Another blast from the past (from the book of cleaning out inbox) On Wed, 29 May 2013 09:52:49 +0200 Peter Zijlstra <pet...@infradead.org> wrote:
> On Tue, May 28, 2013 at 08:01:16PM -0400, Steven Rostedt wrote: > > The function tracer uses preempt_disable/enable_notrace() for > > synchronization between reading registered ftrace_ops and unregistering > > them. > > > > Most of the ftrace_ops are global permanent structures that do not > > require this synchronization. That is, ops may be added and removed from > > the hlist but are never freed, and wont hurt if a synchronization is > > missed. > > > > But this is not true for dynamically created ftrace_ops or control_ops, > > which are used by the perf function tracing. > > > > The problem here is that the function tracer can be used to trace > > kernel/user context switches as well as going to and from idle. > > Basically, it can be used to trace blind spots of the RCU subsystem. > > This means that even though preempt_disable() is done, a > > synchronize_sched() will ignore CPUs that haven't made it out of user > > space or idle. These can include functions that are being traced just > > before entering or exiting the kernel sections. > > Just to be clear, its the idle part that's a problem, right? Being stuck > in userspace isn't a problem since if that CPU is in userspace its > certainly not got a reference to whatever list entry we're removing. > > Now when the CPU really is idle, its obviously not using tracing either; > so only the gray area where RCU thinks we're idle but we're not actually > idle is a problem? > > Is there something a little smarter we can do? Could we use > on_each_cpu_cond() with a function that checks if the CPU really is > fully idle? > > > To implement the RCU synchronization, instead of using > > synchronize_sched() the use of schedule_on_each_cpu() is performed. This > > means that when a dynamically allocated ftrace_ops, or a control ops is > > being unregistered, all CPUs must be touched and execute a ftrace_sync() > > stub function via the work queues. This will rip CPUs out from idle or > > in dynamic tick mode. This only happens when a user disables perf > > function tracing or other dynamically allocated function tracers, but it > > allows us to continue to debug RCU and context tracking with function > > tracing. > > I don't suppose there's anything perf can do to about this right? Since > its all on user demand we're kinda stuck with dynamic memory. If Paul finished his "synchronize_all_tasks_scheduled()" then we could use that instead. Where "synchornize_all_tasks_scheduled()" would return after all tasks have either scheduled, in userspace, or idle (that is, not on the run queue). And scheduled means a non preempted schedule, where the task itself actually called schedule. Paul, how you doing on that? You said you could have something by 3.17. That's coming up quick :-) -- Steve -- 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/
