linux-kernel - Re: [PATCH v2 02/10] freezer: skip waking up tasks with PF_FREEZER

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Date:	Thu, 2 May 2013 16:24:07 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Colin Cross <ccross@...roid.com>
Cc:	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Rafael J. Wysocki" <rjw@...k.pl>, arve@...roid.com,
	Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH v2 02/10] freezer: skip waking up tasks with
 PF_FREEZER_SKIP set

On Wed, May 01, 2013 at 06:35:00PM -0700, Colin Cross wrote:
> Android goes through suspend/resume very often (every few seconds when
> on a busy wifi network with the screen off), and a significant portion
> of the energy used to go in and out of suspend is spent in the
> freezer.  If a task has called freezer_do_not_count(), don't bother
> waking it up.  If it happens to wake up later it will call
> freezer_count() and immediately enter the refrigerator.
> 
> Combined with patches to convert freezable helpers to use
> freezer_do_not_count() and convert common sites where idle userspace
> tasks are blocked to use the freezable helpers, this reduces the
> time and energy required to suspend and resume.
> 
> Signed-off-by: Colin Cross <ccross@...roid.com>
> ---
> v2:  move check to freeze_task()
> 
>  kernel/freezer.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/kernel/freezer.c b/kernel/freezer.c
> index c38893b..8b2afc1 100644
> --- a/kernel/freezer.c
> +++ b/kernel/freezer.c
> @@ -110,6 +110,18 @@ bool freeze_task(struct task_struct *p)
>  {
>  	unsigned long flags;
>  
> +	/*
> +	 * This check can race with freezer_do_not_count, but worst case that
> +	 * will result in an extra wakeup being sent to the task.  It does not
> +	 * race with freezer_count(), the barriers in freezer_count() and
> +	 * freezer_should_skip() ensure that either freezer_count() sees
> +	 * freezing == true in try_to_freeze() and freezes, or
> +	 * freezer_should_skip() sees !PF_FREEZE_SKIP and freezes the task
> +	 * normally.
> +	 */
> +	if (freezer_should_skip(p))
> +		return false;

Maybe a line or two explaining that this matters for power saving?
Other than that,

Acked-by: Tejun Heo <tj@...nel.org>

Oleg, this looks correct to me.  Can you please ack too?

Thanks.

-- 
tejun
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