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

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Date:	Thu, 30 Oct 2008 13:07:27 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Miklos Szeredi <miklos@...redi.hu>
cc:	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.

On Thu, 30 Oct 2008, Miklos Szeredi wrote:

> On Thu, 30 Oct 2008, Alan Stern wrote:
> > On Thu, 30 Oct 2008, Miklos Szeredi wrote:
> > 
> > > On Wed, 29 Oct 2008, Alan Stern wrote:
> > > > I discussed this last summer with Rafael.  It's a lot harder than it 
> > > > looks, for all sorts of reasons.  For example, what about user tasks 
> > > > that have access to memory-mapped I/O regions?
> > > 
> > > What about them?  Freezing doesn't seem to help with that.
> > 
> > Sure it does.  A frozen process can't touch a memory-mapped I/O region, 
> > whereas a non-frozen process can.
> 
> But it can be in the middle of I/O by your definition.

True.  Yet another problem...

> > 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).

Converting it to per-CPU counters later on should be fairly easy.

> > 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.

Okay.  Don't forget things like ioctl for sockets -- they often involve 
doing I/O directly to the network interface device.

What happens to a task accessing a non-regular file on a fuse 
filesystem?  :-)

Alan Stern

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