linux-kernel - Re: [RFC][PATCH 0/7] PM: Asynchronous suspend and resume (updated)

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Date:	Wed, 19 Aug 2009 18:34:11 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	linux-pm <linux-pm@...ts.linux-foundation.org>,
	linux-acpi <linux-acpi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Zhang Rui <rui.zhang@...el.com>, Len Brown <lenb@...nel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Greg KH <gregkh@...e.de>
Subject: Re: [RFC][PATCH 0/7] PM: Asynchronous suspend and resume (updated)

On Wed, 19 Aug 2009, Rafael J. Wysocki wrote:

> On Wednesday 19 August 2009, Alan Stern wrote:
> > On Wed, 19 Aug 2009, Rafael J. Wysocki wrote:
> > 
> > > There's a problem that for safety reasons I maintain the ordering of dpm_list
> > > and the callbacks are scheduled for async execution in the same order in
> > > which they would have been executed synchronously.  If were to change this,
> > > we'd have to be _very_ careful.
> > 
> > Why?  The order in which jobs are scheduled for async execution doesn't 
> > bear any particular relation to the order in which they get run.
> 
> Yes, it does, if all of the async threads are busy and we add more async jobs
> to the queue.  We must ensure that none of the jobs being executed will wait
> for any jobs in the queue.

That's part of the reason for the dependencies: to make sure that where
it matters, things don't run in the wrong order.

> Also, if any devices are handled synchronously, they must not wait for any
> "async" devices that haven't been scheduled yet.

I'm not sure what you mean.  If a device is handled synchronously, it 
must wait for all the devices preceding it on the list, regardless of 
whether those devices are handled asynchronously.

> With a master thread that would do all the waiting that wouldn't be a problem
> any more, but I'm not sure how to implement such a thread efficiently.  The
> problem is that each device may depend on multiple other devices, so even
> if one callback finishes, there's no guarantee there will be any device with
> satisfied dependencies, so it looks like the master thread would have to
> browse dpm_list continuously searching it for devices that are ready for
> suspending.
> 
> Also I don't think we can change the ordering of dpm_list as a result of
> asynchronous execution.

Okay, so I'll try to write something.  We'll see how it comes out.

Alan Stern

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