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

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Date:	Mon, 31 Mar 2014 18:10:07 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Jovi Zhangwei <jovi.zhangwei@...il.com>
Cc:	Ingo Molnar <mingo@...hat.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	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]

(2014/03/28 22:47), Jovi Zhangwei wrote:
> kp_events.c handle ktap events management(registry, destroy, event callback)
> 
> This file is core event management interface between ktap and kernel.
> 
> Exposed functions:
> 1). kp_events_init/kp_events_exit
> 
> 2). kp_event_create_kprobe
>         create kprobe event, for example:
>                 kdebug.kprobe("SyS_futex", function () {})
> 
> 3). kp_event_create_tracepoint
>         create tracepoint event, for example"
>                 kdebug.tracepoint("sys_futex_enter", function () {})
> 
> 4). kp_event_create
>         create perf backend event, for example:
>                 trace sched:sched_switch { print(argstr) }
> 
>         It call kernel function 'perf_event_create_kernel_counter' to
>         register event(tracepoint/kprobe/uprobe)
> 
> 5). kp_event_getarg
>         get argument of event, from arg0 to arg9,
>         only can be called in probe context.
>                 trace sched:sched_switch { print(arg0, arg1) }
> 
> 6). kp_event_stringify/kp_event_tostr
>         stringify argstr, sometimes if store argstr as key to table,
>         then it need to stringify firstly, like below:
>                 var s={} trace sched:sched_switch { s[argstr] += 1 }
>         (This is quite rare usage, but ktap support it)
> 
> 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.

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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