Re: [RFC][PATCH] ftrace: Use schedule_on_each_cpu() as a heavy synchronize_sched()

[Peter Zijlstra <peterz@infradead.org>]

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.