[PATCH 1/1 v2] Honor state disabling in the cpuidle ladder governor - documented

[Carsten Emde <C.Emde@osadl.org>]

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On 07/18/2012 01:48 PM, Deepthi Dharwar wrote:
> On 07/18/2012 04:32 PM, Carsten Emde wrote:
>> On 07/18/2012 08:36 AM, Deepthi Dharwar wrote:
>>> On 07/18/2012 12:29 AM, Carsten Emde wrote:
>>>> There are two cpuidle governors ladder and menu. While the ladder
>>>> governor is always available, if CONFIG_CPU_IDLE is selected, the
>>>> menu governor additionally requires CONFIG_NO_HZ.
>>>>
>>>> A particular C state can be disabled by writing to the sysfs file
>>>> /sys/devices/system/cpu/cpuN/cpuidle/stateN/disable, but this mechanism
>>>> is only implemented in the menu governor. Thus, in a system where
>>>> CONFIG_NO_HZ is not selected, the ladder governor becomes default and
>>>> always will walk through all sleep states - irrespective of whether the
>>>> C state was disabled via sysfs or not. The only way to select a specific
>>>> C state was to write the related latency to /dev/cpu_dma_latency and
>>>> keep the file open as long as this setting was required - not very
>>>> practical and not suitable for setting a single core in an SMP system.
>>>>
>>>> With this patch, the ladder governor only will promote to the next
>>>> C state, if it has not been disabled, and it will demote, if the
>>>> current C state was disabled.
>>>
>>> Yes, I agree that currently that disabling a particular C-state
>>> is not reflected in working of ladder governor. This patch is needed
>>> to fix it on ladder too.
>>>
>>> Also wanted to clarify on the intended implementation here,
>>> if there are say 5 C-states on a system, disabling 2nd
>>> state would also end by disabling all the remaining 3 deeper states too
>>> as ladder governor enters the lightest state first, and will only move
>>> on to the next deeper state if a idle period was long enough as
>>> per the implementation.
>>> If one is disabling only the deepest state, then it would
>>> work as intended.
>> Yes, the patch does not make the setting of the sysfs variable
>> "disable" coherent, i.e. if one is disabling a light state, then all
>> deeper states are disabled as well, but the "disable" variable does not
>> reflect it. Likewise, if one enables a deep state but a lighter state
>> still is disabled, then this has no effect.
>
> Agree, as per the ladder design.
>
>> I could implement a sanitize mechanism of the ladder governor that
>> takes care the "disable" variables of all deeper states are set to 1,
>> if a state is disabled, and those of all lighter states are set to 0,
>> if a state is enabled. Do you wish me to do that?
>
> No, I dont think thats necessary, current code suffices it.
> The disable flag is knob we are giving to the user . So may be just
> document  the  intended use of disable flag working
> alongside design of ladder governor.
Here comes v2 with a related section added to the documentation.

	-Carsten.

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Subject: Honor state disabling in the cpuidle ladder governor
From: Carsten Emde <C.Emde@osadl.org>
Date: Tue, 17 Jul 2012 19:50:13 +0100

There are two cpuidle governors ladder and menu. While the ladder
governor is always available, if CONFIG_CPU_IDLE is selected, the
menu governor additionally requires CONFIG_NO_HZ.

A particular C state can be disabled by writing to the sysfs file
/sys/devices/system/cpu/cpuN/cpuidle/stateN/disable, but this mechanism
is only implemented in the menu governor. Thus, in a system where
CONFIG_NO_HZ is not selected, the ladder governor becomes default and
always will walk through all sleep states - irrespective of whether the
C state was disabled via sysfs or not. The only way to select a specific
C state was to write the related latency to /dev/cpu_dma_latency and
keep the file open as long as this setting was required - not very
practical and not suitable for setting a single core in an SMP system.

With this patch, the ladder governor only will promote to the next
C state, if it has not been disabled, and it will demote, if the
current C state was disabled.

Note that the patch does not make the setting of the sysfs variable
"disable" coherent, i.e. if one is disabling a light state, then all
deeper states are disabled as well, but the "disable" variable does not
reflect it. Likewise, if one enables a deep state but a lighter state
still is disabled, then this has no effect. A related section has been
addded to the documentation.

Signed-off-by: Carsten Emde <C.Emde@osadl.org>

---
 Documentation/cpuidle/sysfs.txt    |   10 +++++++++-
 drivers/cpuidle/governors/ladder.c |    4 +++-
 2 files changed, 12 insertions(+), 2 deletions(-)

Index: linux-3.4.4-rt14-rc2-64/Documentation/cpuidle/sysfs.txt
===================================================================
--- linux-3.4.4-rt14-rc2-64.orig/Documentation/cpuidle/sysfs.txt
+++ linux-3.4.4-rt14-rc2-64/Documentation/cpuidle/sysfs.txt
@@ -76,9 +76,17 @@ total 0
 
 
 * desc : Small description about the idle state (string)
-* disable : Option to disable this idle state (bool)
+* disable : Option to disable this idle state (bool) -> see note below
 * latency : Latency to exit out of this idle state (in microseconds)
 * name : Name of the idle state (string)
 * power : Power consumed while in this idle state (in milliwatts)
 * time : Total time spent in this idle state (in microseconds)
 * usage : Number of times this state was entered (count)
+
+Note:
+The behavior and the effect of the disable variable depends on the
+implementation of a particular governor. In the ladder governor, for
+example, it is not coherent, i.e. if one is disabling a light state,
+then all deeper states are disabled as well, but the disable variable
+does not reflect it. Likewise, if one enables a deep state but a lighter
+state still is disabled, then this has no effect.
Index: linux-3.4.4-rt14-rc2-64/drivers/cpuidle/governors/ladder.c
===================================================================
--- linux-3.4.4-rt14-rc2-64.orig/drivers/cpuidle/governors/ladder.c
+++ linux-3.4.4-rt14-rc2-64/drivers/cpuidle/governors/ladder.c
@@ -88,6 +88,7 @@ static int ladder_select_state(struct cp
 
 	/* consider promotion */
 	if (last_idx < drv->state_count - 1 &&
+	    !drv->states[last_idx + 1].disable &&
 	    last_residency > last_state->threshold.promotion_time &&
 	    drv->states[last_idx + 1].exit_latency <= latency_req) {
 		last_state->stats.promotion_count++;
@@ -100,7 +101,8 @@ static int ladder_select_state(struct cp
 
 	/* consider demotion */
 	if (last_idx > CPUIDLE_DRIVER_STATE_START &&
-	    drv->states[last_idx].exit_latency > latency_req) {
+	    (drv->states[last_idx].disable ||
+	    drv->states[last_idx].exit_latency > latency_req)) {
 		int i;
 
 		for (i = last_idx - 1; i > CPUIDLE_DRIVER_STATE_START; i--) {