netdev - Re: [PATCH v3 linux-trace 1/8] tracing: attach eBPF programs to tracepoints and syscalls

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Date:	Sat, 14 Feb 2015 18:02:41 -0500
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Hekuang <hekuang@...wei.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>, wangnan0@...wei.com
Subject: Re: [PATCH v3 linux-trace 1/8] tracing: attach eBPF programs to
 tracepoints and syscalls

On Wed, Feb 11, 2015 at 11:58 PM, Hekuang <hekuang@...wei.com> wrote:
>
>>> eBPF is very flexible, which means it is bound to have someone use it
>>> in a way you never dreamed of, and that will be what bites you in the
>>> end (pun intended).
>>
>> understood :)
>> let's start slow then with bpf+syscall and bpf+kprobe only.
>
>
> I think BPF + system calls/kprobes can meet our use case
> (https://lkml.org/lkml/2015/2/6/44), but there're some issues to be
> improved.
>
> I suggest that you can improve bpf+kprobes when attached to function
> headers(or TRACE_MARKERS), make it converts pt-regs to bpf_ctx->arg1,
> arg2.., then top models and architectures can be separated by bpf.
>
> BPF bytecode is cross-platform, but what we can get by using bpf+kprobes
> is a 'regs->rdx' kind of information, such information is both
> architecture and kernel version related.

for kprobes in the middle of the function, kernel cannot
convert pt_regs into argN. Placement was decided by compiler
and can only be found in debug info.
I think bpf+kprobe will be using it when it is available.
When there is no debug info, kprobes will be limited
to function entry and mapping of regs/stack into
argN can be done by user space depending on architecture.
So user tracing scripts in some higher level language
can be kernel/arch independent when 'perf probe+bpf'
is loading them on the fly on the given machine.

> We hope to establish some models for describing kernel procedures such
> as IO and network, which requires that it does not rely on architecture
> and does not rely to a specific kernel version as much as possible.

That's obviously a goal, but it requires a new approach to tracepoints.
I think a lot of great ideas were discussed in this thread, so I'm
hopeful that we'll come up with solution that will satisfy even
strictest Peter's requirements :)
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