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

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Date:	Fri, 28 May 2010 04:05:51 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Thomas Gleixner <tglx@...utronix.de>,
	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>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

2010/5/28 Alan Cox <alan@...rguk.ukuu.org.uk>:
>> This is a much harder question to answer that what we need to use
>> opportunistic suspend. The question we ask is more like this: "Is all
>> important work complete?". In the simplest case these can be the same,
>
> I don't believe you can answer that question without telepathy and a
> crystal ball.
>
But we have answered this question.

> The application doesn't know because it has no idea how to balance
> conflicting resource demands or to infer the users requirements and
> wishes. Most apps will misbehave anyway.
>
> The OS doesn't know because it cannot tell what the app wants
>
> So at best you have a heuristic.
>
>> What happens if the user presses the button right before you set QoS
>> of 'user apps' to  QS_NONE?
>
> Read down a paragraph.
>
>> To me it looks like this solution would result in this sequence which
>> may ignore the button press:
>>         Button pushed
>>         Button driver sets QoS of app it wakes to QS_ABOVESUSPEND
>>         Set QoS of 'user apps' to QS_NONE
>>
>>
>> >        That would I think solve the reliable wakeup case although
>> >        drivers raising a QoS parameter is a bit unusual in the kernel.
>> >        That would at least however be specific to a few Android drivers
>> >        and maybe a tiny amount of shared driver stuff so probably not
>> >        unacceptable. (wake_up_pri(&queue, priority); isn't going to
>> >        kill anyone is it - especially if it usually ignores the
>> >        priority argument)
>>
>> Why is "wake_up_pri(&queue, priority)" more acceptable than "suspend_block(..."?
>
> We keep it kernel side
> It expresses policy and wishes rather than enforcing a behaviour.
>
> What for example does "suspend_block" mean on a virtual machine ?
>
> I would prefer "priority" was some kind of resource constraint model
> instead but I'm just trying to think how to be absolutely minimally
> invasible at this point.
>
>> What happens if the button press happend before this line:
>> >        count2 = tasks to QS_NONE | QS_NOTCHANGED
>> >        Screen off
>> >                                        Button Press
>> >                                        task to QS_ABOVESUSPEND
>> >        count = tasks that are QS_NOTCHANGED to QS_NONE
>> >
>> >        if (count != count2) {
>> >                Stuff happened ... rethink
>> >        }
>> >
>> > That is still a bit weird and wonderful but all the logic is in the right
>> > places. The special magic remains in the Android policy code and in the
>> > kernel specifics for Android.
>> >
>> > Thoughts ?
>>
>> I don't think it works. Also, it does not seem much less invasive than
>> suspend blockers.
>
> "I don't think it works" isn't that helpful. I don't think it works
> because .. would help me a lot more.

Did you miss this:
>> What happens if the button press happend before this line:
>> >        count2 = tasks to QS_NONE | QS_NOTCHANGED

As far as I can tell this is the same race I described where you just
told me to read down a paragraph.

-- 
Arve Hjønnevåg
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