Re: Instrumentation and RCU

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Mar 10, 2020, at 11:46 AM, rostedt rostedt@goodmis.org wrote:

> On Tue, 10 Mar 2020 11:31:51 -0400 (EDT)
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> 
>> I think there are two distinct problems we are trying to solve here,
>> and it would be good to spell them out to see which pieces of technical
>> solution apply to which.
>> 
>> Problem #1) Tracer invoked from partially initialized kernel context
>> 
>>   - Moving the early/late entry/exit points into sections invisible from
>>     instrumentation seems to make tons of sense for this.
>> 
>> Problem #2) Tracer recursion
>> 
>>   - I'm much less convinced that hiding entry points from instrumentation
>>     works for this. As an example, with the isntr_begin/end() approach you
>>     propose above, as soon as you have a tracer recursing into itself because
>>     something below do_stuff() has been instrumented, having hidden the entry
>>     point did not help at all.
>> 
>> So I would be tempted to use the "hide entry/exit points" with explicit
>> instr begin/end annotation to solve Problem #1, but I'm still thinking there
>> is value in the per recursion context "in_tracing" flag to prevent tracer
>> recursion.
> 
> The only recursion issue that I've seen discussed is breakpoints. And
> that's outside of the tracer infrastructure. Basically, if someone added a
> breakpoint for a kprobe on something that gets called in the int3 code
> before kprobes is called we have (let's say rcu_nmi_enter()):
> 
> 
> rcu_nmi_enter();
>  <int3>
>     do_int3() {
>        rcu_nmi_enter();
>          <int3>
>             do_int3();
>                [..]
> 
> Where would a "in_tracer" flag help here? Perhaps a "in_breakpoint" could?

An approach where the "in_tracer" flag is tested and set by the instrumentation
(function tracer, kprobes, tracepoints) would work here. Let's say the beginning
of the int3 ISR is part of the code which is invisible to instrumentation, and
before we issue rcu_nmi_enter(), we handle the in_tracer flag:

rcu_nmi_enter();
 <int3>
    (recursion_ctx->in_tracer == false)
    set recursion_ctx->in_tracer = true
    do_int3() {
       rcu_nmi_enter();
         <int3>
            if (recursion_ctx->in_tracer == true)
                iret

We can change "in_tracer" for "in_breakpoint", "in_tracepoint" and
"in_function_trace" if we ever want to allow different types of instrumentation
to nest. I'm not sure whether this is useful or not through.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com