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

From: Miroslav Benes
Date: Fri Feb 13 2015 - 09:40:23 EST

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.

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