linux-kernel - Re: ftrace global trace_pipe

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Message-ID: <20181219113734.56b57f93@gandalf.local.home>
Date:   Wed, 19 Dec 2018 11:37:34 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Claudio <claudio.fontana@...wa.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: ftrace global trace_pipe_raw

On Wed, 19 Dec 2018 12:32:41 +0100
Claudio <claudio.fontana@...wa.com> wrote:

> >>
> >> I would imagine the core functionality is already available, since trace_pipe
> >> in the tracing directory already shows all events regardless of CPU, and so
> >> it would be a matter of doing the same for trace_pipe_raw.  
> > 
> > The difference between trace_pipe and trace_pipe_raw is that trace_pipe
> > is post processed, and reads the per CPU buffers and interleaves them
> > one event at a time. The trace_pipe_raw just sends you the raw
> > unprocessed data directly from the buffers, which are grouped per CPU.  
> 
> I think that what I am looking for, to improve the performance of our system,
> is a post processed stream of binary entry data, already merged from all CPUs
> and sorted per timestamp, in the same way that it is done for textual output
> in __find_next_entry:
> 
>        for_each_tracing_cpu(cpu) {
> 
>                 if (ring_buffer_empty_cpu(buffer, cpu))
>                         continue;
> 
>                 ent = peek_next_entry(iter, cpu, &ts, &lost_events);
> 
>                 /*                                                                
>                  * Pick the entry with the smallest timestamp:                    
>                  */
>                 if (ent && (!next || ts < next_ts)) {
>                         next = ent;
>                         next_cpu = cpu;
>                         next_ts = ts;
>                         next_lost = lost_events;
>                         next_size = iter->ent_size;
>                 }
>         }
> 
> We first tried to use the textual output directly, but this lead to
> unacceptable overheads in parsing the text.
> 
> Please correct me if I do not understand, however it seems to me that it
> would be possible do the same kind of post processing including generating
> a sorted stream of entries, just avoiding the text output formatting,
> and outputting the binary data of the entry directly, which would be way
> more efficient to consume directly from user space correlators.
> 
> But maybe this is not a general enough requirement to be acceptable for
> implementing directly into the kernel?
> 
> We have the requirement of using the OS tracing events, including
> scheduling events, to react from software immediately
> (vs doing after-the-fact analysis).

Have you looked at using the perf event interface? I believe it uses a
single buffer for all events. At least for tracing a single process.

-- Steve