Re: [PATCH RFC tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 29, 2014 at 11:19:49AM -0700, Paul E. McKenney wrote:
> > I feel we're doing far too much async stuff already and it keeps getting
> > worse and worse. Ideally we'd be able to account every cycle of kernel
> > 'overhead' to a specific user action.
> 
> Hmmm...
> 
> In theory, we could transfer the overhead of the kthread for a given grace
> period to the task invoking the corresponding synchronize_rcu_tasks().
> In practice, the overhead might need to be parceled out among several
> tasks that concurrently invoked synchronize_rcu_tasks().  Or I suppose
> that the overhead could be assigned to the first such task that woke
> up, on the theory that things would even out over time.
> 
> So exactly how annoyed are you about the lack of accounting?  ;-)

Its a general annoyance that people don't seem to consider this at all.

And RCU isn't the largest offender by a long shot.

> > Another reason is that I fundamentally dislike polling stuff.. but yes,
> > I'm not really seeing how to do this differently, partly because I'm not
> > entirely sure why we need this to begin with. I'm not sure what problem
> > we're solving.
> 
> As I recall it...
> 
> Steven is working on some sort of tracing infrastructure that involves
> dynamically allocated trampolines being inserted into some/all functions.
> The trampoline code can be preempted, but never does voluntary context
> switches, and presumably never calls anything that does voluntary
> context switches.
> 
> Easy to insert a trampoline, but the trick is removing them.
> 
> The thought is to restore the instructions at the begining of the
> function in question, wait for an RCU-tasks grace period, then dispose
> of the trampoline.
> 
> Of course, you could imagine disabling preemption or otherwise entering
> an RCU read-side critical section before transferring to the trampoline,
> but this was apparently a no-go due to the overhead for small functions.

So why not use the freezer to get the kernel into a known good state and
then remove them trampolines? That would mean a more noticeable
disruption of service, but it might be ok for something like disabling a
tracer or so. Dunno.

Kernel threads are the problem here, lemme ponder this for a bit.