Messages in this thread | | | Date | Wed, 17 Sep 2008 14:41:29 +0200 | From | "Frédéric Weisbecker" <> | Subject | Re: Tracing/ftrace: trouble with trace_entries and trace_pipe |
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2008/9/16 Pekka Paalanen <pq@iki.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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