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

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Date:	Tue, 15 Dec 2009 17:01:18 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Tue, 15 Dec 2009, Rafael J. Wysocki wrote:

> > Ideally we would figure out how to do the slow devices in parallel
> > without interference from fast devices having unknown dependencies.  
> > Unfortunately this may not be possible.
> 
> I really expect to see those "unknown dependencies" in the _noirq
> suspend/resume phases and above.  [The very fact they exist is worrisome,
> because that's why we don't know why things work on one system and don't
> work on another, although they appear to be very similar.]

This is a good reason for keeping the _noirq phases synchronous.  AFAIK 
they don't take long enough to be worth converting, so there's no loss.

> > The real issue is "blockage": synchronous devices preventing 
> > possible concurrency among async devices.  That's what you thought 
> > making PCI bridges async would help.
> > 
> > In general, blockage arises in suspend when you have an async child
> > with a synchronous parent.  The parent has to wait for the child, which
> > might take a long time, thereby delaying other unrelated devices.
> 
> Exactly, but the Linus' point seems to be that's going to be rare and we
> should be able to special case all of the interesting cases.

Maybe that's true.  Without seeing some examples of actual dpm_list
contents, we can't tell.  Can you post the interesting parts of the
lists from some of your test machines?  Maybe with a USB device or two
plugged in?  (The device names together with the names of their parents
should be enough.)

> > (This explains why you wanted to make PCI bridges async -- they are the
> > parents of USB controllers.)  For resume it's the opposite: an async
> > parent with synchronous children.
> 
> Is that really going to happen in practice?  I mean, what would be the point?

I don't know.  It's all speculation until we see some actual lists.

Alan Stern

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