[Skiboot] [PATCH] power9-dd1:slw: Modify the stop0_lite latency & residency.

[Gautham R. Shenoy]

From: "Gautham R. Shenoy" <ego at linux.vnet.ibm.com>

Currently skiboot exposes the exit-latency for stop0_lite as 200ns and
the target-residency to be 2us.

However, the kernel cpu-idle infrastructure rounds up the latency to
microseconds and lists the stop0_lite latency as 0us, putting it on
par with snooze state. As a result, when the predicted latency is
small (< 1us), cpuidle will select stop0_lite instead of snooze. The
difference between these states is that snooze doesn't require an
interrupt to exit from the state, but stop0_lite does. And the value
200ns doesn't include the interrupt latency.

This shows up in the context_switch2 benchmark
(http://ozlabs.org/~anton/junkcode/context_switch2.c) where the number
of context switches per second with the stop0_lite disabled is found
to be roughly 30% more than with stop0_lite enabled.

===============================================================================
x latency_200ns_residency_2us
+ latency_200ns_residency_2us_stop0_lite_disabled
    N           Min           Max        Median           Avg        Stddev
x 100        222784        473466        294510     302295.26       45380.6
+ 100        205316        609420        385198     396338.72     78135.648
Difference at 99.0% confidence
	94043.5 +/- 23276.2
	31.1098% +/- 7.69983%
	(Student's t, pooled s = 63892.8)
===============================================================================

This can be correlated with the number of times cpuidle enters
stop0_lite compared to snooze.
===================================================================
latency=200ns, residency=2us
   stop0_lite enabled.
	* snooze usage      = 7
	* stop0 lite usage  = 3200324
	* stop1 lite usage  = 6
 stop0_lite disabled
	* snooze usage: 287846
	* stop0_lite usage: 0
	* stop1_lite usage: 0
==================================================================

Hence, bump up the exit latency of stop0_lite to 1us. Since the target
residency is chosen to be 10 times the exit latency, set the target
residency to 10us.

With these values, we see a 50% improvement in the number of context
switches:
=====================================================================
x latency_200ns_residency_2us
+ latency_1us_residency_10us
    N           Min           Max        Median           Avg        Stddev
x 100        222784        473466        294510     302295.26       45380.6
+ 100        281790        710784        514878     510224.62     85163.252
Difference at 99.0% confidence
	207929 +/- 24858.3
	68.7835% +/- 8.22319%
	(Student's t, pooled s = 68235.5)
=====================================================================

The cpuidle usage statistics show that we choose stop0_lite less often
in such cases.

latency = 1us, residency = 10us
    stop0_lite enabled
	* snooze usage      = 536808
	* stop0 lite usage  = 3
	* stop1 lite usage  = 7

Reported-by: Anton Blanchard <anton at samba.org>
Signed-off-by: Gautham R. Shenoy <ego at linux.vnet.ibm.com>
---
 hw/slw.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/hw/slw.c b/hw/slw.c
index b879173..bc8e967 100644
--- a/hw/slw.c
+++ b/hw/slw.c
@@ -654,8 +654,8 @@ static struct cpu_idle_states power9_cpu_idle_states[] = {
 static struct cpu_idle_states power9_dd1_cpu_idle_states[] = {
 	{
 		.name = "stop0_lite",
-		.latency_ns = 200,
-		.residency_ns = 2000,
+		.latency_ns = 1000,
+		.residency_ns = 10000,
 		.flags = 0*OPAL_PM_DEC_STOP \
 		       | 0*OPAL_PM_TIMEBASE_STOP  \
 		       | 0*OPAL_PM_LOSE_USER_CONTEXT \
-- 
1.9.4