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

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Fri, Feb 13, 2015 at 03:22:15PM +0100, 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.

Yes, but it would introduce much more latency than locking rq, since
there would be at least some added latency to every schedule() call
during the transition phase.  Locking the rq would only add latency in
those cases where another CPU is trying to do a context switch while
we're holding the lock.

It also seems much more dangerous.  A bug in __switch_to() could easily
do a lot of damage.

> > We'd also have to wake up all the sleeping processes.
> 
> Yes, I don't think there is a way around that.

Actually this patch set is a way around that :-)

-- 
Josh