Re: Instrumentation and RCU

[Thomas Gleixner <tglx@linutronix.de>]

Mathieu Desnoyers <mathieu.desnoyers@efficios.com> writes:
> ----- On Mar 9, 2020, at 3:52 PM, Thomas Gleixner tglx@linutronix.de wrote:
>> In a quick test I did with a invalid syscall number with profiling the
>> trace_hardirqs_off() is pretty prominent and goes down by roughly a
>> factor of 2 when I move it past enter_from_user_mode() and use just the
>> non RCU idle variant.
>
> I think one issue here is that trace_hardirqs_off() is now shared between
> lockdep and tracing. For lockdep, we have the following comment:
>
>         /*
>          * IRQ from user mode.
>          *
>          * We need to tell lockdep that IRQs are off.  We can't do this until
>          * we fix gsbase, and we should do it before enter_from_user_mode
>          * (which can take locks).  Since TRACE_IRQS_OFF is idempotent,
>          * the simplest way to handle it is to just call it twice if
>          * we enter from user mode.  There's no reason to optimize this since
>          * TRACE_IRQS_OFF is a no-op if lockdep is off.
>          */
>         TRACE_IRQS_OFF
>
>         CALL_enter_from_user_mode
>
> 1:
>         ENTER_IRQ_STACK old_rsp=%rdi save_ret=1
>         /* We entered an interrupt context - irqs are off: */
>         TRACE_IRQS_OFF
>
> which seems to imply that lockdep requires TRACE_IRQS_OFF to be performed
> _before_ entering from usermode. I don't expect this to be useful at all for
> other tracers though. I think this should be replaced by a new e.g.
> LOCKDEP_ENTER_FROM_USER_MODE or such which would call into lockdep without
> calling other tracers.

See the entry series I'm working on. Aside of moving all this nonsense
into C-code it splits lockdep and tracing so it looks like this:

            lockdep_hardirqs_off();
            user_exit_irqsoff();
            __trace_hardirqs_off();

The latter uses regular RCU and not the scru/rcu_irq dance.

>> Right, but that still does the whole rcu_irq dance especially in the
>> entry code just to trace 50 or 100 instructions which are turning on RCU
>> anyway.
>
> Agreed. Would changing this to a lockdep-specific call as I suggest above
> solve this ?

That split exist for a few weeks now at least in my patches :)

>>> If a tracer recurses, or if a tracer attempts to trace another tracer, the
>>> instrumentation would break the recursion chain by preventing instrumentation
>>> from firing. If we end up caring about tracers tracing other tracers, we could
>>> have one distinct flag per tracer and let each tracer break the recursion chain.
>>>
>>> Having this flag per kernel stack rather than per CPU or per thread would
>>> allow tracing of nested interrupt handlers (and NMIs), but would break
>>> call chains both within the same stack or going through a trap. I think
>>> it could be a nice complementary safety net to handle mishaps in a non-fatal
>>> way.
>> 
>> That works as long as none of this uses breakpoint based patching to
>> dynamically disable/enable stuff.
>
> I'm clearly missing something here. I was expecting the "in_tracing" flag trick
> to be able to fix the breakpoint recursion issue. What is the problem I'm missing
> here ?

How do you "fix" that when you can't reach the tracepoint because you
trip over a breakpoint and then while trying to fixup that stuff you hit
another one?

Thanks,

        tglx