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

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Message-Id: <201006020024.14220.rjw@sisk.pl>
Date:	Wed, 2 Jun 2010 00:24:14 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	James Bottomley <James.Bottomley@...e.de>
Cc:	Matthew Garrett <mjg59@...f.ucam.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	tytso@....edu, LKML <linux-kernel@...r.kernel.org>,
	Florian Mickler <florian@...kler.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	felipe.balbi@...ia.com, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Alan Stern <stern@...land.harvard.edu>, mark.gross@...el.com,
	Neil Brown <neilb@...e.de>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

On Tuesday 01 June 2010, James Bottomley wrote:
> On Tue, 2010-06-01 at 14:51 +0100, Matthew Garrett wrote:
> > On Mon, May 31, 2010 at 04:21:09PM -0500, James Bottomley wrote:
> > 
> > > You're the one mentioning x86, not me.  I already explained that some
> > > MSM hardware (the G1 for example) has lower power consumption in S3
> > > (which I'm using as an ACPI shorthand for suspend to ram) than any
> > > suspend from idle C state.  The fact that current x86 hardware has the
> > > same problem may be true, but it's not entirely relevant.
> > 
> > As long as you can set a wakeup timer, an S state is just a C state with 
> > side effects. The significant one is that entering an S state stops the 
> > process scheduler and any in-kernel timers. I don't think Google care at 
> > all about whether suspend is entered through an explicit transition or 
> > something hooked into cpuidle - the relevant issue is that they want to 
> > be able to express a set of constraints that lets them control whether 
> > or not the scheduler keeps on scheduling, and which doesn't let them 
> > lose wakeup events in the process.
> 
> Exactly, so my understanding of where we currently are is:

Thanks for the recap.

>      1. pm_qos will be updated to be able to express the android suspend
>         blockers as interactivity constraints (exact name TBD, but
>         probably /dev/cpu_interactivity)

I think that's not been decided yet precisely enough.  I saw a few ideas
here and there in the thread, but which of them are we going to follow?

>      2. pm_qos will be updated to be callable from atomic context
>      3. pm_qos will be updated to export statistics initially closely
>         matching what suspend blockers provides (simple update of the rw
>         interface?)
> 
> After this is done, the current android suspend block patch becomes a
> re-expression in kernel space in terms of pm_qos, with the current
> userspace wakelocks being adapted by the android framework into pm_qos
> requirements expressed to /dev/cpu_interactivity (or whatever name is
> chosen).  Then opportunistic suspend is either a small add-on kernel
> patch they have in their tree to suspend when the interactivity
> constraint goes to NONE, or it could be done entirely by a userspace
> process.  Long term this could migrate to the freezer and suspend from
> idle approach as the various problem timers get fixed.
> 
> I think the big unresolved issue is the stats extension.  For android,
> we need just a name written along with the value, so we have something
> to hang the stats off ... current pm_qos userspace users just write a
> value, so the name would be optional.  From the kernel, we probably just
> need an additional API that takes a stats name or NULL if none
> (pm_qos_add_request_named()?).  Then reading the stats could be done by
> implementing a fops read routine on the misc device.

Is the original idea of having that information in debugfs objectionable?

Rafael
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