Jan Kiszka wrote:
> Hi,
> 
> quite a few limitations and complications of using Linux services over
> non-Linux domains relate to potentially invalid "current" and
> "thread_info". The non-Linux domain could maintain their own kernel
> stacks while Linux tend to derive current and thread_info from the stack
> pointer. This is not an issue anymore on x86-64 (both states are stored
> in per-cpu variables) but other archs (e.g. x86-32 or ARM) still use the
> stack and may continue to do so.

On ARM vanilla tracing capabilities have way too much overhead to be
really usable. I have tested a "standalone" version of mcount which
calls directly the Ipipe tracer, and I get less overhead, something like
20%.

The philosophy of ftrace is well explained by the following sentence,
extracted from the ftrace design text file: "Also keep in mind that this
mcount function will be called *a lot*, so optimizing for the default
case of no tracer will help the smooth running of your system when
tracing is disabled."

When using the I-pipe tracer, we are interested in precisely the reverse
optimization: we do not care about the overhead of mcount when the
tracer is not enabled, we will not keep the tracer enabled in
configuration when not tracint anyway, but we really want mcount to have
as little overhead as possible when the tracer is enabled.

Still on ARM, the perf code requires handling an interrupt when the
performance counters overflow. So, getting this to work with the I-pipe
would mean ironing this irq handler and all the functions it calls, and
all the spinlocks that are involved.

What I am trying to say is that trying to use the vanilla
infrastructures will probably cause many more problems than just the
stack issue, and if we look at the I-pipe tracer example again, it is
really not obvious what the vanilla infrastructure bring us: only the
ftrace_register/ftrace_unregister services, at the expense of 20% more
overhead.

-- 
                                            Gilles.

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