linux-kernel - Re: Tracing configuration review

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Message-ID: <4BFCE5F2.4070906@cn.fujitsu.com>
Date:	Wed, 26 May 2010 17:12:18 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
CC:	Pekka Enberg <penberg@...helsinki.fi>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chase Douglas <chase.douglas@...onical.com>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Soeren Sandmann <sandmann@...mi.au.dk>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: Tracing configuration review

Eduard - Gabriel Munteanu wrote:
> On Wed, May 26, 2010 at 03:20:16PM +0800, Li Zefan wrote:
>> Pekka Enberg wrote:
>>> On Tue, May 25, 2010 at 11:13 PM, Frederic Weisbecker
>>> <fweisbec@...il.com> wrote:
>>>>> # CONFIG_KMEMTRACE is not set (Kconfig says N if unsure)
>>>> Deprecated. We have the kmem trace event that are a full replacement now.
>>>> Pekka, Gabriel, can we remove it now?
>>> I don't think ftrace supports boot-time tracing which kmemtrace did.
>> We can do boot-time tracing by passing "trace_event=" kernel parameter.
>>
>> By passing "ftrace=kmemtrace", kmemtrace can be started when
>> calling tracer_alloc_buffer(), which is an early_initcall.
>> While trace events are inititialized as a fs_initcall,  it can
>> be modified to an early_initcall.
>>
>> Furthermore, I noticed the discussion on perf persistent events, which
>> seems to enable perf trace at boot time, so I think perf-kmem can take
>> advantage of this and will be a full-replacement of kmemtrace soon ?
>>
>>> That said, I was probably the only one actually using the feature so
>>> maybe we can just nuke kmemtrace at this point...
>> If you agree on removing kmemtrace now, I'll re-send the patch.
> 
> I don't understand... wasn't the old kmemtrace (the relayfs based stuff)
> removed a long time ago? Right now it's just the hooks and code that
> sets up tracing and printers.
> 
> Or does it happen that that code isn't necessary for 'perf kmem' to do
> its job?
> 

Yes, actually the code has nothing to do with perf-kmem.

> And yeah, I'm all for the perf stuff, kmemtrace-user (the out-of-tree
> userspace tools) has been obsolete since the shift to ftrace.
> 
> As for early tracing, that was possible with the relayfs variant, it
> required only the SLAB layer to be up IIRC. But that stopped working
> once it got converted to ftrace (kmemtrace_init() is a nop).
> 

Ah, thanks for making this clear, so there's no such early tracing
feature that acts as a barrier in removing kmemtrace.

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