linux-kernel - Re: Async suspend-resume patch w/ completions (was: Re: Async suspend-resume patch w/ rwsems)

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Message-Id: <200912142311.31658.rjw@sisk.pl>
Date:	Mon, 14 Dec 2009 23:11:31 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Zhang Rui <rui.zhang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>
Subject: Re: Async suspend-resume patch w/ completions (was: Re: Async suspend-resume patch w/ rwsems)

On Monday 14 December 2009, Linus Torvalds wrote:
> 
> On Sat, 12 Dec 2009, Rafael J. Wysocki wrote:
> > 
> > One solution that we have discussed on linux-pm is to start a bunch of async
> > threads searching for async devices that can be suspended and suspending
> > them (assuming suspend is considered) out of order with respect to dpm_list.
> 
> Ok, guys, stop the crazy.
> 
> That's another of those "ok, that's just ttoally stupid and clearly too 
> complex" ideas that I would never pull.
> 
> I should seriously suggest that people just stop discussing architectural 
> details on the pm list if they all end up being this level of crazy.
> 
> The sane thing to do is to just totally ignore the async layer on PCI 
> bridges and other things that only have a late-suspend/early-resume thing. 
> No need for the above kind of obviously idiotic crap.
> 
> However, my point was really that we wouldn't even have _needed_ that kind 
> of special case if we had just decided to let the subsystems do it. But 
> whatever. At worst, the PCI layer can even just mark such devices with 
> just late/early suspend/resume as being asynchronous, even though that 
> ends up resulting in some totally pointless async work that doesn't do 
> anything.
> 
> But please guys - reign in the crazy ideas on the pm list. It's not like 
> our suspend/resume has gotten so stable as to be boring, and we want it to 
> become unreliable again.

Indeed.

OK, what about a two-pass approach in which the first pass only inits the
completions and starts async threads for leaf "async" devices?  I think leaf
devices are most likely to take much time to suspend, so this will give us
a chance to save quite some time.

A more aggressive version of this might start the async threads for all async
devices in the first pass and then only handle the sychronous ones in the
second pass - as long as there are only a few async devices that should be
quite efficient.

Rafael
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