linux-kernel - Re: [PATCH 2/4] tracing: add event trace infrastructure

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Date:	Wed, 25 Feb 2009 16:57:09 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Theodore Tso <tytso@....edu>,
	Arjan van de Ven <arjan@...radead.org>,
	Pekka Paalanen <pq@....fi>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jason Baron <jbaron@...hat.com>,
	Martin Bligh <mbligh@...gle.com>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 2/4] tracing: add event trace infrastructure


* Steven Rostedt <rostedt@...dmis.org> wrote:

> > #                _------=> CPU#
> > #               / _-----=> irqs-off
> > #              | / _----=> need-resched
> > #              || / _---=> hardirq/softirq
> > #              ||| / _--=> preempt-depth
> > #              |||| /
> > #              |||||     delay
> > #  cmd     pid ||||| time  |   caller
> > #     \   /    |||||   \   |   /
> >   <idle>-0     0d..1    0us+: trace_hardirqs_off_thunk (apic_timer_interrupt)
> >   <idle>-0     0d.s.   97us : __do_softirq (do_softirq)
> >   <idle>-0     0d.s1   98us : trace_hardirqs_on (do_softirq)
[...]
> > 
> > your time starts now.
> 
> Note, that is a legacy format, that works great for the 
> latency tracers. Those include (the example you used) irqsoff, 
> the preempt off, wake up latencies. This gives us a nice 
> listing of why we are hitting a latency. All other plugins 
> generally use the trace file, although they could also use 
> that file too.
> 
> As for your English comment. I showed the header to my wife, 
> and she had no idea what irqs-off means, nor need-resched, nor 
> preempt-depth ;-) They are not quite English as they are 
> technical terms. Most of the names are hard coded into the 
> kernel too.
> 
> Yeah, maybe that file (latency_trace) is a bit too much. I for 
> one love it. It is great to load a kernel on some remote box, 
> and run the irqs off latency tracer to see where the 
> interrupts are disabled for the longest time. This format is 
> really nice because it shows me when we are in an interrupt, 
> or interrupts are disabled, and when the task should have been 
> rescheduled.
> 
> This has help find places that we miss a preemption check too.

Could we get that, as PeterZ has suggested, as a trace_option 
column in the 'trace' file? It would be default off for 
non-latency tracers, with latency tracing plugins turning it on 
by default. Would that work?

	Ingo
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