===============================================================================
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@samba.org>
Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
---
Changes from v1:
[PATCH] power9-dd1:slw: Modify the stop0_lite latency & residency
https://lists.ozlabs.org/pipermail/skiboot/2017-April/007011.html
Update latency to atleast 1us for both DD1 and DD2 idle states.
--Vaidy
hw/slw.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
@@ -508,8 +508,8 @@ static struct cpu_idle_states power8_cpu_idle_states[] = {
static struct cpu_idle_states power9_cpu_idle_states[] = {
{
.name = "stop0_lite", /* Enter stop0 with no state loss */
- .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 \
@@ -522,8 +522,8 @@ static struct cpu_idle_states power9_cpu_idle_states[] = {
.pm_ctrl_reg_mask = OPAL_PM_PSSCR_MASK },
{
.name = "stop0",
- .latency_ns = 300,
- .residency_ns = 3000,
+ .latency_ns = 2000,
+ .residency_ns = 20000,
.flags = 0*OPAL_PM_DEC_STOP \
| 0*OPAL_PM_TIMEBASE_STOP \
| 0*OPAL_PM_LOSE_USER_CONTEXT \
@@ -653,8 +653,8 @@ static struct cpu_idle_states power9_cpu_idle_states[] = {
static struct cpu_idle_states power9_ndd1_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 \
From: "Gautham R. Shenoy" <ego@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.