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Date:   Tue, 12 Mar 2019 11:13:26 +0000
From:   Qais Yousef <qais.yousef@arm.com>
To:     Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "bpf@vger.kernel.org" <bpf@vger.kernel.org>
Subject: Re: [RFC][PATCH] Export supported trace features in debugfs
Message-ID: <20190312111326.nz3wgdy64g2nk44v@e107158-lin.cambridge.arm.com>
References: <20190312110437.5977-1-qais.yousef@arm.com>
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On 03/12/19 11:07, Qais Yousef wrote:
> eBPF tools like bcc-tools have hard time figuring out when features like
> raw_tracepoint are supported in the kernel on which we are running. At
> the moment a fragile mechanism of matching bpf_find_raw_tracepoint()
> function in /proc/kallsyms is used to find out whether raw tracepoints
> can be used or not. But when this function was renamed recently to
> bpf_get_raw_tracepoint() the tool started to fail to use raw
> tracepoints.
> 
> To help in providing a more reliable way to detect features like
> RAW_TRACEPOINT, add a new file in trace debugfs to export the supported
> features.
> 
> $cat /sys/kernel/debug/tracing/supported_features
> RAW_TRACEPOINT
> EXAMPLE_FEATURE_1
> EXAMPLE_FEATURE_2
> 
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> ---
> 
> This is a half baked patch to probe the potential of this solution.
> 
> The breakage mentioned in the commit message is here:
> 
> https://github.com/iovisor/bcc/pull/2241/commits/0f5849187972a50adf0d9eaa8788c11f9fd926ea
> 
> I am not sure what else beside raw_tracepoint makes sense to expose right now.
> 
>  kernel/trace/trace.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index c4238b441624..daae09238e62 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -6670,6 +6670,28 @@ static const struct file_operations snapshot_raw_fops = {
> 
>  #endif /* CONFIG_TRACER_SNAPSHOT */
> 
> +#define TRACE_FEATURE(feat)__stringify(feat) "\n"
> +
> +#define TRACE_FEATURES\
> +TRACE_FEATURE(RAW_TRACEPOINT)\
> +TRACE_FEATURE(EXAMPLE_FEATUTE_1)\
> +TRACE_FEATURE(EXAMPLE_FEATUTE_2)
> +
> +static ssize_t
> +tracing_read_trace_features(struct file *filp, char __user *ubuf,
> +size_t cnt, loff_t *ppos)
> +{
> +char *buf = TRACE_FEATURES;
> +size_t len = sizeof(TRACE_FEATURES);
> +
> +return simple_read_from_buffer(ubuf, cnt, ppos, buf, len);
> +}
> +
> +static const struct file_operations show_trace_features_fops = {
> +.read           = tracing_read_trace_features,
> +.llseek         = no_llseek,
> +};
> +
>  static int tracing_buffers_open(struct inode *inode, struct file *filp)
>  {
>  struct trace_array *tr = inode->i_private;
> @@ -8242,6 +8264,9 @@ static __init int tracer_init_tracefs(void)
>  &ftrace_update_tot_cnt, &tracing_dyn_info_fops);
>  #endif
> 
> +trace_create_file("trace_features", 0444, d_tracer,
> +NULL, &show_trace_features_fops);
> +
>  create_trace_instances(d_tracer);
> 
>  update_tracer_options(&global_trace);
> --
> 2.17.1
> 
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Apologies that shouldn't have appeared. I can resend the patch if it matters.

--
Qais Yousef