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

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Message-Id: <200707051449.55004.rjw@sisk.pl>
Date:	Thu, 5 Jul 2007 14:49:54 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Miklos Szeredi <miklos@...redi.hu>, stern@...land.harvard.edu,
	oliver@...kum.org, mjg59@...f.ucam.org,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [linux-pm] [PATCH] Remove process freezer from suspend to RAM pathway

On Thursday, 5 July 2007 02:43, Paul Mackerras wrote:
> Miklos Szeredi writes:
> 
> > OK, let me summarize the situation as I see it now: there are two
> > camps, the pro-freezers and the anti-freezers.
> > 
> > Pro-freezers say:
> > 
> >   - don't remove the freezer, otherwise we'll have to deal with
> >     numerous problems in drivers
> > 
> > Anti-freezers say:
> > 
> >   - let's remove the freezer, which causes numerous problems
> > 
> > Alan summerized the pro-freezer arguments well I think.  What are the
> > anti-freezer arguments then?
> 
> 1. The freezer cannot be guaranteed deadlock-free without constructing
>    a dependency graph between tasks (both user and kernel), which is
>    virtually impossible since the dependencies are not externally
>    observable.

I don't agree with that.

The freezer only fails to handle uninterruptible tasks, so we need to take
the situations in which an uninterruptible task waits for a frozen task into
consideration.  Now, if both tasks are from the user land, this is highly
unusual.

> 2. As a consequence of (1), we try to make a crude approximation of
>    the graph by saying "only kernel threads that want to be frozen
>    will be frozen" or some other similar statement.

No.  The rule is that kernel threads should not be freezable, but there are
some for which that is useful.

> 3. However, (2) means that we can no longer guarantee that drivers
>    will not get any I/O requests after their suspend method has been
>    called, and therefore the freezer fails in its main objective.

This is a very general statement.  Can you please give some examples?

> 4. We have an existence proof that reliable suspend can be achieved
>    without the freezer.

No.  We only know that it might work if the nonboot CPUs are disabled before
suspending devices, which is not the case in the generic suspend code.

> To summarize, the argument is that the freezer is deadlock-prone and
> ineffective.

I remain unconvinced. ;-)

Greetings,
Rafael


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