linux-kernel - Re: [PATCH] pm_qos: remove unnecessary list-traversal

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Date:	Tue, 1 Jun 2010 07:08:12 -0700
From:	mark gross <640e9920@...il.com>
To:	florian@...kler.org
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, 640e9920@...il.com,
	markgross@...gnar.org, Arve Hj?nnev?g <arve@...roid.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Brian Swetland <swetland@...gle.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] pm_qos: remove unnecessary list-traversal

On Tue, Jun 01, 2010 at 10:46:54AM +0200, florian@...kler.org wrote:
> The new extreme value is only depending on the old extreme value and
> the changed value.
> 
> Signed-off-by: Florian Mickler <florian@...kler.org>
> ---
>  kernel/pm_qos_params.c |   19 +++++++++++++++++--
>  1 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/pm_qos_params.c b/kernel/pm_qos_params.c
> index f42d3f7..5d71c12 100644
> --- a/kernel/pm_qos_params.c
> +++ b/kernel/pm_qos_params.c
> @@ -136,6 +136,16 @@ static s32 min_compare(s32 v1, s32 v2)
>  }
>  
>  
> +static void update_target_val(int pm_qos_class, s32 val)
> +{
> +	s32 extreme_value;
> +	s32 new_value;
> +	extreme_value = atomic_read(&pm_qos_array[pm_qos_class]->target_value);
> +	new_value = pm_qos_array[pm_qos_class]->comparitor(val,extreme_value);
> +	if (extreme_value != new_value)
> +		atomic_set(&pm_qos_array[pm_qos_class]->target_value,new_value);
> +}
> +

only works right when the new val is a more stringent constraint
(request) than what is already there.  When removing or reducing a
constraint (request) the list still needs to be walked to get the new
aggregate request.

But its a really good optimization for 1/2 of the calls to pm_qos I
should have seen and included a long time ago!  Thanks!

--mgross


>  static void update_target(int pm_qos_class)
>  {
>  	s32 extreme_value;
> @@ -253,19 +263,24 @@ void pm_qos_update_request(struct pm_qos_request_list *pm_qos_req,
>  	s32 temp;
>  
>  	if (pm_qos_req) { /*guard against callers passing in null */
> +		int target = pm_qos_req->pm_qos_class;
>  		spin_lock_irqsave(&pm_qos_lock, flags);
>  		if (new_value == PM_QOS_DEFAULT_VALUE)
> -			temp = pm_qos_array[pm_qos_req->pm_qos_class]->default_value;
> +			temp = pm_qos_array[target]->default_value;
>  		else
>  			temp = new_value;
>  
>  		if (temp != pm_qos_req->value) {
>  			pending_update = 1;
>  			pm_qos_req->value = temp;
> +			update_target_val(target,temp);
>  		}
>  		spin_unlock_irqrestore(&pm_qos_lock, flags);
> +
>  		if (pending_update)
> -			update_target(pm_qos_req->pm_qos_class);
> +			blocking_notifier_call_chain(
> +				pm_qos_array[pm_qos_class]->notifiers,
> +					(unsigned long) temp, NULL);
>  	}
>  }
>  EXPORT_SYMBOL_GPL(pm_qos_update_request);
> -- 
> 1.7.1
> 
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