[PATCH] x86,sched: Fix sched_smt_power_savings totally broken

From: Youquan Song
Date: Mon Jan 09 2012 - 04:21:25 EST

sched_smt_power_savings is totally broken at lastest linux and -tip tree.

sched_smt_power_savings is set to 1, the scheduler tries to schedule processes
on the least number of hyper-threads on a core as possible. In other words,
the process load is distributed such that all the hyper-threads in a core and
all the cores within the same processor are busy before the load is distributed
to other hyper-threads and cores in another processor.

Test On Intel Xeon machine with 2 physical CPUs and each CPU has 8 cores / 16
threads. physical CPU 0 includes cpu[0~7] and cpu[16~23]; while physical CPU 1
includes cpu[8~15] and cpu[24~31].

At latest -tip tree:
echo 1 > /sys/devices/system/cpu/sched_smt_power_savings
./ebizzy -t 16 -S 100 & sleep 10 ; cat /proc/sched_debug | grep -A 1 cpu# > tmp.log

cpu#0, 2693.564 MHz
.nr_running : 0
--
cpu#1, 2693.564 MHz
.nr_running : 1
--
cpu#2, 2693.564 MHz
.nr_running : 0
--
cpu#3, 2693.564 MHz
.nr_running : 1
--
cpu#4, 2693.564 MHz
.nr_running : 1
--
cpu#5, 2693.564 MHz
.nr_running : 1
--
cpu#6, 2693.564 MHz
.nr_running : 1
--
cpu#7, 2693.564 MHz
.nr_running : 1
--
cpu#8, 2693.564 MHz
.nr_running : 1
--
cpu#9, 2693.564 MHz
.nr_running : 1
--
cpu#10, 2693.564 MHz
.nr_running : 1
--
cpu#11, 2693.564 MHz
.nr_running : 0
--
cpu#12, 2693.564 MHz
.nr_running : 1
--
cpu#13, 2693.564 MHz
.nr_running : 1
--
cpu#14, 2693.564 MHz
.nr_running : 0
--
cpu#15, 2693.564 MHz
.nr_running : 0
--
cpu#16, 2693.564 MHz
.nr_running : 0
--
cpu#17, 2693.564 MHz
.nr_running : 0
--
cpu#18, 2693.564 MHz
.nr_running : 1
--
cpu#19, 2693.564 MHz
.nr_running : 0
--
cpu#20, 2693.564 MHz
.nr_running : 0
--
cpu#21, 2693.564 MHz
.nr_running : 0
--
cpu#22, 2693.564 MHz
.nr_running : 0
--
cpu#23, 2693.564 MHz
.nr_running : 0
--
cpu#24, 2693.564 MHz
.nr_running : 0
--
cpu#25, 2693.564 MHz
.nr_running : 1
--
cpu#26, 2693.564 MHz
.nr_running : 1
--
cpu#27, 2693.564 MHz
.nr_running : 1
--
cpu#28, 2693.564 MHz
.nr_running : 0
--
cpu#29, 2693.564 MHz
.nr_running : 0
--
cpu#30, 2693.564 MHz
.nr_running : 1
--
cpu#31, 2693.564 MHz
.nr_running : 1

>From above, we notice 16 threads are distributed among 2 physical CPUs.
After apply the patch, 16 threads are only distributed at one physical CPU.
In this case, we can notice 30% power saving.
Following are the result after apply the patch:

cpu#0, 2693.384 MHz
.nr_running : 1
--
cpu#1, 2693.384 MHz
.nr_running : 1
--
cpu#2, 2693.384 MHz
.nr_running : 1
--
cpu#3, 2693.384 MHz
.nr_running : 1
--
cpu#4, 2693.384 MHz
.nr_running : 1
--
cpu#5, 2693.384 MHz
.nr_running : 1
--
cpu#6, 2693.384 MHz
.nr_running : 1
--
cpu#7, 2693.384 MHz
.nr_running : 1
--
cpu#8, 2693.384 MHz
.nr_running : 0
--
cpu#9, 2693.384 MHz
.nr_running : 0
--
cpu#10, 2693.384 MHz
.nr_running : 0
--
cpu#11, 2693.384 MHz
.nr_running : 0
--
cpu#12, 2693.384 MHz
.nr_running : 0
--
cpu#13, 2693.384 MHz
.nr_running : 0
--
cpu#14, 2693.384 MHz
.nr_running : 0
--
cpu#15, 2693.384 MHz
.nr_running : 0
--
cpu#16, 2693.384 MHz
.nr_running : 1
--
cpu#17, 2693.384 MHz
.nr_running : 1
--
cpu#18, 2693.384 MHz
.nr_running : 1
--
cpu#19, 2693.384 MHz
.nr_running : 1
--
cpu#20, 2693.384 MHz
.nr_running : 1
--
cpu#21, 2693.384 MHz
.nr_running : 1
--
cpu#22, 2693.384 MHz
.nr_running : 1
--
cpu#23, 2693.384 MHz
.nr_running : 1
--
cpu#24, 2693.384 MHz
.nr_running : 0
--
cpu#25, 2693.384 MHz
.nr_running : 0
--
cpu#26, 2693.384 MHz
.nr_running : 0
--
cpu#27, 2693.384 MHz
.nr_running : 0
--
cpu#28, 2693.384 MHz
.nr_running : 0
--
cpu#29, 2693.384 MHz
.nr_running : 0
--
cpu#30, 2693.384 MHz
.nr_running : 0
--
cpu#31, 2693.384 MHz
.nr_running : 1


This patch will set SMT sibling power capability to SCHED_POWER_SCALE
(1024) when sched_smt_power_savings set. So when there is possible do power
saving during scheduling, scheduler will truly schedule processes as
sched_smt_power_savings should do.


Signed-off-by: Youquan Song <youquan.song@xxxxxxxxx>
Tested-by: Anhua Xu <anhua.xu@xxxxxxxxx>
---
kernel/sched/fair.c | 3 +++
1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a4d2b7a..5be1d43 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3715,6 +3715,9 @@ unsigned long default_scale_smt_power(struct sched_domain *sd, int cpu)
unsigned long weight = sd->span_weight;
unsigned long smt_gain = sd->smt_gain;

+ if (sched_smt_power_savings)
+ return SCHED_POWER_SCALE;
+
smt_gain /= weight;

return smt_gain;
--
1.6.4.2

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