linux-kernel - Re: Tracing/ftrace: trouble with trace_entries and trace

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Date:	Wed, 17 Sep 2008 14:41:29 +0200
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Pekka Paalanen" <pq@....fi>
Cc:	"Steven Rostedt" <rostedt@...dmis.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>
Subject: Re: Tracing/ftrace: trouble with trace_entries and trace_pipe

2008/9/16 Pekka Paalanen <pq@....fi>:
> Ok, so the problem is probably the commit I mentioned. It makes the
> no_tracer tracer to set tracer_enabled to 0, and I can't find where
> it would be set to 1 for mmiotrace. And this interferes with
> tracing_read_pipe(), making it quit when iter->pos is non-zero.
> See no_trace_init() in trace.c. According to this, the cat-quit
> occurs when the buffer gets empty after first data, but this isn't
> totally in agreement with what I recall from my experiments. And it
> does happen also on other times than injecting markers.
>
> So either it is wrong to check tracer_enabled in tracing_read_pipe(),
> or no_trace_init() should not touch it.

Indeed that could be the problem.
None_tracer is chosen as the default tracer when the tracer engine
loads. But actually no_trace_init is not called at this time.

But if you set another tracer and reset current_tracer to none, you
will call no_trace_init. Since tracer_enabled is not reset to 1 with
other tracer's init functions, the problem occurs when you choose one
more time another tracer.

I think that mmiotrace receives a smaller flow of entries (depends on
the debugged module) whereas sched_switch or function's tracer, as an
example, are continuously fed and never enter the "while
(trace_empty(iter))" block. That's why I only see this bug in
mmiotrace.

Perhaps it would be a good idea to reenable tracer_enabled on
tracing_set_trace_write() just before calling the init callback of the
tracer chosen.
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