Re: [PATCH] freezer: configure user space process frozen along with kernel threads

Alan Stern
2013-02-20T21:58:05Z

On Wed, 20 Feb 2013, Eric W. Biederman wrote:

> > Oh, okay. But it's no different from any other filesystem in that
> > respect. Processes generally can't be frozen while they are waiting
> > for filesystem I/O to complete. In many cases they can't receive
> > signals either (they are in an uninterruptible wait state).
>
> Ick. So the process freezer and all network filesystems has problems?
> Especially nfs?

I don't know any of the details. On the other hand, it is not exactly
hot, up-to-the-minute news to learn that NFS has problems...

> > There's a big difference between preemption and freezing: Preemption
> > is involuntary whereas freezing is voluntary. It's like the difference
> > between preemptive and cooperative multitasking.
>
> I hadn't realized freezing was voluntary. That certainly seems like a
> pain.

More precisely, it's voluntary when processes are running in kernel
mode. When they're in user mode there is no problem; they get sent a
signal and then go into the freezer when they switch to kernel mode to
process the signal.

> >> At most I would suggest that processes be frozen in reverse priority
> >> order. Which unless there is a priority inversion should solve this
> >> problem without an additional proc file.
> >
> > Do fuse daemons (and the processes they rely upon) run with elevated
> > priority?
>
> I don't know if the daemons are of an elevated scheduling priority today
> but if they aren't it is as easy to require an elevated scheduling
> priority as it is to require a magic freezer priority. Furthermore if
> they don't run at an elevated priority there is the possibility of
> priority inversion.

This seems like a reasonable thing to try out.

Alan Stern