linux-kernel - Re: [RFC][PATCH -mm 2/3] freezer: Introduce freezer

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Date:	Mon, 23 Apr 2007 20:49:58 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	ego@...ibm.com
Cc:	Satyam Sharma <satyam.sharma@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Oleg Nesterov <oleg@...sign.ru>,
	linux-kernel@...r.kernel.org, vatsa@...ibm.com, paulmck@...ibm.com,
	pavel@....cz
Subject: Re: [RFC][PATCH -mm 2/3] freezer: Introduce freezer_flags

On Monday, 23 April 2007 16:19, Gautham R Shenoy wrote:
> Hi Satyam, 
> On Mon, Apr 23, 2007 at 09:39:30AM +0530, Satyam Sharma wrote:
> > Hi Rafael,
> > 
> > >+/*
> > >+ *     Per task flags used by the freezer
> > >+ *
> > >+ *     They should not be referred to directly outside of this file.
> > >+ */
> > >+#define TFF_NOFREEZE   0       /* task should not be frozen */
> > >+#define TFF_FREEZE     8       /* task should go to the refrigerator ASAP 
> > >*/
> > >+#define TFF_SKIP       9       /* do not count this task as freezable */
> > >+#define TFF_FROZEN     10      /* task is frozen */
> > 
> > Aren't NOFREEZE and SKIP doing the same thing? One of them appears
> > superfluous. I'm looking at 21-rc6-mm1 and vfork(2) seems to be its
> > only user. Seeing how vfork(2) used it, can't the call to
> > freezer_do_not_count() be replaced with a call to freezer_exempt()?
> > Similarly, the freezer_count() after the wait_for_completion might
> > just as well be a clear of the NOFREEZE bit followed by a
> > try_to_freeze(). Could you please explain the rationale behind the
> > SKIP flag?
> 
> The difference between the NOFREEZE and the SKIP flag is a subtle one.
> 
> When a task (say p) sets it's NOFREEZE flag, it tells the freezer not to
> consider it for freezing. Which means freezeable(p) will return 0.
> So the freezer will not even mark it for freezing. 
> 
> However, when a task sets it SKIP flag, it tells the freezer - "I might
> block at a safe place. So when you are counting the processes which
> have been marked as freezeable, but have not frozen yet, please don't
> count me in. IOW, please skip me."  
> Thus such a task can still be marked for freezing.
> 
> The typical usage is 
> freezer_do_not_count(current);
> /* currents goes to an uninterruptible sleep, like wait_for_completion. */
> freezer_count(current);
> 
> Once the task wakes up from it's uninterruptible sleep, it will 
> call freezer_count which in turn calls try_to_freeze.
> If the task was marked for freezing, it will be frozen now.
> 
> You may want to check the thread http://lkml.org/lkml/2007/2/18/47 
> on how it came into existance.

Very well explained.  Thanks!

Greetings,
Rafael

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