linux-kernel - Re: [RFD] Automatic suspend

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Date:	Fri, 20 Feb 2009 15:11:28 -0800
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Oliver Neukum <oliver@...kum.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Alan Stern <stern@...land.harvard.edu>,
	"Woodruff, Richard" <r-woodruff2@...com>,
	Arjan van de Ven <arjan@...radead.org>,
	Kyle Moffett <kyle@...fetthome.net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Pavel Machek <pavel@....cz>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	mark gross <mgross@...ux.intel.com>,
	Uli Luckas <u.luckas@...d.de>,
	Igor Stoppa <igor.stoppa@...ia.com>,
	Brian Swetland <swetland@...gle.com>,
	Len Brown <lenb@...nel.org>
Subject: Re: [RFD] Automatic suspend

On Fri, Feb 20, 2009 at 2:05 PM, Oliver Neukum <oliver@...kum.org> wrote:
> Am Freitag 20 Februar 2009 11:46:55 schrieb Rafael J. Wysocki:
>> On Thursday 19 February 2009, Oliver Neukum wrote:
>> > Am Donnerstag 19 Februar 2009 23:21:46 schrieb Rafael J. Wysocki:
>> > > > That's the whole point behind userspace wakelocks!  They provide a
>> > > > mechanism for userspace to tell the kernel when (as far as userspace
>> >
>> > is
>> >
>> > > > concerned) it is or is not okay to auto-sleep.
>> > >
>> > > Still, one can go further and observe that the user space can in fact
>> > > start automatic suspend by itself whenever it knows it's appropriate
>> > > ...
>> >
>> > User space initiating this is a race condition.
>>
>> Do you mean a race with the other user space processes or with the kernel?
>
> With the set of runnable processes.There's always a window between
> evaluating the current set of runnable tasks and telling the kernel to sleep.
> IMO the most elegant solution would be a task attribute that would signal
> the kernel that a task should not count as keeping the system busy even
> if it is runnable and trigger the sleep in kernel space.

It is not always safe to enter suspend when no tasks are runnable. For
instance, a key event could be on a user space queue, but the code
that reads from that queue has been paged out.

-- 
Arve Hjønnevåg
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