linux-kernel - Re: [PATCH] - race-free suspend. Was: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

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Date:	Thu, 3 Jun 2010 16:33:07 +1000
From:	Neil Brown <neilb@...e.de>
To:	Brian Swetland <swetland@...gle.com>
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Alan Stern <stern@...land.harvard.edu>,
	Felipe Balbi <felipe.balbi@...ia.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul@...p1.linux-foundation.org" <Paul@...p1.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Mickler <florian@...kler.org>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	James Bottomley <James.Bottomley@...e.de>
Subject: Re: [PATCH] - race-free suspend. Was: Re: [linux-pm] [PATCH 0/8] 
 Suspend block api (version 8)

On Wed, 2 Jun 2010 11:05:18 -0700
Brian Swetland <swetland@...gle.com> wrote:

> On Wed, Jun 2, 2010 at 1:06 AM, Neil Brown <neilb@...e.de> wrote:
> > On Wed, 2 Jun 2010 00:05:14 -0700
> > Arve Hjønnevåg <arve@...roid.com> wrote:
> >> > The user-space suspend daemon avoids losing wake-events by using
> >> > fcntl(F_OWNER) to ensure it gets a signal whenever any important wake-event
> >> > is ready to be read by user-space.  This may involve:
> >> >  - the one daemon processing all wake events
> >>
> >> Wake up events are not all processed by one daemon.
> >
> > Not with your current user-space code, no.  Are you saying that you are not
> > open to any significant change in the Android user-space code?  That would
> > make the situation a lot harder to resolve.
> 
> There are many wakeup events possible in a typical system --
> keypresses or other input events, network traffic, telephony events,
> media events (fill audio buffer, fill video decoder buffer, etc), and
> I think requiring that all wakeup event processing bottleneck through
> a single userspace process is non-optimal here.

Just to be clear: I'm not suggesting all wake-events need to go through one
process.  That was just one example of how the interface I proposed could be
used.  There were two other examples.
However one process would need to know about any wakeup event that happens.
I don't think that needs to be a significant bottleneck, but I don't really
know enough about all the requirement to try devising a demonstration.

> 
> The current suspend-blocker proposal already involves userspace
> changes (it's different than our existing wakelock interface), and
> we're certainly not opposed to any/all userspace changes on principle,
> but on the other hand we're not interested in significant reworks of
> userspace unless they actually improve the situation somehow.  I think
> bottlenecking events through a central daemon would represent a step
> backwards.

I guess it becomes an question of economics for you then.  Does the cost of
whatever user-space changes are required exceed the value of using an upstream
kernel?  Both the cost and the value would be very hard to estimate in
advance.  I don't envy you the decision...

NeilBrown
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