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

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Date:	Wed, 20 Feb 2013 16:58:03 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
cc:	Li Fei <fei.li@...el.com>, <pavel@....cz>, <rjw@...k.pl>,
	<len.brown@...el.com>, <mingo@...hat.com>, <peterz@...radead.org>,
	<akpm@...ux-foundation.org>, <viro@...iv.linux.org.uk>,
	<gorcunov@...nvz.org>, <rientjes@...gle.com>,
	<linux-kernel@...r.kernel.org>, <linux-pm@...r.kernel.org>,
	<chuansheng.liu@...el.com>, <biao.wang@...el.com>
Subject: Re: [PATCH] freezer: configure user space process frozen along with
 kernel threads

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

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