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

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

[Alexei Starovoitov]

On Tue, Mar 17, 2020 at 11:34 AM Steven Rostedt <rostedt@goodmis.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.