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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
