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Subject[RFC PATCH 0/7] Early task context tracking
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Note: do not take it too seriously, it is just a proof of concept.

Some time ago, while using perf to check the automaton model, I noticed
that perf was losing events. The same was reproducible with ftrace.

See: https://www.spinics.net/lists/linux-rt-users/msg19781.html

Steve pointed to a problem in the identification of the context
execution used by the recursion control.

Currently, recursion control uses the preempt_counter to
identify the current context. The NMI/HARD/SOFT IRQ counters
are set in the preempt_counter in the irq_enter/exit functions.

In a trace, they are set like this:
-------------- %< --------------------
0) ==========> |
0) | do_IRQ() { /* First C function */
0) | irq_enter() {
0) | /* set the IRQ context. */
0) 1.081 us | }
0) | handle_irq() {
0) | /* IRQ handling code */
0) + 10.290 us | }
0) | irq_exit() {
0) | /* unset the IRQ context. */
0) 6.657 us | }
0) + 18.995 us | }
0) <========== |
-------------- >% --------------------

As one can see, functions (and events) that take place before the set
and after unset the preempt_counter are identified in the wrong context,
causing the miss interpretation that a recursion is taking place.
When this happens, events are dropped.

To resolve this problem, the set/unset of the IRQ/NMI context needs to
be done before the execution of the first C execution, and after its
return. By doing so, and using this method to identify the context in the
trace recursion protection, no more events are lost.

A possible solution is to use a per-cpu variable set and unset in the
entry point of NMI/IRQs, before calling the C handler. This possible
solution is presented in the next patches as a proof of concept, for
x86_64. However, other ideas might be better than mine... so...

Daniel Bristot de Oliveira (7):
x86/entry: Add support for early task context tracking
trace: Move the trace recursion context enum to trace.h and reuse it
trace: Optimize trace_get_context_bit()
trace/ring_buffer: Use trace_get_context_bit()
trace: Use early task context tracking if available
events: Create an trace_get_context_bit()
events: Use early task context tracking if available

arch/x86/entry/entry_64.S | 9 ++++++
arch/x86/include/asm/irqflags.h | 30 ++++++++++++++++++++
arch/x86/kernel/cpu/common.c | 4 +++
include/linux/irqflags.h | 4 +++
kernel/events/internal.h | 50 +++++++++++++++++++++++++++------
kernel/softirq.c | 5 +++-
kernel/trace/ring_buffer.c | 28 ++----------------
kernel/trace/trace.h | 46 ++++++++++++++++++++++--------
8 files changed, 129 insertions(+), 47 deletions(-)

--
2.20.1

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