Re: [PATCH net-next 2/8] perf, bpf: allow bpf programs attach to tracepoints

From: Alexei Starovoitov
Date: Mon Apr 18 2016 - 21:16:31 EST

On 4/18/16 3:16 PM, Steven Rostedt wrote:
On Mon, 18 Apr 2016 14:43:07 -0700
Alexei Starovoitov <ast@xxxxxx> wrote:

I was worried about this too, but single 'if' and two calls
(as in commit 98b5c2c65c295) is a better way, since it's faster, cleaner
and doesn't need to refactor the whole perf_trace_buf_submit() to pass
extra event_call argument to it.
perf_trace_buf_submit() is already ugly with 8 arguments!

Right, but I solved that in ftrace by creating an on-stack descriptor
that can be passed by a single parameter. See the "fbuffer" in the
trace_event_raw_event* code.

Yes. That what I referred to in below 'a struct to pass args'...
But, fine, will try to optimize the size further.
Frankly much bigger .text savings will come from combining
trace_event_raw_event_*() with perf_trace_*()
Especially if you're ok with copying tp args into perf's percpu
buffer first and then copying into ftrace's ring buffer.
Then we can half the number of such auto-generated functions.

Passing more args or creating a struct to pass args only going to
hurt performance without much reduction in .text size.
tinyfication folks will disable tracepoints anyway.
Note that the most common case is bpf returning 0 and not even
calling perf_trace_buf_submit() which is already slow due
to so many args passed on stack.
This stuff is called million times a second, so every instruction

Note, that doesn't matter if you are bloating the kernel for the 99.9%
of those that don't use bpf.

Please remember this! Us tracing folks are second class citizens! If
there's a way to speed up tracing by 10%, but in doing so we cause
mainline to be hurt by over 1%, we shouldn't be doing it. Tracing and
hooks on tracepoints are really not used by many people. Don't fall
into Linus's category of "my code is the most important". That's
especially true for tracing.

tracing was indeed not used that often in the past, but
bpf+tracing completely changed the picture. It's no longer just
debugging. It is the first class citizen that runs 24/7 in production
and its performance and lowest overhead are crucial.