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

[Jason Baron]

On Tue, Mar 24, 2009 at 06:50:49PM +0100, Ingo Molnar wrote:
> * Jason Baron <jbaron@xxxxxxxxxx> 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

maybe i wasn't clear. Entry and exit as I proposed and as in the tracing
tree are for entry and exit into each handler per irq. Mathieu is
proposing an entry/exit tracepoint per irq, and a 3rd tracepoint to
tell us which handler is being called and its return code. hope this is
clear.

thanks,

-Jason
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