Re: Instrumentation and RCU

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, Mar 09, 2020 at 06:40:45PM -0700, Alexei Starovoitov wrote:
> On Mon, Mar 09, 2020 at 02:37:40PM -0400, Mathieu Desnoyers wrote:
> > > 
> > >    But what's relevant is the tracer overhead which is e.g. inflicted
> > >    with todays trace_hardirqs_off/on() implementation because that
> > >    unconditionally uses the rcuidle variant with the scru/rcu_irq dance
> > >    around every tracepoint.
> > 
> > I think one of the big issues here is that most of the uses of
> > trace_hardirqs_off() are from sites which already have RCU watching,
> > so we are doing heavy-weight operations for nothing.
> 
> I think kernel/trace/trace_preemptirq.c created too many problems for the
> kernel without providing tangible benefits. My understanding no one is using it
> in production.

Hi Alexei,
There are various people use the preempt/irq disable tracepoints for last 2
years at Google and ARM. There's also a BPF tool (in BCC) that uses those for
tracing critical sections. Also Daniel Bristot's entire Preempt-IRQ formal
verification stuff depends on it.

> It's a tool to understand how kernel works. And such debugging
> tool can and should be removed.

If we go by that line of reasoning, then function tracing also should be
removed from the kernel.

I am glad Thomas and Peter are working on it and looking forward to seeing
the patches,

thanks,

 - Joel


> One of Thomas's patches mentioned that bpf can be invoked from hardirq and
> preempt tracers. This connection doesn't exist in a direct way, but
> theoretically it's possible. There is no practical use though and I would be
> happy to blacklist such bpf usage at a minimum.
> 
> > We could use the approach proposed by Peterz's and Steven's patches to basically
> > do a lightweight "is_rcu_watching()" check for rcuidle tracepoint, and only enable
> > RCU for those cases. We could then simply go back on using regular RCU like so:
> > 
> > #define __DO_TRACE(tp, proto, args, cond, rcuidle)                      \
> >         do {                                                            \
> >                 struct tracepoint_func *it_func_ptr;                    \
> >                 void *it_func;                                          \
> >                 void *__data;                                           \
> >                 bool exit_rcu = false;                                  \
> >                                                                         \
> >                 if (!(cond))                                            \
> >                         return;                                         \
> >                                                                         \
> >                 if (rcuidle && !rcu_is_watching()) {                    \
> >                         rcu_irq_enter_irqson();                         \
> >                         exit_rcu = true;                                \
> >                 }                                                       \
> >                 preempt_disable_notrace();                              \
> >                 it_func_ptr = rcu_dereference_raw((tp)->funcs);         \
> >                 if (it_func_ptr) {                                      \
> >                         do {                                            \
> >                                 it_func = (it_func_ptr)->func;          \
> >                                 __data = (it_func_ptr)->data;           \
> >                                 ((void(*)(proto))(it_func))(args);      \
> >                         } while ((++it_func_ptr)->func);                \
> >                 }                                                       \
> >                 preempt_enable_notrace();                               \
> >                 if (exit_rcu)                                           \
> >                         rcu_irq_exit_irqson();                          \
> >         } while (0)
> 
> I think it's a fine approach interim.
> 
> Long term sounds like Paul is going to provide sleepable and low overhead
> rcu_read_lock_for_tracers() that will include bpf.
> My understanding that this new rcu flavor won't have "idle" issues,
> so rcu_is_watching() checks will not be necessary.
> And if we remove trace_preemptirq.c the only thing left will be Thomas's points
> 1 (low level entry) and 2 (breakpoints) that can be addressed without
> creating fancy .text annotations and teach objtool about it.
> 
> In the mean time I've benchmarked srcu for sleepable bpf and it's quite heavy.
> srcu_read_lock+unlock roughly adds 10x execution cost to trivial bpf prog.
> I'm proceeding with it anyway, but really hoping that
> rcu_read_lock_for_tracers() will materialize soon.
> 
> In general I'm sceptical that .text annotations will work. Let's say all of
> idle is a red zone. But a ton of normal functions are called when idle. So
> objtool will go and mark them as red zone too. This way large percent of the
> kernel will be off limits for tracers. Which is imo not a good trade off. I
> think addressing 1 and 2 with explicit notrace/nokprobe annotations will cover
> all practical cases where people can shot themselves in a foot with a tracer. I
> realize that there will be forever whack-a-mole game and these annotations will
> never reach 100%. I think it's a fine trade off. Security is never 100% either.
> Tracing is never going to be 100% safe too.