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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Jun 18, 2014 at 09:56:26PM -0400, Steven Rostedt wrote:
> 
> Another blast from the past (from the book of cleaning out inbox)
> 
> On Wed, 29 May 2013 09:52:49 +0200
> Peter Zijlstra <peterz@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 :-)

I am still expecting to, depite my misadventures with performance
regressions.

							Thanx, Paul