Re: [patch 2/9] LTTng instrumentation - irq

[Jason Baron <jbaron@redhat.com>]

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 :)

thanks,

-Jason