linux-kernel - Re: [linux-pm] Freezer: Don't count threads waiting for frozen filesystems.

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Message-ID: <20081115165851.GA1523@ucw.cz>
Date:	Sat, 15 Nov 2008 17:58:51 +0100
From:	Pavel Machek <pavel@...e.cz>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	stern@...land.harvard.edu, rjw@...k.pl,
	linux-kernel@...r.kernel.org, ncunningham@...a.org.au,
	linux-pm@...ts.linux-foundation.org
Subject: Re: [linux-pm] Freezer: Don't count threads waiting for frozen
	filesystems.

Hi!

> > > > I don't know.  There are other interfaces too, like sysfs attributes, 
> > > > that would have to be handled specially.  On the whole, the freezer 
> > > > seems much, much simpler.
> > > 
> > > OK, then non-device files on "regular" filesystems.
> > 
> > Would you like to write a first-pass patch?  I don't think it will 
> > work.
> 
> If somebody doesn't beat me to it, I'll do that (first implemented
> with a global rw-sem).

Cool!
l
> > Doing that seems like a lot of work, just as modifying every driver 
> > does.  Changing a few kernel entry points is simpler, but I'm pretty
> > sure it won't work.  For instance, tasks can block arbitrarily long on 
> > read calls (waiting for data to arrive); you can't allow such things to 
> > prevent the system from suspending.
> 
> But we already do: either
> 
>  a) it's in interruptible sleep (I/O on sockets, pipes, etc), and
>     freezing simply interrupts it, or
> 
>  b) it's in uninterruptible sleep and suspend will wait it out (or
>     time out).
> 
> In the new scheme we could retain that part of the freezer: interrupt
> all tasks which are inside the critical region and wait for them to
> exit the critical region.
> 
> To put it in another way: it's still the freezer, it does all the same
> things as the old freezer, except that the condition for freezing is
> not that the task is out of the kernel, rather that it's out of the
> disable_supend - enable_suspend region.  As such it's not a big change
> to the whole suspend system, and so there shouldn't be anything big
> going wrong there.

Disadvantage is it will add overhead to regular syscalls, at least
initialy. That's why I implemented freezer initialy...

Of course, suspend is more important than it was back then, and
disadvantages of freezer are now well known, so maybe a little
overhead in exchange of cleaner design is worth it...?
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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