linux-kernel - Re: [linux-pm] [PATCH 1/8] PM: Opportunistic suspend support.

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Message-ID: <AANLkTimVhC3X68uN3krrkkAAAK4a4D5yTK1V8i_x_Vfy@mail.gmail.com>
Date:	Fri, 28 May 2010 21:04:53 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	markgross@...gnar.org
Cc:	Neil Brown <neilb@...e.de>, 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 Fri, May 28, 2010 at 7:52 PM, mark gross <640e9920@...il.com> wrote:
> 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.
>

Can you be more specific? If you are talking about only letting
drivers abort suspend, not block it, then the main argument against
that is that you are forcing user-space to poll until the driver stops
aborting suspend (which according to people arguing against us using
suspend would make the power-manager a "bad" process). Or are you
talking about blocking the request from user-space until all other
suspend-blockers have been released and then doing a single suspend
cycle before returning. This would not be as bad, but it would force
the user-space power manager to be multi-threaded since it now would
have way to cancel the request. Either way, what problem are you
trying to solve by making it a one-shot request?

-- 
Arve Hjønnevåg
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