Messages in this thread | | | Date | Wed, 29 May 2013 09:52:49 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC][PATCH] ftrace: Use schedule_on_each_cpu() as a heavy synchronize_sched() |
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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.
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