linux-kernel - Re: [PATCH -tip 5/7] perf probe: Query basic types from debuginfo

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Message-ID: <4BB119F8.7020206@redhat.com>
Date:	Mon, 29 Mar 2010 17:22:00 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Arnaldo Carvalho de Melo <acme@...radead.org>
CC:	Ingo Molnar <mingo@...e.hu>, lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH -tip 5/7] perf probe: Query basic types from debuginfo

Arnaldo Carvalho de Melo wrote:
> Em Mon, Mar 29, 2010 at 04:38:29PM -0400, Masami Hiramatsu escreveu:
>> +static void convert_variable_type(Dwarf_Die *vr_die,
>> +				  struct kprobe_trace_arg *targ)
>> +{
>> +	Dwarf_Die type;
>> +	char buf[16];
>> +	int ret;
>> +
>> +	if (die_get_real_type(vr_die, &type) == NULL)
>> +		die("Failed to get a type information of %s.",
>> +		    dwarf_diename(vr_die));
>> +
>> +	ret = die_get_byte_size(&type) * 8;
>> +	if (ret) {
>> +		/* Check the bitwidth */
>> +		if (ret > MAX_BASIC_TYPE_BITS) {
>> +			pr_warning("  Warning: %s exceeds max-bitwidth."
>> +				   " Cut down to %d bits.\n",
>> +				   dwarf_diename(&type), MAX_BASIC_TYPE_BITS);
>> +			ret = MAX_BASIC_TYPE_BITS;
>> +		}
>> +
>> +		ret = snprintf(buf, 16, "%c%d",
>> +			       die_is_signed_type(&type) ? 's' : 'u', ret);
>> +		if (ret < 0 || ret >= 16)
>> +			die("Failed to convert variable type.");
>> +		targ->type = xstrdup(buf);
> 
> Question, should we use the equivalent to panic'ing the kernel in the
> userspace bits in tools/?
> 
> I tend to see all code there as potentially part of a library, i.e. to
> be callable by some unantecipated new tool or library that would rather
> receive a return value telling it that the operation can't be performed
> for some reason so that it can inform the user instead of having the
> whole tool exit to the command line.

OK, so that you want to see 

ret = library_func();
if (ret < 0)	/* Something wrong happened */
	return ret;

instead of 

library_func();
hopefully_executed_next_func();



> It may well be that some specific operation needs lots of resources but
> many other don't, panic'c because the one that requires lots of
> resources can't be performed, bringing down a gui/tui is really nasty.

Hmm, agreed. But I'd like to have some API for storing dying^H^H^H^H^Herror
message. (maybe can we use setjump/longjump approach - like try/cache - for die()?)

Thank you,

> 
> Perhaps the answer will be the same as for when people added panic calls
> in the kernel in the past, "you're screwed up anyway if that happens",
> but it just feels wrong :-\
> 
> In the DWARF bits it has even the added twist namespace collapse with
> DIE 8-)
> 
> - Arnaldo
> --
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-- 
Masami Hiramatsu
e-mail: mhiramat@...hat.com
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