Re: [PATCH RFC tip/core/rcu 09/16] rcu-tasks: Add an RCU-tasks rude variant

[Lai Jiangshan]

On Tue, Mar 17, 2020 at 6:03 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Mon, 16 Mar 2020 17:45:40 -0400
> Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> > >
> > > Same for the function side (if not even more so). This would require adding
> > > a srcu_read_lock() to all functions that can be traced! That would be a huge
> > > kill in performance. Probably to the point no one would bother even using
> > > function tracer.
> >
> > Point well taken! Thanks,
>
> Actually, it's worse than that. (We talked about this on IRC but I wanted
> it documented here too).
>
> You can't use any type of locking, unless you insert it around all the
> callers of the nops (which is unreasonable).
>
> That is, we have gcc -pg -mfentry that creates at the start of all traced
> functions:
>
>  <some_func>:
>     call __fentry__
>     [code for function here]
>
> At boot up (or even by the compiler itself) we convert that to:
>
>  <some_func>:
>     nop
>     [code for function here]
>
>
> When we want to trace this function we use text_poke (with current kernels)
> and convert it to this:
>
>  <some_func>:
>     call trace_trampoline
>     [code for function here]
>
>
> That trace_trampoline can be allocated, which means when its no longer
> needed, it must be freed. But when do we know it's safe to free it? Here's
> the issue.
>
>
>  <some_func>:
>     call trace_trampoline  <- interrupt happens just after the jump
>     [code for function here]
>
> Now the task has just executed the call to the trace_trampoline. Which
> means the instruction pointer is set to the start of the trampoline. But it
> has yet executed that trampoline.
>
> Now if the task is preempted, and a real time hog is keeping it from
> running for minutes at a time (which is possible!). And in the mean time,
> we are done with that trampoline and free it. What happens when that task
> is scheduled back? There's no more trampoline to execute even though its
> instruction pointer is to execute the first operand on the trampoline!
>
> I used the analogy of jumping off the cliff expecting a magic carpet to be
> there to catch you, and just before you land, it disappears. That would be
> a very bad day indeed!
>
> We have no way to add a grace period between the start of a function (can
> be *any* function) and the start of the trampoline.

Hello

I think adding a small number of instructions to preempt_schedule_irq()
is sufficient to create the needed protected region between the start
of a function and the trampoline body.

preempt_schedule_irq() {
+  if (unlikely(is_trampoline_page(page_of(interrupted_ip)))) {
+      return; // don't do preempt schedule
+
+  }
  preempt_schedule_irq() original body
}

// generated on trampoline pages
trace_trampoline() {
   preempt_disable();
   trace_trampoline body
   jmp preempt_enable_traced(clobbers)
}

asm(kernel text):
preempt_enable_traced:
   preempt_enable_notrace();
   restore cobblers
   return(the return ip on the stack is traced_function_start_code)


If the number of instructions added in preempt_schedule_irq() and
the complexity to make trampoline ip detectable(is_trampoline_page(),
or is_trampoline_range()) are small, and tasks_rcu is rendered useless,
I think it will be win-win.

Thanks

Lai

> Since the problem is
> that the task was non-voluntarily preempted before it could execute the
> trampoline, and that trampolines are not allowed (suppose) to call
> schedule, then we have our quiescent state to track (voluntary scheduling).
> When all tasks have either voluntarily scheduled, or entered user space
> after disconnecting a trampoline from a function, we know that it is safe to
> free the trampoline.
>
> -- Steve