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Date: Tue, 19 May 2009 14:11:55 +0800
From: Li Zefan <lizf@cn.fujitsu.com>
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To: Frederic Weisbecker <fweisbec@gmail.com>
CC: Ingo Molnar <mingo@elte.hu>, Jens Axboe <jens.axboe@oracle.com>,
       Steven Rostedt <rostedt@goodmis.org>, Tom Zanussi <tzanussi@gmail.com>,
       "Theodore Ts'o" <tytso@mit.edu>,
       Steven Whitehouse <swhiteho@redhat.com>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC][PATCH] convert block trace points to TRACE_EVENT()
References: <4A0BB813.9080807@cn.fujitsu.com> <20090518130514.GB4704@nowhere>
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Frederic Weisbecker wrote:
> On Thu, May 14, 2009 at 02:20:03PM +0800, Li Zefan wrote:
>> TRACE_EVENT is a more generic way to define tracepoints. Doing so adds
>> these new capabilities to this tracepoint:
>>
>>   - zero-copy and per-cpu splice() tracing
>>   - binary tracing without printf overhead
>>   - structured logging records exposed under /debug/tracing/events
>>   - trace events embedded in function tracer output and other plugins
>>   - user-defined, per tracepoint filter expressions
>>   ...
>>
>> Cons and problems:
>>
>>   - no dev_t info for the output of plug, unplug_timer and unplug_io events.
>>     no dev_t info for getrq and sleeprq events if bio == NULL.
>>     no dev_t info for rq_abort,...,rq_requeue events if rq->rq_disk == NULL.
>>
>>   - for large packet commands, only 16 bytes of the command will be output.
>>     Because TRACE_EVENT doesn't support dynamic-sized arrays, though it
>>     supports dynamic-sized strings.
>>
>>   - a packet command is converted to a string in TP_assign, not TP_print.
>>     While blktrace do the convertion just before output.
>>
>>   - in blktrace, an event can have 2 different print formats, but a TRACE_EVENT
>>     has a unique format. (see the output of getrq and rq_insert)
> 
> I'm starting to think it would be nice to choose between several outputs
> in a trace event.
> Ie: perhaps we need a kind of per event flag, something simple, just to
> choose between several TP_printk output. Not sure how much it would
> (non) trivial to implement though...
> 

If a trace event wants several TP_printk output, probably it wants different
structs of trace entry, otherwise we'll be wasting memory.