linux-kernel - Re: [PATCH] Remove process freezer from suspend to RAM pathway

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Message-Id: <200707082345.27859.rjw@sisk.pl>
Date:	Sun, 8 Jul 2007 23:45:26 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Kyle Moffett <mrmacman_g4@....com>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Pavel Machek <pavel@....cz>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org
Subject: Re: [PATCH] Remove process freezer from suspend to RAM pathway

On Sunday, 8 July 2007 23:03, Benjamin Herrenschmidt wrote:
> On Sun, 2007-07-08 at 21:15 +0200, Rafael J. Wysocki wrote:
> > On Sunday, 8 July 2007 07:14, Benjamin Herrenschmidt wrote:
> > [--snip--]
> > > 
> > > I just think that the freezer approach, as it is, is backward. We can't
> > > have a 3rd party try to discriminate what to freeze and what not, it
> > > will always get something wrong, and in some cases with the wrong timing
> > > or ordering.
> > 
> > Nice discussion, except for one thing: the freezer doesn't decide what to
> > freeze.  For example, even right now kernel threads decide if they want to be
> > frozen.
> 
> Somewhat... userspace doesn't and workqueues are a gray area.

Workqueues are kernel threads and the creator decides if they are going to
freeze.  There are only two freezable worqueues in the entire tree right now.

> Also, I've been thinking this "icebox" idea a bit more and it seems in
> fact a bit racy in some areas, at least for use by things like drivers,
> unless we end up doing something aking to an RCU on suspend, waiting for
> all tasks to reach userland once, but that has the same annoyances as
> the current freezer.
> 
> Thus I'm tempted to go back to saying that driver can handle things
> locally :-)

Actaully, I'm perfectly fine with that, as long as each task blocked by the
driver due to suspend has PF_FROZEN (or something similar) set.  Then, at
least theoretically, we'll be able to drop the freezer from the suspend code
path and move it after device_suspend() (or the hibernation-specific
equivalent) for hibernation (in that case there shouldn't be a problem with
any task waiting on I/O while the freezer is running ;-)).

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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