Re: [patch 1/2] cpuidle: use last_state which can reflect the actual state entered

[Kevin Hilman <khilman@deeprootsystems.com>]

Venkatesh Pallipadi <venkatesh.pallipadi@intel.com> writes:

> cpuidle accounts the idle time for the C-state it was trying to enter and
> not to the actual state that the driver eventually entered. The driver may
> select a different state than the one chosen by cpuidle due to
> constraints like bus-mastering, etc.
>
> Change the time acounting code to look at the dev->last_state after
> returning from target_state->enter(). Driver can modify dev->last_state
> internally, inside the enter routine to reflect the actual C-state
> entered.
>
> Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
>
> ---
>  drivers/cpuidle/cpuidle.c |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> Index: tip/drivers/cpuidle/cpuidle.c
> ===================================================================
> --- tip.orig/drivers/cpuidle/cpuidle.c	2008-09-23 13:52:59.000000000 -0700
> +++ tip/drivers/cpuidle/cpuidle.c	2008-09-23 14:22:43.000000000 -0700
> @@ -71,8 +71,11 @@ static void cpuidle_idle_call(void)
>  	target_state = &dev->states[next_state];
>  
>  	/* enter the state and update stats */
> -	dev->last_residency = target_state->enter(dev, target_state);
>  	dev->last_state = target_state;
> +	dev->last_residency = target_state->enter(dev, target_state);
> +	if (dev->last_state)
> +		target_state = dev->last_state;
> +
>  	target_state->time += (unsigned long long)dev->last_residency;
>  	target_state->usage++;

Under what conditions would the enter hook set dev->last_sate to NULL?
Having the check seems to indicate it's possilble.

A minor nit-pick...  why not explicitly do the accounting using
'last_state' like below.  While functionally the same as above, this
makes it makes it more explicit when reading the code that the
accounting is done using 'last_state' and not 'target_state.'

Kevin


diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index 5ce07b5..c1294f5 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -67,10 +67,10 @@ static void cpuidle_idle_call(void)
 	target_state = &dev->states[next_state];
 
 	/* enter the state and update stats */
-	dev->last_residency = target_state->enter(dev, target_state);
 	dev->last_state = target_state;
-	target_state->time += (unsigned long long)dev->last_residency;
-	target_state->usage++;
+	dev->last_residency = target_state->enter(dev, target_state);
+	dev->last_state->time += (unsigned long long)dev->last_residency;
+	dev->last_state->usage++;
 
 	/* give the governor an opportunity to reflect on the outcome */
 	if (cpuidle_curr_governor->reflect)