linux-kernel - Re: [PATCH 2/2] perf: Fix ftrace builtin when kernel doesn't have function

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Message-ID: <20171101124433.1a6ed9c6@gandalf.local.home>
Date:   Wed, 1 Nov 2017 12:44:33 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Julien Thierry <julien.thierry@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, takahiro.akashi@...aro.org,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] perf: Fix ftrace builtin when kernel doesn't have
 function_graph

On Wed,  1 Nov 2017 14:33:44 +0000
Julien Thierry <julien.thierry@....com> wrote:

> When linux is built without support for function graph tracer, the ftrace
> builtin of perf will fail when trying to reset max_graph_depth because the
> file does not exist. This prevents the use of function tracer from perf.
> 
> Do not attempt to write this file when the tracer in use is not
> function_graph.
> 
> Signed-off-by: Julien Thierry <julien.thierry@....com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: linux-kernel@...r.kernel.org
> ---
>  tools/perf/builtin-ftrace.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
> index 25a42ac..48120f2 100644
> --- a/tools/perf/builtin-ftrace.c
> +++ b/tools/perf/builtin-ftrace.c
> @@ -130,7 +130,8 @@ static int reset_tracing_files(struct perf_ftrace *ftrace __maybe_unused)
>  	if (reset_tracing_cpu() < 0)
>  		return -1;
> 
> -	if (write_tracing_file("max_graph_depth", "0") < 0)
> +	if (!strcmp(ftrace->tracer, "function_graph") &&
> +	    write_tracing_file("max_graph_depth", "0") < 0)

Hmm, instead of doing this, could we just do a stat on the file first.
As with trace-cmd, I like to reset files like this even when not
enabling function_graph tracer.

-- Steve


>  		return -1;
> 
>  	reset_tracing_filters();
> --
> 1.9.1