Re: [PATCH -tip 3/3] perf-probe: Use the actual address as a hint for uprobes

[Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>]

(2013/12/21 3:03), Arnaldo Carvalho de Melo wrote:
> Em Fri, Dec 20, 2013 at 10:03:02AM +0000, Masami Hiramatsu escreveu:
>> Use the actual address of tracepoint as a hint to find
>> different local symbols. Since sometimes there are local
>> symbols which have same name, it is impossible to determine
>> which symbol should be used. This saves the actual address
>> from debuginfo and use it as a hint later.
>>
>> The reason why we don't use the address directly is that
>> the address stored in the debuginfo has some section offset.
>> Thus this just uses the last 12 bits of the address as a
>> hint when searching the symbol in the map.
> 
> Again, can you provide an example of cases where before this patch the
> problem happens, with actual command output?

Ok, here is what the problem I've met,

Without this patch;
# perf probe -vfn -x perf scnprintf 2>&1
[...]
Writing event: p:probe_perf/scnprintf /kbuild/ksrc/linux-2.6/tools/perf/perf:0x8fd10
Writing event: p:probe_perf/scnprintf_1 /kbuild/ksrc/linux-2.6/tools/perf/perf:0xc02d0
Writing event: p:probe_perf/scnprintf_2 /kbuild/ksrc/linux-2.6/tools/perf/perf:0xc02d0
Writing event: p:probe_perf/scnprintf_3 /kbuild/ksrc/linux-2.6/tools/perf/perf:0x9ea00
Writing event: p:probe_perf/scnprintf_4 /kbuild/ksrc/linux-2.6/tools/perf/perf:0x25f36
Writing event: p:probe_perf/scnprintf_5 /kbuild/ksrc/linux-2.6/tools/perf/perf:0x29f90
Writing event: p:probe_perf/scnprintf_6 /kbuild/ksrc/linux-2.6/tools/perf/perf:0x29f90
Writing event: p:probe_perf/scnprintf_7 /kbuild/ksrc/linux-2.6/tools/perf/perf:0xc02d0
Writing event: p:probe_perf/scnprintf_8 /kbuild/ksrc/linux-2.6/tools/perf/perf:0xc02d0
Writing event: p:probe_perf/scnprintf_9 /kbuild/ksrc/linux-2.6/tools/perf/perf:0xc02d0

You can see there are many 0xc02d0, which is the last symbol of scnprintf in perf.
# nm perf | grep scnprintf
[...]
00000000004a3a30 t scnprintf
000000000041dad0 t scnprintf
0000000000425480 t scnprintf
0000000000442b20 t scnprintf
0000000000448b20 t scnprintf
00000000004525e0 t scnprintf
00000000004555f0 t scnprintf
00000000004678c0 t scnprintf
00000000004c02d0 t scnprintf

With this patch,
# perf probe -vfn -x perf scnprintf 2>&1
[...]
Writing event: p:probe_perf/scnprintf /kbuild/ksrc/linux-2.6/tools/perf/perf:0x1a820
Writing event: p:probe_perf/scnprintf_1 /kbuild/ksrc/linux-2.6/tools/perf/perf:0x1dad0
Writing event: p:probe_perf/scnprintf_2 /kbuild/ksrc/linux-2.6/tools/perf/perf:0x25480
Writing event: p:probe_perf/scnprintf_3 /kbuild/ksrc/linux-2.6/tools/perf/perf:0x25560
Writing event: p:probe_perf/scnprintf_4 /kbuild/ksrc/linux-2.6/tools/perf/perf:0x25f36
Writing event: p:probe_perf/scnprintf_5 /kbuild/ksrc/linux-2.6/tools/perf/perf:0x29f90
Writing event: p:probe_perf/scnprintf_6 /kbuild/ksrc/linux-2.6/tools/perf/perf:0x3e050
Writing event: p:probe_perf/scnprintf_7 /kbuild/ksrc/linux-2.6/tools/perf/perf:0x48b20
Writing event: p:probe_perf/scnprintf_8 /kbuild/ksrc/linux-2.6/tools/perf/perf:0x48b20
Writing event: p:probe_perf/scnprintf_9 /kbuild/ksrc/linux-2.6/tools/perf/perf:0x525e0

Oops, I thought 12bits are enough, but it seems not...

BTW, I'm not sure why debuginfo and nm shows symbol address + 0x400000,
and why the perf's map/symbol can remove this offset. Could you tell me
how it works?
If I can get the offset (0x400000) from binary, I don't need this kind
of ugly hacks...

Thank you,

> 
> Then output from the same command with the patch applied, showing how it
> works after the fix.
> 
> This way users can more quickly use your work and, among others, I can
> help you by validating it in my test machines.
> 
> Thanks,
> 
> - Arnaldo
>  
>> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
>> ---
>>  tools/perf/util/probe-event.c  |   16 +++++++++++++---
>>  tools/perf/util/probe-event.h  |    1 +
>>  tools/perf/util/probe-finder.c |    1 +
>>  3 files changed, 15 insertions(+), 3 deletions(-)
>>
>> diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
>> index e27cecb..e5fbda3 100644
>> --- a/tools/perf/util/probe-event.c
>> +++ b/tools/perf/util/probe-event.c
>> @@ -187,11 +187,18 @@ static int init_user_exec(void)
>>  }
>>  
>>  static const char *__target_symbol;
>> +static unsigned long __target_hint;
>>  static struct symbol *__result_sym;
>> +#define HINT_MASK	0xfff
>>  
>>  static int filter_target_symbol(struct map *map __maybe_unused,
>>  				struct symbol *sym)
>>  {
>> +	/* Check the last bits is same */
>> +	if (__target_hint)
>> +		if ((sym->start & HINT_MASK) != (__target_hint & HINT_MASK))
>> +			return 0;
>> +
>>  	if (strcmp(__target_symbol, sym->name) == 0) {
>>  		__result_sym = sym;
>>  		return 0;
>> @@ -201,7 +208,7 @@ static int filter_target_symbol(struct map *map __maybe_unused,
>>  
>>  /* Find the offset of the symbol in the executable binary */
>>  static int find_symbol_offset(const char *exec, const char *function,
>> -			      unsigned long *offs)
>> +			      unsigned long hint, unsigned long *offs)
>>  {
>>  	struct symbol *sym;
>>  	struct map *map = NULL;
>> @@ -218,6 +225,7 @@ static int find_symbol_offset(const char *exec, const char *function,
>>  	pr_debug("Search %s in %s\n", function, exec);
>>  	__target_symbol = function;
>>  	__result_sym = NULL;
>> +	__target_hint = hint;
>>  	if (map__load(map, filter_target_symbol)) {
>>  		pr_err("Failed to find %s in %s.\n", function, exec);
>>  		goto out;
>> @@ -357,8 +365,10 @@ static int add_exec_to_probe_trace_events(struct probe_trace_event *tevs,
>>  		return 0;
>>  
>>  	for (i = 0; i < ntevs && ret >= 0; i++) {
>> +		offset = tevs[i].point.address - tevs[i].point.offset;
>>  		/* Get proper offset */
>> -		ret = find_symbol_offset(exec, tevs[i].point.symbol, &offset);
>> +		ret = find_symbol_offset(exec, tevs[i].point.symbol,
>> +					 offset, &offset);
>>  		if (ret < 0)
>>  			break;
>>  		offset += tevs[i].point.offset;
>> @@ -2418,7 +2428,7 @@ static int convert_name_to_addr(struct perf_probe_event *pev, const char *exec)
>>  		goto out;
>>  	}
>>  
>> -	ret = find_symbol_offset(exec, pp->function, &vaddr);
>> +	ret = find_symbol_offset(exec, pp->function, 0, &vaddr);
>>  	if (ret < 0)
>>  		goto out;
>>  
>> diff --git a/tools/perf/util/probe-event.h b/tools/perf/util/probe-event.h
>> index f9f3de8..d481c46 100644
>> --- a/tools/perf/util/probe-event.h
>> +++ b/tools/perf/util/probe-event.h
>> @@ -12,6 +12,7 @@ struct probe_trace_point {
>>  	char		*symbol;	/* Base symbol */
>>  	char		*module;	/* Module name */
>>  	unsigned long	offset;		/* Offset from symbol */
>> +	unsigned long	address;	/* Actual address of the trace point */
>>  	bool		retprobe;	/* Return probe flag */
>>  };
>>  
>> diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
>> index ffb657f..7db7e05 100644
>> --- a/tools/perf/util/probe-finder.c
>> +++ b/tools/perf/util/probe-finder.c
>> @@ -729,6 +729,7 @@ static int convert_to_trace_point(Dwarf_Die *sp_die, Dwfl_Module *mod,
>>  		return -ENOENT;
>>  	}
>>  	tp->offset = (unsigned long)(paddr - sym.st_value);
>> +	tp->address = (unsigned long)paddr;
>>  	tp->symbol = strdup(symbol);
>>  	if (!tp->symbol)
>>  		return -ENOMEM;
>>
> 


-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com