Re: [RFC] tracing: Adding cgroup aware tracing functionality

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Apr 07, 2011 at 01:22:30PM -0700, David Sharp wrote:
> On Thu, Apr 7, 2011 at 5:06 AM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> Perf doesn't have the same latency characteristics as ftrace. It costs
> a full microsecond for every trace event.
> 
> https://lkml.org/lkml/2010/10/28/261
> 
> It's possible these results need to be updated. Has any effort been
> made to improve the tracing latency of perf?

Nothing significant since then, I believe. But the hotspots are known
and some are relatively low hanging fruits if you want to get closer to
ftrace throughput:

* When an event triggers, we do a double copy. A first one in a temporary
buffer and a second one from the temporary buffer to the event'ss one.
This is because we don't have the same discard feature than in ftrace
buffer. We need to first filter on the temporary buffer and give up if the filter
matched instead of copying to the main buffer.

As a short term solution: have a fast path tracing for the case where we
don't have a filter: directly copy to the main buffer.

In the longer term I think we want to filter on tracepoint parameters
rather than in the ending trace.

* We save more things in perf, because we have the perf headers. So we
save the pid twice: once in trace event headers, second in perf headers.
We need to drop the one from the trace event.
Also in the case of pure tracing, we don't need to save the ip in the perf
headers.

* We have lots of conditionals in the fast path, due to some exclusion options,
overflow count tracking, etc... We probably want a fastpath tracing function
for the high volume tracing case, something that goes quickly to the buffer
saving.

And there are things common to ftrace and perf that we probably want to have:
like tracking of pids using sched switch event if one is running, instead
of saving the pid on each traces. And get rid of the preempt_count in the
trace event headers, at least have the possibility to choose whether we want
it.


Any help in any of these tasks would be very welcome.