linux-kernel - Re: [PATCH 1/1 v3] Honor state disabling in the cpuidle ladder governor

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Date:	Thu, 19 Jul 2012 23:48:05 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Carsten Emde <C.Emde@...dl.org>
Cc:	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>,
	Len Brown <len.brown@...el.com>, Kevin Hilman <khilman@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PM mailing list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 1/1 v3] Honor state disabling in the cpuidle ladder governor

On Thursday, July 19, 2012, 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.
> 
> 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
> added to the documentation.
> 
> Signed-off-by: Carsten Emde <C.Emde@...dl.org>

This looks fine to me, but it's too late for v3.6.  I can queue it up
for v3.7, though.

Thanks,
Rafael


> ---
>  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 &&
> +	    !dev->states_usage[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) {
> +	    (dev->states_usage[last_idx].disable ||
> +	    drv->states[last_idx].exit_latency > latency_req)) {
>  		int i;
>  
>  		for (i = last_idx - 1; i > CPUIDLE_DRIVER_STATE_START; i--) {
> 
> 
> 

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