(n >= SCHED_AVG_HALFLIFE)) {
+		val >>= n / SCHED_AVG_HALFLIFE;
+		n %= SCHED_AVG_HALFLIFE;
 	}
 
-	val = mul_u64_u32_shr(val, __decay_inv_multiply_N[local_n], 32);
+	val = mul_u64_u32_shr(val, __decay_inv_multiply_N[n], 32);
 	return val;
 }
 
@@ -2654,7 +2649,7 @@ static __always_inline u64 __decay_sum(u64 val, u64 n)
  * We can compute this efficiently by combining:
  * y^32 = 1/2 with precomputed \Sum 1024*y^n   (where n < 32)
  */
-static u32 __accumulate_sum(u64 n)
+static u32 __accumulate_sum(u32 n)
 {
 	u32 contrib = 0;
 
@@ -2708,8 +2703,8 @@ static __always_inline int
 __update_sched_avg(u64 now, int cpu, struct sched_avg *sa,
 		  unsigned long weight, int running, struct cfs_rq *cfs_rq)
 {
-	u64 delta, scaled_delta, periods;
-	u32 contrib;
+	u64 delta, scaled_delta;
+	u32 contrib, periods;
 	unsigned int delta_w, scaled_delta_w, decayed = 0;
 	unsigned long scale_freq, scale_cpu;
 
@@ -2762,7 +2757,11 @@ __update_sched_avg(u64 now, int cpu, struct sched_avg *sa,
 
 		delta -= delta_w;
 
-		/* Figure out how many additional periods this update spans */
+		/*
+		 * Figure out how many additional periods this update spans.
+		 * A period is 1024*1024ns or ~1ms, so a 32bit integer can hold
+		 * approximately a maximum of 49 (=2^32/1000/3600/24) days.
+		 */
 		periods = delta / 1024;
 		delta %= 1024;
 
-- 
1.7.9.5