linux-kernel - Re: [PATCH] perf tools: Add sample types for bpf-output event

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Date:	Fri, 1 Apr 2016 12:16:10 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Wang Nan <wangnan0@...wei.com>
Cc:	linux-kernel@...r.kernel.org, pi3orama@....com,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...nel.org>,
	Milian Wolff <milian.wolff@...b.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Brendan Gregg <brendan.d.gregg@...il.com>
Subject: Re: [PATCH] perf tools: Add sample types for bpf-output event

Em Fri, Apr 01, 2016 at 01:26:42PM +0000, Wang Nan escreveu:
> Before this patch we can see very large time in the events before
> bpf-output event. For example:
> 
>  # perf trace --ev bpf-output/no-inherit,name=evt/ \
>               --ev ./test_bpf_trace.c/map:channel.event=evt/ \
>               usleep 10

Thanks, applied and tested, now one idea that occurred to me to shorten
the above command line: automagically create a
"__perf_trace_bpf_stdout__" bpf-output event when a .c bpf event is
specified and no bpf-output is present, i.e. the following command line
would produce the same result as the one above:

  # trace --ev test_bpf_trace.c usleep 10

Well, it would have to build test_bpf_trace.c and see if it references
the equivalent of a "stdout", i.e. it expects a bpf-output event to be
present to send output to.

I.e. in this example we have a:

 struct bpf_map_def SEC("maps") channel = {

That later on you use to do "puts(msg)" like operations, i.e. to a
"stdout" of sorts:

 func(void *ctx, int type)
 {
	char output_str[] = "Raise a BPF event!";
	char err_str[] = "BAD %d\n";
	int err;

        err = perf_event_output(ctx, &channel, get_smp_processor_id(),
			        &output_str, sizeof(output_str));
	if (err)
		trace_printk(err_str, sizeof(err_str), err);
        return 1;
 }

Perhaps, to make all more familiar we could even define equivalents to
stdio.h functions like puts, printf, fputs, etc, that would send to this
bpf-output based "stdout" "channel", then the above would end up being:

 func(void *ctx, int type)
 {
        char err_str[] = "BAD %d\n";
        int err;

        err = puts("Raise a BPF event!");
        if (err)
                trace_printk(err_str, sizeof(err_str), err);
        return 1;
 }

This trace_printk() in turn could become error() (glibc's error.h header), i.e.
the error mechanism would use the equivalent to userland's "syslog", i.e.
trace_printk :-)

In general trying to make BPF C scriptlets fed via perf to be as compact as
possible, hiding all these details while allowing them to be used, if desired.

- Arnaldo