linux-kernel - Re: [patch 2/9] LTTng instrumentation

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Date:	Tue, 24 Mar 2009 18:50:49 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jason Baron <jbaron@...hat.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	ltt-dev@...ts.casi.polymtl.ca,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Russell King <rmk+lkml@....linux.org.uk>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Hideo AOKI <haoki@...hat.com>,
	Takashi Nishiie <t-nishiie@...css.fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
Subject: Re: [patch 2/9] LTTng instrumentation - irq


* Jason Baron <jbaron@...hat.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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