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

[Paul E. McKenney]

On Sun, May 10, 2020 at 05:59:27PM +0800, Lai Jiangshan wrote:
> 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.

It certainly would provide a nice reduction in code size!

This would provide a zero-instructions preempt_disable() at the beginning
of the trampoline and a zero-instructions preempt_enable_no_resched() at
the end, correct?  If so, wouldn't this create a potentially long (though
"weak") preempt-disable region extending to the next preempt_enable(),
local_bh_enable(), schedule(), interrupt, transition to userspace,
or similar?  This could be quite some time.  Note that cond_resched()
wouldn't help, given that this is only in PREEMPT=y kernels.

The "weak" refers to the fact that if a second resched IPI arrived in the
meantime, preemption would then happen.  But without that second IPI,
the request for preemption could be ignored for quite some time.

Or am I missing something here?

							Thanx, Paul

> 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