linux-kernel - Re: [RFC][PATCH 00/11] ring-buffer/tracing: Remove disabling of ring buffer while reading trace file

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Message-ID: <CAADnVQ+hpVnsfKUNNeJWs8X0ogvsR8uNKrEzz5CK2XRdqn+80A@mail.gmail.com>
Date:   Wed, 18 Mar 2020 19:27:28 -1000
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>, bpf <bpf@...r.kernel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Wu <peter@...ensteyn.nl>,
        Jonathan Corbet <corbet@....net>,
        Tom Zanussi <zanussi@...nel.org>
Subject: Re: [RFC][PATCH 00/11] ring-buffer/tracing: Remove disabling of ring
 buffer while reading trace file

On Tue, Mar 17, 2020 at 11:34 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> When the ring buffer was first written for ftrace, there was two
> human readable files to read it. One was a standard "producer/consumer"
> file (trace_pipe), which would consume data from the ring buffer as
> it read it, and the other was a "static iterator" that would not
> consume the events, such that the file could be read multiple times
> and return the same output each time.
>
> The "static iterator" was never meant to be read while there was an
> active writer to the ring buffer. If writing was enabled, then it
> would disable the writer when the trace file was opened.
>
> There has been some complaints about this by the BPF folks, that did
> not realize this little bit of information and it was requested that
> the "trace" file does not stop the writing to the ring buffer.
>
> This patch series attempts to satisfy that request, by creating a
> temporary buffer in each of the per cpu iterators to place the
> read event into, such that it can be passed to users without worrying
> about a writer to corrupt the event while it was being written out.
> It also uses the fact that the ring buffer is broken up into pages,
> where each page has its own timestamp that gets updated when a
> writer crosses over to it. By copying it to the temp buffer, and
> doing a "before and after" test of the time stamp with memory barriers,
> can allow the events to be saved.

Awesome. Thank you so much for working on it.
Looks like it addresses all the issues bpf folks reported.
cc-ing bpf list for visibility.