linux-kernel - Re: [PATCH 14/28] ktap: add runtime/kp

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Date:	Tue, 01 Apr 2014 17:05:43 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Jovi Zhangwei <jovi.zhangwei@...il.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 14/28] ktap: add runtime/kp_events.[c|h]

(2014/04/01 16:28), Jovi Zhangwei wrote:

>>>>> Note:
>>>>> Why ktap support 'kdebug.kprobe' and 'kdebug.tracepoint' when
>>>>> it already support perf backend event(trace xxx {})?
>>>>>
>>>>> Because benchmark shows raw kprobe and tracpoint interface is faster
>>>>> than perf backed tracing, nearly 10+%, it's more fair to compare
>>>>> with Systemtap by raw tracing syntax, not perf backend tracing.
>>>>>
>>>>
>>>> Do we really need it just for a +10% performance? I doubt that.
>>>> I think the benefit point of ktap is "dynamic & simple programmable
>>>> tracer in kernel", not the good performance at least at this point.
>>>> Thus I think we should start ktap only with perf backend.
>>>>
>>> Yeah, agreed, most people like the perf-backed tracing syntax,
>>> that raw trace interface is just for benchmark when I wanted to look
>>> overhead compare with stap, the result is very inspiring, ktap table
>>> operation overhead is lower than stap.
>>>
>>> On the performance overhead of dynamic tracing tools(ktap/stap/dtrace),
>>> it's interesting enough that dtrace was used in production many year,
>>> _but_ IMO the runtime of dtrace is slow after I checked dtrace source
>>> code :),  system workload does big matter than tracing tool overhead.
>>
>> Yeah, I see that less overhead is also required especially for enterprise
>> people. I just doubt that it is solved by ktap itself. Should we improve
>> perf(or ftrace) to export more effective interfaces for this kind of
>> tracers?
>>
> Yes, I also think it would be better to improve perf/ftrace unified callback
> overhead, not to let each tracer(stap/ktap/lttng) develop its own raw
> trace callback for performance reason.
> 
> Those raw trace interfaces(only designed for benchmark) will be remove
> in next version, if we think it's worth to continue.

Of course, I think ktap scripting flexibility should be merged to upstream :)
I just surprised that the size of this series. If you reform this series
for incremental build (this means that we can do git-bisect on this series),
I think that will be much easier to test and review.

Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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