linux-kernel - Re: [PATCH 04/22] tools lib traceevent: Add jbd2 plugin

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Date:	Sat, 23 Nov 2013 03:52:21 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	David Ahern <dsahern@...il.com>,
	"Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH 04/22] tools lib traceevent: Add jbd2 plugin

On Fri, 22 Nov 2013 23:27:57 +0900
Namhyung Kim <namhyung@...nel.org> wrote:

> [SNIP]
> > +#define MINORBITS	20
> > +#define MINORMASK	((1U << MINORBITS) - 1)
> > +
> > +#define MAJOR(dev)	((unsigned int) ((dev) >> MINORBITS))
> > +#define MINOR(dev)	((unsigned int) ((dev) & MINORMASK))
> > +
> > +unsigned long long process_jbd2_dev_to_name(struct trace_seq *s,
> > +					    unsigned long long *args)
> 
> Isn't it better to make these functions static?

Yeah, but that was my mistake. Lets keep the fix separate from the
backport though. Thanks!

> 
> 
> > +{
> > +	unsigned int dev = args[0];
> > +
> > +	trace_seq_printf(s, "%d:%d", MAJOR(dev), MINOR(dev));
> > +	return 0;
> > +}
> > +
> > +unsigned long long process_jiffies_to_msecs(struct trace_seq *s,
> > +					    unsigned long long *args)
> > +{
> > +	unsigned long long jiffies = args[0];
> > +
> > +	trace_seq_printf(s, "%lld", jiffies);
> > +	return jiffies;
> > +}
> > +
> > +int PEVENT_PLUGIN_LOADER(struct pevent *pevent)
> > +{
> > +	pevent_register_print_function(pevent,
> > +				       process_jbd2_dev_to_name,
> > +				       PEVENT_FUNC_ARG_STRING,
> 
> Actually the function returns long long not string.  But it seems the
> current code doesn't care about the return type.

Actually it's not representing what process_jbd2_dev_to_name() returns
(which will always return unsigned long long), but what
"jbd2_dev_to_name()" returns that is (was) defined in the kernel. That
was:

	const char *jbd2_dev_to_name(dev_t device)

When registering a function to handle, you need to express the
prototype of that function (not the handler). The third argument is the
ret_type of that function.

But this is interesting, the ret_type doesn't seem to be used in
event_parse.c. The return value of the callback is only done in
eval_num_arg() where we could put a warning if the ret_type is not a
number.

Funny thing is, this patch is irrelevant with todays kernel due to this
commit:

commit 4862fd6047ed02e2726667c54d35f538eecc56aa
Author: Theodore Ts'o <tytso@....edu>
Date:   Sun Jul 10 22:05:08 2011 -0400

    jbd2: remove jbd2_dev_to_name() from jbd2 tracepoints
    
    Using function calls in TP_printk causes perf heartburn, so print the
    MAJOR/MINOR device numbers instead.
    
    Signed-off-by: "Theodore Ts'o" <tytso@....edu>



> 
> 
> > +				       "jbd2_dev_to_name",
> > +				       PEVENT_FUNC_ARG_INT,
> > +				       PEVENT_FUNC_ARG_VOID);
> > +
> > +	pevent_register_print_function(pevent,
> > +				       process_jiffies_to_msecs,
> > +				       PEVENT_FUNC_ARG_LONG,
> > +				       "jiffies_to_msecs",
> > +				       PEVENT_FUNC_ARG_LONG,
> > +				       PEVENT_FUNC_ARG_VOID);
> > +	return 0;
> > +}
> 
> Shouldn't it unregister these functions when unloaded?

Probably ;-)

-- Steve

> 
> Thanks,
> Namhyung
> 

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