linux-kernel - Re: Attempted summary of suspend-blockers LKML thread

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Message-ID: <alpine.DEB.2.00.1008041222280.6545@asgard.lang.hm>
Date:	Wed, 4 Aug 2010 12:29:36 -0700 (PDT)
From:	david@...g.hm
To:	Matthew Garrett <mjg59@...f.ucam.org>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	pavel@....cz, florian@...kler.org, rjw@...k.pl,
	stern@...land.harvard.edu, swetland@...gle.com,
	peterz@...radead.org, tglx@...utronix.de, alan@...rguk.ukuu.org.uk
Subject: Re: Attempted summary of suspend-blockers LKML thread

On Wed, 4 Aug 2010, Matthew Garrett wrote:

> On Wed, Aug 04, 2010 at 12:15:59PM -0700, david@...g.hm wrote:
>> On Wed, 4 Aug 2010, Matthew Garrett wrote:
>>> No! And that's precisely the issue. Android's existing behaviour could
>>> be entirely implemented in the form of binary that manually triggers
>>> suspend when (a) the screen is off and (b) no userspace applications
>>> have indicated that the system shouldn't sleep, except for the wakeup
>>> event race. Imagine the following:
>>>
>>> 1) The policy timeout is about to expire. No applications are holding
>>> wakelocks. The system will suspend providing nothing takes a wakelock.
>>> 2) A network packet arrives indicating an incoming SIP call
>>> 3) The VOIP application takes a wakelock and prevents the phone from
>>> suspending while the call is in progress
>>>
>>> What stops the system going to sleep between (2) and (3)? cgroups don't,
>>> because the voip app is an otherwise untrusted application that you've
>>> just told the scheduler to ignore.
>>
>> Even in the current implementation (wakelocks), Since the VOIP
>> application isn't allowed to take a wakelock, wouldn't the system go to
>> sleep immediatly anyway, even if the application gets the packet and
>> starts the call? What would ever raise the wakelock to keep the phone
>> from sleeping in the middle of the call?
>
> There's two parts of that. The first is that the voip application is
> allowed to take a wakelock - but that doesn't mean that you trust it the
> rest of the time.

why would you trust it to take a wakelock, but not trust it the rest of 
the time?

in my proposal I'm saying that if you would trust the application to take 
a wakelock, you instead trust it to be sane in the rest of it's power 
activity (avoiding polling, etc) and so you consider it for sleep 
decisions.

> The second is that the incoming network packet causes
> the kernel to take a wakelock that will be released once userspace has
> processed the network packet. This ensures that at least one wakelock is
> held for the entire relevant period of time.

how do you determine that userspace has processed the network packet so 
that the kernel can release the wakelock (or is this one of the cases 
where there is a timer related to the wakelock)

two things here,

on the dirty networks that I see as common, refusing to sleep if network 
packets are arriving will mean that you never go to sleep.

secondly, nothing stops the code doing the idle/suspend decision from 
considering network activity. I would be surprised if there weren't 
already options to support this today.

David Lang
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