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SubjectRe: Instrumentation and RCU
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.

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