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

Ingo Molnar
2009-03-24T19:14:15Z

* Jason Baron <jbaron@redhat.com> wrote:

> On Tue, Mar 24, 2009 at 06:50:49PM +0100, Ingo Molnar wrote:
> > * 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
>
> 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.

Ok, i misunderstood that.

Mathieu's is slightly more compact, but yours is more logical.

I believe your pre/post IRQ handler callback is the right model - it
decouples device IRQ handling from any notion of 'IRQ'.

For example, we could correctly express "handler got executed by an
IRQ thread" via it - while via Mathieu's scheme it does not really
map to that.

So if then i think there should be a third tracepoint in addition to
your two existing tracepoints: a 'raw vector' type of tracepoint.
It's added both to do_IRQ() entry point, but also to the various
common SMP IPI entry points: reschedule, TLB flush and local timer
IRQ tick.

The best information there to pass to the probe is the raw vector
number, and the ptregs structure.

Hm?

Ingo