linux-kernel - Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

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Date:	Thu, 27 May 2010 16:28:51 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Matthew Garrett <mjg59@...f.ucam.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arve Hjønnevåg <arve@...roid.com>,
	Florian Mickler <florian@...kler.org>,
	Vitaly Wool <vitalywool@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul@...p1.linux-foundation.org, felipe.balbi@...ia.com,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

On Thu, 2010-05-27 at 15:06 +0100, Matthew Garrett wrote:

> I don't entirely see how this works. In order to deal with poorly 
> written applications, it's necessary to (optionally, based on some 
> policy) ignore them when it comes to the scheduler. The problem is how 
> to implement the optional nature of this in a race-free manner. This is 
> obviously a pathological case, but imagine an application that does 
> something along the following lines:
> 
> int input = open ("/dev/input", O_RDONLY|O_NONBLOCK);
> char foo;
> 
> while (1) {
> 	suspend_block();
> 	if (read(input, &foo, 1) > 0) {
> 		(do something)
> 		suspend_unblock();
> 	} else {
> 		suspend_unblock();
> 		(draw bouncing cows and clouds and tractor beams briefly)
> 	}
> }
> 
> Now, if the user is playing this game, you want it to be scheduled. If 
> the user has put down their phone and the screen lock has kicked in, you 
> don't want it to be scheduled. So we could imagine some sort of cgroup 
> that contains untrusted tasks - when the session is active we set a flag 
> one way which indicates to the scheduler that tasks in TASK_RUNNING 
> should be scheduled, and when the session is idle we set the flag the 
> other way and all processes in that cgroup get shifted to 
> TASK_INTERRUPTIBLE or something.

What's wrong with simply making the phone beep loudly and displaying:
bouncing cows is preventing your phone from sleeping!


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