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From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: stern@rowland.harvard.edu, linux-kernel@vger.kernel.org,
	ncunningham@crca.org.au, linux-pm@lists.linux-foundation.org
Subject: Re: [linux-pm] Freezer: Don't count threads waiting for frozen filesystems.
Date: Thu, 30 Oct 2008 21:17:48 +0100	[thread overview]
Message-ID: <200810302117.49419.rjw@sisk.pl> (raw)
In-Reply-To: <E1KvYgR-0002Cx-AZ@pomaz-ex.szeredi.hu>

On Thursday, 30 of October 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.
> 
> > > > 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).
> 
> > 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.

I like this idea.

I was thinking about using task flags to mark processes as "not freezable
at the moment", which would make the freezer to wait for such tasks
(currently a task may only be always freezable or not freezable at all,
which is not very flexible).

Unfortunately, I didn't have the time to implement this.

Thanks,
Rafael

  parent reply	other threads:[~2008-10-30 20:13 UTC|newest]

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <1224886068.6478.21.camel@nigel-laptop>
2008-10-26 20:01 ` Rafael J. Wysocki
2008-10-27 11:12   ` Miklos Szeredi
2008-10-27 11:20     ` Nigel Cunningham
2008-10-27 11:37       ` Rafael J. Wysocki
2008-10-27 11:40         ` Nigel Cunningham
2008-10-27 12:38           ` Miklos Szeredi
2008-10-27 20:59             ` Nigel Cunningham
2008-10-27 21:09               ` Miklos Szeredi
2008-10-27 22:13                 ` Nigel Cunningham
2008-10-28 20:25                   ` Miklos Szeredi
2008-10-28 21:29                     ` Nigel Cunningham
2008-10-28 21:51                       ` Miklos Szeredi
2008-10-28 22:00                         ` Rafael J. Wysocki
2008-10-28 22:02                           ` Miklos Szeredi
2008-10-28 22:21                             ` Rafael J. Wysocki
2008-10-28 23:21                               ` Miklos Szeredi
2008-10-28 23:59                                 ` Rafael J. Wysocki
2008-10-29  8:10                                   ` Miklos Szeredi
2008-10-29 13:51                                     ` Alan Stern
2008-10-29 14:50                                       ` Miklos Szeredi
2008-10-29 15:28                                         ` Alan Stern
2008-10-29 15:50                                           ` Miklos Szeredi
2008-10-29 16:17                                             ` Alan Stern
2008-10-29 16:10                                           ` Miklos Szeredi
2008-10-29 20:37                                             ` Alan Stern
2008-10-29 21:11                                               ` Rafael J. Wysocki
2008-10-29 21:45                                                 ` Nigel Cunningham
2008-10-29 22:07                                                   ` Rafael J. Wysocki
2008-10-29 23:53                                                     ` Miklos Szeredi
2008-11-09 13:44                                                       ` Pavel Machek
2008-10-29 23:48                                                   ` Miklos Szeredi
2008-10-30 13:04                                                     ` Nigel Cunningham
2008-10-30 13:56                                                       ` Alan Stern
2008-10-30 21:44                                                         ` Nigel Cunningham
2008-10-31  8:49                                                           ` Miklos Szeredi
2008-10-31  9:10                                                             ` Nigel Cunningham
2008-10-31  9:16                                                               ` Miklos Szeredi
2008-10-31 11:28                                                                 ` Nigel Cunningham
2008-10-31 12:44                                                                   ` Miklos Szeredi
2008-10-31 21:11                                                                     ` Nigel Cunningham
2008-10-29 23:43                                               ` Miklos Szeredi
2008-10-30 13:54                                                 ` Alan Stern
2008-10-30 14:39                                                   ` Miklos Szeredi
2008-10-30 17:07                                                     ` Alan Stern
2008-10-30 17:43                                                       ` Miklos Szeredi
2008-10-30 20:17                                                     ` Rafael J. Wysocki [this message]
2008-11-15 16:58                                                     ` Pavel Machek
2008-10-29 23:56                                               ` Matthew Garrett
2008-10-28 22:03                         ` Nigel Cunningham
2008-10-28 23:04                     ` Nigel Cunningham
2008-10-28 23:12                       ` Miklos Szeredi
2008-10-28 23:17                         ` Nigel Cunningham
2008-10-28 23:24                           ` Miklos Szeredi
2008-10-28 23:41                             ` Nigel Cunningham
2008-10-28 23:45                               ` Miklos Szeredi
2008-10-28 23:50                                 ` Miklos Szeredi
2008-10-28 23:58                                   ` Nigel Cunningham
2008-10-28 23:54                                 ` Nigel Cunningham
2008-10-27 11:37       ` Miklos Szeredi

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