Date:	Fri, 28 May 2010 19:52:15 -0700
From:	mark gross <640e9920@...il.com>
To:	Neil Brown <neilb@...e.de>
Cc:	Matthew Garrett <mjg59@...f.ucam.org>, Greg KH <gregkh@...e.de>,
	linux-doc@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Len Brown <len.brown@...el.com>,
	James Bottomley <James.Bottomley@...e.de>, tytso@....edu,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [linux-pm] [PATCH 1/8] PM: Opportunistic suspend support.

On Thu, May 27, 2010 at 05:23:54PM +1000, Neil Brown wrote:
> On Wed, 26 May 2010 14:20:51 +0100
> Matthew Garrett <mjg59@...f.ucam.org> wrote:
> 
> > On Wed, May 26, 2010 at 02:57:45PM +0200, Peter Zijlstra wrote:
> > 
> > > I fail to see why. In both cases the woken userspace will contact a
> > > central governing task, either the kernel or the userspace suspend
> > > manager, and inform it there is work to be done, and please don't
> > > suspend now.
> > 
> > Thinking about this, you're right - we don't have to wait, but that does 
> > result in another problem. Imagine we get two wakeup events 
> > approximately simultaneously. In the kernel-level universe the kernel 
> > knows when both have been handled. In the user-level universe, we may 
> > have one task schedule, bump the count, handle the event, drop the count 
> > and then we attempt a suspend again because the second event handler 
> > hasn't had an opportunity to run yet. We'll then attempt a suspend and 
> > immediately bounce back up. That's kind of wasteful, although it'd be 
> > somewhat mitigated by checking that right at the top of suspend entry 
> > and returning -EAGAIN or similar.
> > 
> 
> (I'm coming a little late to this party, so excuse me if I say something that
> has already been covered however...)
> 
> The above triggers a sequence of thoughts which (When they settled down) look
> a bit like this.
> 
> At the hardware level, there is a thing that we could call a "suspend
> blocker".  It is an interrupt (presumably level-triggered) that causes the
> processor to come out of suspend, or not to go into it.
> 
> Maybe it makes sense to export a similar thing from the kernel to user-space.
> When any event happens that would wake the device (and drivers need to know
> about these already), it would present something to user-space to say that
> the event happened.
> 
> When user-space processes the event, it clears the event indicator.

we did I proposed making the suspend enabling a oneshot type of thing
and all sorts of weak arguments came spewing forth.  I honestly couldn't
tell if I was reading valid input or fanboy BS.

--mgross


> 
> When there are no more current event indicators, userspace is allowed to
> request a suspend.  Obviously this could fail as an event could happen at any
> moment, but the same is true when the kernel asks the device to suspend, an
> interrupt might happen immediately to stop it.  But in either case an event
> will be reported.  So when userspace requests a suspend and it fails, it
> will see events reported and so will wait for them to be handled.
> 
> I imagine a sysfs directory with files that appear when events are pending.
> We could have some separate mechanism for user-space processes to request
> that the suspend-daemon not suspend.  Then it suspends whenever there are no
> pending requests from user-space or from the kernel.
> 
> The advantage of this model of suspend-blockers is that it is a close
> analogue for something that already exists in hardware so it isn't really
> creating new concepts, just giving the Linux virtual-machine features that
> have proved themselves in physical machines.
> 
> The cost is that any wake-up event needs to not only be handled, but also
> explicitly acknowledged by clearing the relevant suspend-blocker (i.e.
> removing the file from sysfs, or whatever interface was ultimately chosen).
> I'm hoping that isn't a big cost.
> 
> NeilBrown
> _______________________________________________
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> linux-pm@...ts.linux-foundation.org
> https://lists.linux-foundation.org/mailman/listinfo/linux-pm
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