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

Mathieu Desnoyers
2009-06-15T21:34:36Z

* Ingo Molnar (mingo@elte.hu) wrote:
>
> * Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
>
> > Just for the sake of making NMI handlers less tricky, supporting
> > page faults caused by faulting kernel instructions (rather than
> > only supporting explicit faulting from get_user_pages_inatomic)
> > would be rather nice design-wise if it only costs 2-3 cycles.
> >
> > And I would not want to touch the page fault handler itself to
> > write the saved cr2 value before the handler exits, because this
> > would add a branch on a very hot path.
>
> _That_ path is not hot at all - it's the 'we are in atomic section
> and faulted' rare path (laced with an exception table search - which
> is extremely slow compared to other bits of the pagefault path).
>
> But ... it's not an issue: a check can be made in the NMI code too,
> as we always know about pagefaults there, by virtue of getting
> -EFAULT back from the attempted-user-copy.

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.

Requiring that code to be either built-in or to call vmalloc_sync_all()
after any vmalloc or after module load is just painful and error-prone.
Plus, tracing is a feature that some users will only use in specific
occasions. I don't see why it should be built-into the kernel at all.
That's just a waste of memory in many cases. (I am not talking about
users who want to do continuous system tracing here, which is a totally
different scenario).

Mathieu

>
> Ingo

--
Mathieu Desnoyers
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