Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods

[Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>]

* H. Peter Anvin (hpa@xxxxxxxxx) wrote:
> Mathieu Desnoyers wrote:
> > 
> > As the maintainer of the out-of-tree LTTng tracer, which hooks in the
> > page fault handler with tracepoints, and which can build almost entirely
> > as modules, I am very tempted to argue that having the nmi-code entirely
> > robust wrt in-kernel page faults would be a very-nice-to-have feature.
> > 
> 
> I doubt that is ever going to be reliable, due to reentrancy issues.
> 
> 	-hpa

Do you mean the page fault handler code is no ever going to be reliable
or the tracer code ?

I spent a great deal of effort making LTTng lockless and reentrant wrt
NMIs. It would be great if the low-level kernel exception handlers would
do the same, therefore I would not have to isolate the tracer from the
kernel as I currently do. Well, I would still continue to isolate the
tracer from the kernel, but at least I would not have to spend as much
effort controlling what exceptions and faults paths the tracer is
executing.

Mathieu

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Mathieu Desnoyers
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