Re: Tasks RCU vs Preempt RCU

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

On Tue, May 22, 2018 at 08:38:32AM -0400, Steven Rostedt wrote:
> On Mon, 21 May 2018 21:54:14 -0700
> Joel Fernandes <joel@joelfernandes.org> wrote:
> 
> 
> > Yes, lets brain storm this if you like. One way I was thinking if we can
> > manually check every CPU and see what state its in (usermode, kernel, idle
> > etc) using an IPI mechanism. Once all CPUs have been seen to be in usermode,
> > or idle atleast once - then we are done. You have probably already thought
> 
> Nope, it has nothing to do with CPUs, it really has to do with tasks.
> 
> 	CPU0
> 	----
>  task 1: (pinned to CPU 0)
> 	 call func_tracer_trampoline
> 	 [on trampoline]
> 	 [timer tick, schedule ]
> 
>  task 2: (higher priority, also pinned to CPU 0)
> 	 goes to user space
> 	 [ Runs for along time ]
> 
> We cannot free the trampoline until task 2 releases the CPU and lets
> task 1 run again to get off the CPU.

What Steven said!  IPIs get to CPUs, but we need to handle the
(unlikely, but very real) case where a bunch of tasks are preempted
within trampolines.

> > about this so feel free to say why its not a good idea, but to me there are 3
> > places that a tasks quiescent state is recorded: during the timer tick,
> > during task sleep and during rcu_note_voluntary_context_switch in
> > cond_resched_rcu_qs. Of these, I feel only the cond_resched_rcu_qs case isn't
> > trackable with IPI mechanism which may make the detection a bit slower, but
> > tasks-RCU in mainline is slow right now anyway (~ 1 second delay if any task
> > was held).
> 
> The way I was originally going to handle this was with a per task
> counter, where it can be incremented at certain points via tracepoints.
> 
> Thus my synchronize tasks, would have connected to a bunch of
> tracepoints at known quiescent states that would increment the counter,
> and then check each task until they all pass a certain point, or are in
> a quiescent state (userspace or idle). But this would be doing much of
> what RCU does today, and that is why we decided to hook with the RCU
> infrastructure.

Just for the record, if you guys realy want to take over Tasks RCU,
I have no objections.  For one thing, I don't anticipate any other use
cases for it (famous last words!).  But you break it, you buy it!  ;-)

							Thanx, Paul

> I have to ask, what's your motivation for getting rid of RCU tasks?
> 
> -- Steve
>