Date:	Sat, 29 May 2010 22:41:21 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Florian Mickler <florian@...kler.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	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: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

On Saturday 29 May 2010, Alan Stern wrote:
> On Sat, 29 May 2010, Florian Mickler wrote:
> 
> > On Sat, 29 May 2010 12:42:37 +0200
> > Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > > Now, all I'm interested in is providing interfaces from the kernel where
> > > needed, so that userspace can be optimally frugal with power usage, and
> > > can monitor/contain badly behaving tasks.
> > > 
> > 
> > I think this is a sensible approach.
> 
> Here is an attempt to satisfy everyone as much as possible.  But first 
> an explicit disclaimer: When I say "suspend", I mean it as in 
> "suspend-to-RAM"; i.e., a forced suspend and not a cpuidle mode.
> 
> In place of in-kernel suspend blockers, there will be a new type of QoS 
> constraint -- call it QOS_EVENTUALLY.  It's a very weak constraint, 
> compatible with all cpuidle modes in which runnable threads are allowed 
> to run (which is all of them), but not compatible with suspend.
> 
> The Android people want debugging and accountability.  So in the most 
> objectionable part of this proposal, we add a new way of registering 
> QoS constraints: monitored constraints.  The "monitored" implies that:
> 
> 	The constraint has a name, which can be used for debugging
> 	and accounting;
> 
> 	The kernel maintains statistics on the constraint's use and
> 	makes them available to userspace; and
> 
> 	The PM core is notified whenever the number of active monitored
> 	constraints drops to 0.
> 
> There is no /sys/power/policy file.  In place of opportunistic suspend,
> we have "QoS-based suspend".  This is initiated by userspace writing
> "qos" to /sys/power/state, and it is very much like suspend-to-RAM.
> However a QoS-based suspend fails immediately if there are any active
> normal QoS constraints incompatible with system suspend, in other
> words, any constraints requiring a throughput > 0 or an interrupt
> latency shorter than the time required for a suspend-to-RAM/resume
> cycle.
> 
> If no such constraints are active, the QoS-based suspend blocks in an
> interruptible wait until the number of active QOS_EVENTUALLY
> constraints drops to 0.  When that happens, it carries out a normal
> suspend-to-RAM -- except that it checks along the way to make sure that
> no new QoS constraints are activated while the suspend is in progress.  
> If they are, the PM core backs out and fails the QoS-based suspend.
> 
> Userspace suspend blockers don't exist at all, as far as the kernel is 
> concerned.  In their place, the Android runs a power-manager program 
> that receives IPC requests from other processes when they need to 
> prevent the system from suspending or allow it to suspend.  The power 
> manager's main loop looks like this:
> 
> 	for (;;) {
> 		while (any IPC requests remain)
> 			handle them;
> 		if (any processes need to prevent suspend)
> 			sleep;
> 		else
> 			write "qos" to /sys/power/state;
> 	}
> 
> The idea is that receipt of a new IPC request will cause a signal to be 
> sent, interrupting the sleep or the "qos" write.
> 
> There remains a question as to which kernel drivers should create 
> monitored QOS_EVENTUALLY constraints.  Perhaps userspace could be 
> allowed to specify this (I don't know how).  In any case, this is a 
> relatively minor point.
> 
> The advantages of this scheme are that this does everything the Android
> people need, and it does it in a way that's entirely compatible with 
> pure QoS/cpuidle-based power management.  It even starts along the path
> of making suspend-to-RAM just another kind of dynamic power state.
> 
> If people such as Peter still want to complain that using
> suspend-to-RAM in Android phones isn't a good way to do power
> management, that's okay -- it's the designers' decision to program
> their phones the way they want.  At least the kernel can give them the
> ability to do so in a way that doesn't compromise everybody else.

This sounds reasonable to me.

Rafael
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