Re: ktap inclusion in drivers/staging/?

From: Jovi Zhangwei
Date: Thu Oct 24 2013 - 08:32:46 EST

Hi Ingo,

On Thu, Oct 24, 2013 at 3:58 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> Greg,
> I was surprised to see 'ktap' appear in the staging tree silently,
> via these commits that are visible in today's staging-next:
> 2c856b9e3e06 staging: ktap: remove unused <asm/syscall.h> header file
> 687b63a3bfd5 staging: ktap: update email name in MAINTAINERS
> c63a164271f8 staging: ktap: add to the kernel tree
> ktap is pretty fresh instrumentation code, announced on lkml a
> couple of months ago, and so far I haven't seen much technical
> discussion of integrating ktap upstream, mostly I suspect because
> not a _single_ patch was sent to linux-kernel for review. (!)
I accept Greg revert this in staging-next tree, It's entirely my fault, sorry.

> An announcement of a Git tree was made (which Git tree is not very
> structured), and some very minimal discussion ensued, but no actual
> patches were sent with an intent to merge, no technical arguments
> were made in favor of merging and nothing conclusive was achieved.
> A couple of very quick (and incomplete) technical objections:
> - The Git commits in staging an absolutely unstructured,
> unreviewable mess, due to a single commit adding 16 KLOCs (!) of
> code:
> 80 files changed, 16376 insertions(+)
> (I looked at the ktap Git tree as well, it's not much better.)
> - Most of the kernel code comes with near zero explanations in the
> code itself. I looked at the kernel code in
> drivers/staging/ktap/interpreter/. I have not found a _single_
> substantial in-code comment about design details and
> implementational considerations. (!!)
I will add more comments for it, also will draft a design detail in
doc/ directory.

> - From the little I've been able to decode I get the impression
> that the design should be much more integrated into the rest of
> instrumentation: the in-kernel Lua bytecode interpreter looks
> interesting, it could be an intelligent upgrade (or even outright
> replacement) for the current 'filter' interpreter concept we have
> for tracepoints - instead of putting a parallel interpreter
> implementation into the kernel.
> - In a similar fashion, it would be nice to see it integrated with
> 'perf probe' or 'perf ktap', so that users can create probes from
> a single place, with coherent syntax and integrated analysis
> capabilities. I.e. there's no reason to not make this a
> relatively pain-less yet very useful transition.
Yes, I also mentioned this in my RFC email post before, that's the
reason why I use perf-like interface in ktap as much as I can, like
perf-tracepoints and perf-probe, also ktap can reuse perf debuginfo
handling code in future, we are on the same page at this technical point.

> - In a similar vein, it creates Yet Another Debugfs ABI, instead of
> trying to extend existing instrumentation.
I tried to create file in /sys/kernel/debug/tracing/ktap or use perf syscall,
that's looks more reasonable, but need some patches for kernel, so
independent debugfs interface was chosen in "initial stage".

> Despite my criticism, I'm actually a big proponent of safe kernel
> probing concepts and this code does have many of the qualities that
> I always wanted the tracepoint filter code to have in the long run.
Thanks, I'm glad to hear more and more people says ktap is useful,
of course the code is still need to improve.

> So it does look potentially useful, but _please_ don't merge ktap
> via the staging tree yet, until the code becomes reviewable, until
> it gets a proper review and until the instrumentation guys (I tried
> to Cc: folks who might be interested in it) ack it.
> Kernel instrumentation code should follow established procedures to
> gain Acks from interested kernel maintainers.
> Just because we've made it easy to create instrumentation callbacks
> and made it possible to hide it all in a separate driver doesn't
> mean the whole thing should explode into zillions of disjunct,
> incoherent approaches. It's not like kernel instrumentation is an
> under-maintained subsystem!
> So until it's all cleared up:
> Nacked-by: Ingo Molnar <mingo@xxxxxxxxxx>
Accept, really sorry about this mistake action, entirely my fault.

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