linux-kernel - Re: [RFC Patch 1/1] trace_printk and trace_dump interface

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Message-ID: <op.ubvmdr1i03j166@prasadkr_t60p.in.ibm.com>
Date:	Wed, 28 May 2008 23:46:19 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	"Andrew Morton" <akpm@...ux-foundation.org>, zanussi@...cast.net
Cc:	linux-kernel@...r.kernel.org, dwilder@...ibm.com,
	hch@...radead.org, prasad@...ux.vnet.ibm.com
Subject: Re: [RFC Patch 1/1] trace_printk and trace_dump interface - v2

On Fri, May 23, 2008 at 10:07:50AM +0530, K.Prasad wrote:
> On Tue, May 20, 2008 at 01:12:25PM -0700, Andrew Morton wrote:
>> On Wed, 21 May 2008 01:23:09 +0530
>> "K.Prasad" <prasad@...ux.vnet.ibm.com> wrote:
>>>> The name 'trace' (previously GTSC), I gather that it was the chosen
>> after
>> > much deliberation (http://tinyurl.com/6odoh4), however I'm open to the
>> > idea of changing the name (say dbg_printk/dbg_dump?).
>> >> Kindly let me know of your suggestions for this, and I will change them
>> > during the next version.
>> Well I was just putting it out there for consideration.  Yes, I think
>> the whole idea of consuming the "trace_*" namespace in this patchset
>> was ill-advised.
> Since "trace_*" uses relay infrastructure underneath, I am thinking, if it
> would be acceptable to rename them to "relay_*", say relay_printk() and
> relay_dump(). I would be glad to hear from the 'relay' folks about this
> thought.
>
Hi Andrew,
    Given that "trace_*" consists of wrapper functions around "relay"
(relay + debugfs filesystem), I'm sending out the following patches
which rename lib/trace.[ch] to kernel/relay_debugfs.[ch]. The "trace_*"
functions are renamed to "relay_*" functions without any name-space
clashes with existing "relay" functions.

Now the new functions relay_printk() and relay_dump() will provide an
easy-to-use interface for "relay" and will also reduce the amount of
code require to setup/cleanup relay.

>> Also, I don't know how to move forward with the whole feature - I
>> haven't seen a lot of interest from others and I haven't seen much
>> discussion of how this feature differs from all the other tracing
>> things which have been floating about.
> More than a tracing mechanism, this is a tool that aids in tracing, by
> providing a powerful function that directs its output onto the debugfs
> mount path which could be later harnessed by user-space applications
> too. A potential in-kernel user could be the 'marker' handlers which
> more often would be interested in logging data.
>
>> And even if the proposed patches presently offer unique and useful
>> features, will one of the other tracing implementations (eg: ltt) later
>> grow to close that gap?
>> I'm also a bit dubious about the whole thing based on past experience
>> with kernel-developer-only in-kernel tools.  People just don't use them
>> much.  One example: fault injection.
>
> Among the other enhancements that we were contemplating for this
> mechanism, to make it more powerful and unique, is the ability to
> a)Define callback functions typically invoked everytime before accessing/
>   printing each variable (which may say, acquire a lock or prefix a
>   timestamp) by adding fields to the trace_printk_data structure.
> b)Provide sequencing information for the output, along with ability to
>   prefix the output with essential data such as PID, Timestamp, CPUID,
>   etc.
>
Along with the above mentioned points, the sole in-kernel user of
"relay" which is "blktrace" was also converted to use (the erstwhile)
"trace_*", which resulted in significant code reduction. I will now migrate
"blktrace" to use "relay_*".

Kindly let us know what you think about the patches.

Thanks,
K.Prasad
P.S.: The second patch which actually effects the name change, along
with addition of relay_printk()/relay_dump() interface is found to be
quite lengthy. Since the patches have been previously reviewed I'm
sending them as a single chunk of patch.
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