Commit-ID: 7b20b916e953cabef569541f991a0a583bc344cb
Gitweb: http://git.kernel.org/tip/7b20b916e953cabef569541f991a0a583bc344cb
Author: Yuyang Du <yuyang.du@xxxxxxxxx>
AuthorDate: Tue, 3 May 2016 05:54:27 +0800
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 5 May 2016 09:41:08 +0200
sched/fair: Optimize sum computation with a lookup table
__compute_runnable_contrib() uses a loop to compute sum, whereas a
table lookup can do it faster in a constant amount of time.
The program to generate the constants is located at:
Documentation/scheduler/sched-avg.txt
Signed-off-by: Yuyang Du <yuyang.du@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Morten Rasmussen <morten.rasmussen@xxxxxxx>
Acked-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: bsegall@xxxxxxxxxx
Cc: dietmar.eggemann@xxxxxxx
Cc: juri.lelli@xxxxxxx
Cc: pjt@xxxxxxxxxx
Link: http://lkml.kernel.org/r/1462226078-31904-2-git-send-email-yuyang.du@xxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/sched/fair.c | 21 +++++++++++++--------
1 file changed, 13 insertions(+), 8 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e148571..8c381a6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2603,6 +2603,16 @@ static const u32 runnable_avg_yN_sum[] = {
};
/*
+ * Precomputed \Sum y^k { 1<=k<=n, where n%32=0). Values are rolled down to
+ * lower integers. See Documentation/scheduler/sched-avg.txt how these
+ * were generated:
+ */
+static const u32 __accumulated_sum_N32[] = {
+ 0, 23371, 35056, 40899, 43820, 45281,
+ 46011, 46376, 46559, 46650, 46696, 46719,
+};
+
+/*
* Approximate:
* val * y^n, where y^32 ~= 0.5 (~1 scheduling period)
*/
@@ -2650,14 +2660,9 @@ static u32 __compute_runnable_contrib(u64 n)
else if (unlikely(n >= LOAD_AVG_MAX_N))
return LOAD_AVG_MAX;
- /* Compute \Sum k^n combining precomputed values for k^i, \Sum k^j */
- do {
- contrib /= 2; /* y^LOAD_AVG_PERIOD = 1/2 */
- contrib += runnable_avg_yN_sum[LOAD_AVG_PERIOD];
-
- n -= LOAD_AVG_PERIOD;
- } while (n > LOAD_AVG_PERIOD);
-
+ /* Since n < LOAD_AVG_MAX_N, n/LOAD_AVG_PERIOD < 11 */
+ contrib = __accumulated_sum_N32[n/LOAD_AVG_PERIOD];
+ n %= LOAD_AVG_PERIOD;
contrib = decay_load(contrib, n);
return contrib + runnable_avg_yN_sum[n];
}
--
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