Re: [RFC PATCH tip 4/5] use BPF in tracing filters

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2013/12/05 14:11), Alexei Starovoitov wrote:
> On Wed, Dec 4, 2013 at 4:05 PM, Masami Hiramatsu
> <masami.hiramatsu.pt@hitachi.com> wrote:
>> (2013/12/04 10:11), Steven Rostedt wrote:
>>> On Wed, 04 Dec 2013 09:48:44 +0900
>>> Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> wrote:
>>>
>>>> fetch functions and actions. In that case, we can continue
>>>> to use current interface but much faster to trace.
>>>> Also, we can see what filter/arguments/actions are set
>>>> on each event.
>>>
>>> There's also the problem that the current filters work with the results
>>> of what is written to the buffer, not what is passed in by the trace
>>> point, as that isn't even displayed to the user.
>>
>> Agreed, so I've said I doubt this implementation is a good
>> shape to integrate. Ktap style is better, since it just gets
>> parameters from perf buffer entry (using event format).
> 
> Are you saying always store all arguments into ring buffer and let
> filter run on it?

Yes, it is what ftrace does. I doubt your way fits all of the existing
trace-event macros. However, I think just for dynamic events, you can
integrating the argument fetching and filtering.

> It's slower, but it's cleaner, because of human readable? since ktap
> arg1 matches first
> argument of tracepoint is better than doing ctx->regs.di ? Sure.
> si->arg1 is easy to fix.
> With si->arg1 tweak the bpf will become architecture independent. It
> will run through JIT on x86 and through interpreter everywhere else.
> but for kprobes user have to specify 'var=cpu_register' during probe
> creation… how is it better than doing the same in filter?

Haven't you used perf-probe yet? It already supports such kind of
translation from kernel local variable name to registers, offsets,
and dereference. :) And kprobe-events can parse such arguments into
method chain. See Documentation/trace/kprobetrace.txt and
tools/perf/Documentation/perf-probe.txt for more detail.
Anyway, I'd like to use the bpf for re-implementing fetch method. :)

Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com