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
