Message-ID: <1275451342.21962.1777.camel@mulgrave.site>
Date:	Tue, 01 Jun 2010 23:02:22 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	Neil Brown <neilb@...e.de>, tytso@....edu,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Mickler <florian@...kler.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, mark.gross@...el.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	felipe.balbi@...ia.com
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

On Tue, 2010-06-01 at 18:10 -0700, Arve Hjønnevåg wrote:
> On Tue, Jun 1, 2010 at 3:36 PM, James Bottomley <James.Bottomley@...e.de> wrote:
> > On Wed, 2010-06-02 at 00:24 +0200, Rafael J. Wysocki wrote:
> >> 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?
> >
> > Well, android only needs two states (block and don't block), so that
> > gets translated as 2 s32 values (say 0 and INT_MAX).  I've seen defines
> > like QOS_INTERACTIVE and QOS_NONE (or QOS_DRECKLY or QOS_MANANA) to
> > describe these, but if all we're arguing over is the define name, that's
> > progress.
> 
> I think we need separate state constraints for suspend and idle low
> power modes. On the msm platform only a subset of the interrupts can
> wake up from the low power mode, so we block the use if the low power
> mode from idle while other interrupts are enabled. We do not block
> suspend however if those interrupts are not marked as wakeup
> interrupts. Most constraints that prevent suspend are not hardware
> specific and should not prevent entering low power modes from idle. In
> other words we may need to prevent low power idle modes while allowing
> suspend, and we may need to prevent suspend while allowing low power
> idle modes.

Well, as I said, pm_qos is s32 ... it's easy to make the constraint
ternary instead of binary.

> It would also be good to not have an implementation that gets slower
> and slower the more clients you have. With binary constraints this is
> trivial.

Well, that's an implementation detail ... ordering the list or using a
btree would significantly fix that.  However, the most number of
constraint users I've seen in android is around 60 ... that's not huge
from a kernel linear list perspective, so is this really a concern? ...
particularly when most uses don't necessarily change the constrain, so a
list search isn't done.

> > The other piece they need is the suspend block name, which comes with
> > the stats API, and finally we need to decide what the actual constraint
> > is called (which is how the dev node gets its name) ...
> >
> >> >      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?)
> 
> 4. It would be useful to change pm_qos_add_request to not allocate
> anything so can add constraints from init functions that currently
> cannot fail.

Sure .. we do that for the delayed work queues, it's just an API which
takes the structure as an argument leaving it the responsibility of the
caller to free.

> >> > 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?
> >
> > Well ... debugfs is usually used to get around the sysfs rules.  In this
> > case, pm_qos has a dev interface ... I don't specifically object to
> > using debugfs, but I don't see any reason to forbid it from being a
> > simple dev read interface either.
> >
> 
> We don't currently have a dev interface for stats so this is not an
> immediate requirement. The suspend blocker debugfs interface is just
> as good as the proc interface we have for wakelocks.

OK, great ... what actually exports the statistics is just an
implementation detail. 

James



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