On Sun, Jul 21, 2019 at 04:31:13PM -0700, Paul E. McKenney wrote: > On Sun, Jul 21, 2019 at 02:08:37PM -0700, Matthew Wilcox wrote: > > On Sun, Jul 21, 2019 at 06:17:25AM -0700, Paul E. McKenney wrote: > > > Also, the overhead is important. For example, as far as I know, > > > current RCU gracefully handles close(open(...)) in a tight userspace > > > loop. But there might be trouble due to tight userspace loops around > > > lighter-weight operations. > > > > I thought you believed that RCU was antifragile, in that it would scale > > better as it was used more heavily? > > You are referring to this? https://paulmck.livejournal.com/47933.html > > If so, the last few paragraphs might be worth re-reading. ;-) > > And in this case, the heuristics RCU uses to decide when to schedule > invocation of the callbacks needs some help. One component of that help > is a time-based limit to the number of consecutive callback invocations > (see my crude prototype and Eric Dumazet's more polished patch). Another > component is an overload warning. > > Why would an overload warning be needed if RCU's callback-invocation > scheduling heurisitics were upgraded? Because someone could boot a > 100-CPU system with the rcu_nocbs=0-99, bind all of the resulting > rcuo kthreads to (say) CPU 0, and then run a callback-heavy workload > on all of the CPUs. Given the constraints, CPU 0 cannot keep up. > > So warnings are required as well. > > > Would it make sense to have call_rcu() check to see if there are many > > outstanding requests on this CPU and if so process them before returning? > > That would ensure that frequent callers usually ended up doing their > > own processing. > > Unfortunately, no. Here is a code fragment illustrating why: > > void my_cb(struct rcu_head *rhp) > { > unsigned long flags; > > spin_lock_irqsave(&my_lock, flags); > handle_cb(rhp); > spin_unlock_irqrestore(&my_lock, flags); > } > > . . . > > spin_lock_irqsave(&my_lock, flags); > p = look_something_up(); > remove_that_something(p); > call_rcu(p, my_cb); > spin_unlock_irqrestore(&my_lock, flags); > > Invoking the extra callbacks directly from call_rcu() would thus result > in self-deadlock. Documentation/RCU/UP.txt contains a few more examples > along these lines. We could add an option that simply fails if overloaded, right? Have caller recover... -- MST
