linux-kernel - Re: [PATCH] tracing/ftrace: better manage the context info for events

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Date:	Fri, 30 Jan 2009 11:54:11 -0200
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Frédéric Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing/ftrace: better manage the context info for
	events

Em Fri, Jan 30, 2009 at 11:21:26AM -0200, Arnaldo Carvalho de Melo escreveu:
> Em Fri, Jan 30, 2009 at 01:55:50PM +0100, Frédéric Weisbecker escreveu:
> > 2009/1/30 Arnaldo Carvalho de Melo <acme@...stprotocols.net>:
> > > Em Fri, Jan 30, 2009 at 10:34:46AM -0200, Arnaldo Carvalho de Melo escreveu:
> > >> Em Fri, Jan 30, 2009 at 12:59:05AM -0800, Frederic Weisbecker escreveu:
> > >> > Impact: make trace_event more convenient for tracers
> > >> >
> > >> > All tracers (for the moment) that use the struct trace_event want to have
> > >> > the context info printed before their own output: the pid/cmdline, cpu, and timestamp.
> > >> >
> > >> > But some other tracers that want to implement their trace_event callbacks will
> > >> > not necessary need these information or they may want to format them as they want.
> > >> >
> > >> > This patch adds a new default-enabled trace option: TRACE_ITER_CONTEXT_INFO
> > >> > When disabled through:
> > >> >
> > >> > echo nocontext-info > /debugfs/tracing/trace_options
> > >> >
> > >> > The pid, cpu and timestamps headers will not be printed.
> > >> >
> > >> > IE with the sched_switch tracer with context-info (default):
> > >> >
> > >> >             bash-2935  [001]   100.356561:   2935:120:S ==> [001]     0:140:R <idle>
> > >> >           <idle>-0     [000]   100.412804:      0:140:R   + [000]    11:115:S events/0
> > >> >           <idle>-0     [000]   100.412816:      0:140:R ==> [000]    11:115:R events/0
> > >> >         events/0-11    [000]   100.412829:     11:115:S ==> [000]     0:140:R <idle>
> > >> >
> > >> > Without context-info:
> > >> >
> > >> >   2935:120:S ==> [001]     0:140:R <idle>
> > >> >      0:140:R   + [000]    11:115:S events/0
> > >> >      0:140:R ==> [000]    11:115:R events/0
> > >> >     11:115:S ==> [000]     0:140:R <idle>
> > >> >
> > >> > A tracer can disable it at runtime by clearing the bit TRACE_ITER_CONTEXT_INFO in trace_flags.
> > >> >
> > >> > Moreover, two callbacks have been added inside trace_event:
> > >> > _ context_info() which let a tracer override the format of the context info
> > >> > _ lat_context_info() which do the same with the latency_trace file
> > >> >
> > >> > If they are not defined, the usual way of printing the context info will be used.
> > >>
> > >> Please wait a bit, I'm trying to use it and the first thing I notice was
> > >> that I need the trace_iterator in the context callback, to get the
> > >> timestamp.
> > >
> > > Sorry, false alarm, the iterator is there, I was trying to build the
> > > whole binary trace record in the trace_event->bin(), but its not
> > > possible, so I'll do it in two function calls context + trace.
> > >
> > > - Arnaldo
> > >
> > 
> > Do you need the trace_iterator from one of your event print callback?
> > Such as in bin() ?
> 
> Yes, I'm finding the callbacks a complicating factor, I think that we
> should have instead a flag in the struct trace_event and struct tracer
> telling that we don't need the context printed and the default routine
> exported to tracers.
> 
> That way minimise change on the old tracers but allow new ones to use
> the default context to be called from trace_event->trace() and not from
> trace_event->bin().
> 
> So, to summarize, for blktrace I would use:
> 
> struct trace_event {
> 	.flags	= DONT_PRINT_CONTEXT | OTHER_FUTURE_FLAGS,
> 	.bin	= blktrace_print_bin,
> 	.trace	= blktrace_print_trace,
> }
> 
> blktrace_print_bin(iter)
> {
> 	full control with access to iter to get timestamp, cpu, etc
> }
> 
> blktrace_print_trace(iter)
> {
> 	print_default_context()
> 	print tracer specific stuff
> }
> 
> Back to trying to use your patch to show the problems in the form of a
> working blktrace synthesizer to use with:
> 
> cat /d/tracing/trace_pipe | blkparse -i -

Definetely, as it is it doesn't caters to the blktrace binary printing
needs.

The ->context info callback is not called if TRACE_ITER_BIN is selected,
it should, as doing:

	if (trace_flags & TRACE_ITER_CONTEXT_INFO) {
		SEQ_PUT_FIELD_RET(s, entry->pid);
		SEQ_PUT_FIELD_RET(s, entry->cpu);
		SEQ_PUT_FIELD_RET(s, iter->ts);
	}

is only the default context info for binary mode, what if I want to
provide a different context info?

I'll try to rework your patch into something that is useful for my needs
and post here.

- Arnaldo
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