linux-kernel - Re: disabling group leader perf

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Date:	Mon, 6 Sep 2010 23:37:06 +0300
From:	Pekka Enberg <penberg@...nel.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Avi Kivity <avi@...hat.com>, Pekka Enberg <penberg@...helsinki.fi>,
	Tom Zanussi <tzanussi@...il.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-perf-users@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: disabling group leader perf_event

On Mon, Sep 6, 2010 at 6:47 PM, Ingo Molnar <mingo@...e.hu> wrote:
>>> The actual language doesn't really matter.
>>
>> There are 3 basic categories:
>>
>>  1- Most (least abstract) specific code: a block of bytecode in the form
>>    of a simplified, executable, kernel-checked x86 machine code block -
>>    this is also the fastest form. [yes, this is actually possible.]
>>
>>  2- Least specific (most abstract) code: A subset/sideset of C - as it's
>>    the most kernel-developer-trustable/debuggable form.
>>
>>  3- Everything else little more than a dot on the spectrum between the
>>    first two points.
>>
>> I lean towards #2 - but #1 looks interesting too. #3 is distinctly
>> uninteresting as it cannot be as fast as #1 and cannot be as convenient
>> as #2.

2010/9/6 Pekka Enberg <penberg@...nel.org>:
> It's a question where you want to push the complexity of parsing the
> language and verifying the executed code. I'd image it's easier to
> evolve an ABI if we use an intermediate form ("bytecode") on the
> kernel side. Supporting multiple versions of a C-like language is
> probably going to be painful. You also probably don't want to put
> heavy-weight compiler optimization passes in the kernel so with an
> intermediate form, you can do much of that in user-space.
>
> I'm guessing this thing is expected to work on all architectures? If
> that's true, I'd forget about JIT'ing for the time being and write an
> interpreter first because it's much easier to port. There are
> techniques in making an interpreter pretty fast too. Google for
> "inlining interpreter" if you're interested.
>
> As for the intermediate form, you might want to take a look at Dalvik:
>
> http://www.netmite.com/android/mydroid/dalvik/docs/dalvik-bytecode.html
>
> and probably ParrotVM bytecode too. The thing to avoid is stack-based
> instructions like in Java bytecode because although it's easy to write
> interpreters for them, it makes JIT'ing harder (which needs to convert
> stack-based representation to register-based) and probably doesn't
> lend itself well to stack-constrained kernel code.

Btw, the alternative route is to imitate how compilers like tcc and
lcc (and IIRC go and one of the plan9 languages) go about compiling C
language code into native code directly. They are pretty light-weight,
fast, and generate decent code. The down-side is that verification is
going to be more tricky and ABI issues might turn out to be nasty.

                        Pekka
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