Re: [RFC PATCH 0/9] livepatch: consistency model

Josh Poimboeuf
2015-02-13T14:55:45Z

On Fri, Feb 13, 2015 at 03:40:14PM +0100, Miroslav Benes wrote:
> On Fri, 13 Feb 2015, Jiri Kosina wrote:
>
> > On Fri, 13 Feb 2015, Josh Poimboeuf wrote:
> >
> > > > How about we take a slightly different aproach -- put a probe (or ftrace)
> > > > on __switch_to() during a klp transition period, and examine stacktraces
> > > > for tasks that are just about to start running from there?
> > > >
> > > > The only tasks that would not be covered by this would be purely CPU-bound
> > > > tasks that never schedule. But we are likely in trouble with those anyway,
> > > > because odds are that non-rescheduling CPU-bound tasks are also
> > > > RT-priority tasks running on isolated CPUs, which we will fail to handle
> > > > anyway.
> > > >
> > > > I think Masami used similar trick in his kpatch-without-stopmachine
> > > > aproach.
> > >
> > > Yeah, that's definitely an option, though I'm really not too crazy about
> > > it. Hooking into the scheduler is kind of scary and disruptive.
> >
> > This is basically about running a stack checking for ->next before
> > switching to it, i.e. read-only operation (admittedly inducing some
> > latency, but that's the same with locking the runqueue). And only when in
> > transition phase.
> >
> > > We'd also have to wake up all the sleeping processes.
> >
> > Yes, I don't think there is a way around that.
>
> I think there are two options how to do it if I understand you correctly.
>
> 1. we would put a probe on __switch_to and afterwards wake up all the
> sleeping processes.
>
> 2. we would do it in an asynchronous manner. We would put a probe and let
> the processes to wake themselves. The transition delayed workqueue
> would only check if there is some non-migrated process. Of course if
> some process sleeps for a long time it would take a long time to
> complete the patching. It would be up to the user to send a signal to
> the process to wake up.
>
> Does it make sense? If yes, I cannot decide which approach is better.

Option 2 wouldn't really work for kthreads because you can't signal them
to wake up from user space. And I really want to avoid having to leave
the system in a partially patched state for a long period of time.

But also option 1 wouldn't necessarily result in the system being
immediately patched, since you could have some CPU-bound tasks. So some
asynchronous patching is still needed.

--
Josh