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authorYuyang Du <yuyang.du@intel.com>2016-05-03 05:54:27 +0800
committerIngo Molnar <mingo@kernel.org>2016-05-05 09:41:08 +0200
commit7b20b916e953cabef569541f991a0a583bc344cb (patch)
treed2893671351afb263793839f6b854b8be1d64be5 /kernel/sched
parent7b5953345efe4f226bb52cbea04558d16ec7ebfa (diff)
downloadop-kernel-dev-7b20b916e953cabef569541f991a0a583bc344cb.zip
op-kernel-dev-7b20b916e953cabef569541f991a0a583bc344cb.tar.gz
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@intel.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Morten Rasmussen <morten.rasmussen@arm.com> Acked-by: Vincent Guittot <vincent.guittot@linaro.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mike Galbraith <efault@gmx.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: bsegall@google.com Cc: dietmar.eggemann@arm.com Cc: juri.lelli@arm.com Cc: pjt@google.com Link: http://lkml.kernel.org/r/1462226078-31904-2-git-send-email-yuyang.du@intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/sched')
-rw-r--r--kernel/sched/fair.c21
1 files 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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