Messages in this thread | | | Date | Tue, 24 Mar 2009 18:50:49 +0100 | From | Ingo Molnar <> | Subject | Re: [patch 2/9] LTTng instrumentation - irq |
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* Jason Baron <jbaron@redhat.com> wrote:
> On Tue, Mar 24, 2009 at 11:56:27AM -0400, Mathieu Desnoyers wrote: > > Instrumentation of IRQ related events : irq_entry, irq_exit and > > irq_next_handler. > > > > It allows tracers to perform latency analysis on those various types of > > interrupts and to detect interrupts with max/min/avg duration. It helps > > detecting driver or hardware problems which cause an ISR to take ages to > > execute. It has been shown to be the case with bogus hardware causing an mmio > > read to take a few milliseconds. > > > > Those tracepoints are used by LTTng. > > > > About the performance impact of tracepoints (which is comparable to markers), > > even without immediate values optimizations, tests done by Hideo Aoki on ia64 > > show no regression. His test case was using hackbench on a kernel where > > scheduler instrumentation (about 5 events in code scheduler code) was added. > > See the "Tracepoints" patch header for performance result detail. > > > > irq_entry and irq_exit not declared static because they appear in x86 arch code. > > > > The idea behind logging irq/softirq/tasklet/(and eventually syscall) entry and > > exit events is to be able to recreate the kernel execution state at a given > > point in time. Knowing which execution context is responsible for a given trace > > event is _very_ valuable in trace data analysis. > > > > The IRQ instrumentation instruments the IRQ handler entry and exit. Jason > > instrumented the irq notifier chain calls (irq_handler_entry/exit). His approach > > provides information about which handler is being called, but does not map > > correctly to the fact that _multiple_ handlers are being called from within the > > same interrupt handler. From an interrupt latency analysis POV, this is > > incorrect. > > > > Since we are passing back the irq number, and we can not be > interrupted by the same irq, I think it should be pretty clear we > are in the same handler. That said, the extra entry/exit > tracepoints could make the sequence of events simpler to decipher, > which is important. The code looks good, and provides at least as > much information as the patch that I proposed. So i'll be happy > either way :)
We already have your patch merged up in the tracing tree and it gives entry+exit tracepoints.
Ingo
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