idea: set a niceness value under which a process won't awaken suspended CPU

Lennert Buytenhek buytenh at
Fri Mar 2 05:06:03 EST 2012

On Fri, Mar 02, 2012 at 01:50:16AM -0800, Jon Nettleton wrote:

> > One problem you can get into with this scheme is a kind of priority
> > inversion.  If the low priority process does:
> >
> >        fd = open("/foo/bar", O_RDWR);
> >        flock(fd, LOCK_EX);
> >
> > and the high priority process then also does:
> >
> >        fd = open("/foo/bar", O_RDWR);
> >        flock(fd, LOCK_EX);
> >
> > and the low priority process then does:
> >
> >        sleep(1);
> >        flock(fd, LOCK_UN);
> >
> > and the system then goes into suspend, you'll not wake up again
> > after that second expires, and you might not wake up again soon
> > at all.  The low priority process doesn't even have to sleep
> > explicitly, if it takes a page fault you get basically the same
> > thing.
> I believe the cgroup method will catch this though.  I would have
> to double check, but I think that the cgroup becomes partially
> frozen returning EBUSY until all the processes can properly be
> frozen.  At that time I believe it is up to userspace to thaw, or
> refreeze the group, unless the blocking process ends in which case
> the entire cgroup is set to the frozen state.

You can still suffer from priority inversion if you freeze a cgroup
with a low priority process in it that holds a resource that another
high priority process needs.

You indeed can't suffer from priority inversion between processes in
the same cgroup if you are going to freeze all of them, but then you
don't care about CPU wakeups for the high priority process either.

I guess that what I don't see yet is how you would use cgroups to
implement the goals of John's scheme.

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