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

From: Paul E. McKenney
Date: Tue Jul 29 2014 - 16:11:34 EST

On Tue, Jul 29, 2014 at 09:25:04PM +0200, Peter Zijlstra wrote:
> 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.

A challenge! ;-)

> > > 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.

There was a debate about what points in a kernel thread were "safe
points" a few months back, which might be related.

Thanx, Paul

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