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

[Jovi Zhangwei]

On Fri, Dec 6, 2013 at 4:43 PM, Masami Hiramatsu
<masami.hiramatsu.pt@xxxxxxxxxxx> wrote:
> (2013/12/05 14:11), Alexei Starovoitov wrote:
>> On Wed, Dec 4, 2013 at 4:05 PM, Masami Hiramatsu
>> <masami.hiramatsu.pt@xxxxxxxxxxx> wrote:
>>> (2013/12/04 10:11), Steven Rostedt wrote:
>>>> On Wed, 04 Dec 2013 09:48:44 +0900
>>>> Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx> 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.
>
Does this will affect the user interface of perf-probe argument fetching?

I mean if use bpf backend, do we must need gcc to compile bpf source
for perf-probe argument fetching? as we known, current argument
fetching is go through kprobe_events/uprobe_events debugfs file, and
ktap is based on this behavior.

Thanks.

Jovi.

>> 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@xxxxxxxxxxx
>
>
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