Re: [RFC PATCH tip 0/5] tracing filters with BPF

[Alexei Starovoitov]

On Thu, Dec 5, 2013 at 3:37 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Thu, 5 Dec 2013 14:36:58 -0800
> Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:
>
>> On Thu, Dec 5, 2013 at 5:46 AM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>> >
>> > I know that it would be great to have the bpf filter run before
>> > recording of the tracepoint, but as that becomes quite awkward for a
>> > user interface, because it requires intimate knowledge of the kernel
>> > source, this speed up on the filter itself may be worth while to have
>> > it happen after the recording of the buffer. When it happens after the
>> > record, then the bpf has direct access to the event entry and its
>> > fields as described by the trace event format files.
>>
>> I don't understand that 'awkward' part yet. What do you mean by 'knowledge of
>> the kernel'? By accessing pt_regs structure? Something else ?
>> Can we try fixing the interface first before compromising on performance?
>
> Let me ask you this. If you do not have the source of the kernel on
> hand, can you use BPF to filter the sched_switch tracepoint on prev pid?
>
> The current filter interface allows you to filter with just what the
> running kernel provides. No need for debug info from the vmlinux or
> anything else.

Understood and agreed. For the users that are satisfied with amount of info
that single trace_event provides (like sched_switch) there is probably
little reason to do complex filtering. Either they're fine with all
the events or will
just filter based on pid only.

> I'm fine if it becomes a requirement to have a vmlinux built with
> DEBUG_INFO to use BPF and have a tool like perf to translate the
> filters. But it that must not replace what the current filters do now.
> That is, it can be an add on, but not a replacement.

Of course. tracing filters via bpf is an additional tool for kernel debugging.
bpf by itself has use cases beyond tracing.
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