Messages in this thread | | | Date | Thu, 26 Mar 2009 14:28:59 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [patch 2/9] LTTng instrumentation - irq |
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* Steven Rostedt (rostedt@goodmis.org) wrote: > > On Tue, 24 Mar 2009, Mathieu Desnoyers wrote: > > > > This third type of tracepoint for IRQs you are talking about is actually > > what I had in LTTng. I decided to add the irq_next_handler and to add a > > action field to irq_entry to include the irq handler information needed > > by Jason. > > > > If we want to do this logically, without thinking about tracer > > performance impact, we could/should do : > > > > trace_irq_entry(irqno, pt_regs) > > for_each_handler() { > > trace_irq_handler_entry(action) > > action->handler() > > trace_irq_handler_exit(ret) > > } > > trace_irq_exit(retval) > > > > And add the irq_entry/irq_exit events to the arch-specific reschedule, > > tlb flush, local timer irq, as I have in my lttng tree already. > > > > But given the trace_irq_handler_entry/trace_irq_handler_exit events > > could be combined, given we can record action and ret in the > > irq_entry/exit events, I decided to remove 2 tracepoints (out of 4) from > > the single-handler fast path by adding this information to the irq > > entry/exit events, and decided to combine the irq_handler entry/exit > > into a single next_handler event, which records the previous ret value > > and the next action to execute. > > > > On an interrupt-driven workload, it will have a significant impact. > > (2 events vs 4). > > I thought tracepoints while not active are very low overhead. > > If you only want to use 2 of the 4, would that be just as fast? >
Probably, but I was talking about active tracing overhead here.
> > > > If we add interrupt threads to the kernel, then we can switch to the > > following scheme : > > > > * instrumentation of the real interrupt handler : > > > > trace_irq_entry(irqno, pt_regs) > > > > trace_irq_exit(ret) > > > > * instrumentation of the irq threads : > > > > trace_irq_thread_entry() > > > > trace_irq_thread_exit() > > > > I don't see why we should mind trying to make the tracepoints "logical", > > especially if it hurts performance considerably. Doing these > > implementation-specific versions of irq tracepoints would provide the > > best performance we can get when tracing. It's up to the tracers to > > specialize their analysis based on the underlying IRQ mechanism > > (non-threaded vs threaded). > > Perhaps we want to make them logical so that other things might hook into > these trace points besides a tracer. I do not agree that the code should > be modified just to make the trace points faster. The trace points are > just hooks into code, and should have no effect when disabled. Once the > code starts to change due to better placement of tracepoints for tracers, > that's when those trace points should be NACKed. > > -- Steve >
If it makes the code messy, then yes, I agree that those tracepoints should not go in.
Mathieu
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