* Ingo Molnar <mi...@kernel.org> wrote:
> > But yes, we can do that userspace ring buffer when we really need it. At
> > very
> > first we can start working on perf side and assume overwrite mode is ready.
>
> I don't think Peter asked for much: pick up the patch he has already written
> and
> use it, to have an even lower overhead always-enabled background tracing mode
> of
> perf.
>
> Resizing shouldn't be much of an issue with existing features: if events
> start
> overflowing or some other threshold for dynamic increase of the ring-buffer
> is
> met then the daemon should open a new set of events with a larger
> ring-buffer,
> and close the old events once the new tracing ring-buffer is up and running.
>
> Use event multiplexing to output all interesting events into the same single
> (per CPU) ring-buffer.
Btw., there's another trick we could use to support ftrace-alike workflows even
better: we could expose a task's active perf ring-buffers under /proc/<PID>/
and
could make it readable.
So if an overwrite-mode background tracing session is running, you don't even
have
to signal it to capture the ring-buffer: just open the ring-buffer fd in
procfs,
under /proc/XYZ/perf/ring-buffers/5.trace or so, and dump its current contents,
assuming the task doing that has sufficient permissions - i.e.
ptrace_may_access().
We could even pretty-print some very basic version of the records from the
kernel,
via /proc/XYZ/perf/ring-buffers/5.txt, to support a tooling-less tracing modes.
This way perf based tracing could be supported even on systems that have no
writable filesystems.
I.e. in this regard perf can be made to match ftrace's tracing workflow as well
-
in addition to the more traditional perf profiling workflow we all love and
know!
Thanks,
Ingo
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