> On Apr 2, 2019, at 2:03 PM, Daniel Bristot de Oliveira <[email protected]> > wrote: > > 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.
I would much rather do the opposite: completely remove context tracking from the asm and, instead, stick it into the C code. We'd need to make sure that the C code is totally immune from tracing, kprobes, etc, but it would be a nice cleanup. And then you could fix this bug in C! > > 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 >
