Re: [PATCH v4] perf record: collect user registers set jointly with dwarf stacks

[Alexey Budankov]

On 30.05.2019 16:13, Arnaldo Carvalho de Melo wrote:
> Em Thu, May 30, 2019 at 11:24:49AM +0300, Alexey Budankov escreveu:
>> On 29.05.2019 22:25, Arnaldo Carvalho de Melo wrote:
>>> Em Wed, May 29, 2019 at 05:30:49PM +0300, Alexey Budankov escreveu:
>> <SNIP>
>>>> +++ b/tools/perf/util/evsel.c
>>>> +#define DWARF_REGS_MASK ((1ULL << PERF_REG_IP) | \
>>>> +			 (1ULL << PERF_REG_SP))
>>>> +
>>>>  static void __perf_evsel__config_callchain(struct perf_evsel *evsel,
>>>>  					   struct record_opts *opts,
>>>>  					   struct callchain_param *param)
>>>> @@ -702,7 +705,13 @@ static void __perf_evsel__config_callchain(struct perf_evsel *evsel,
>>>>  		if (!function) {
>>>>  			perf_evsel__set_sample_bit(evsel, REGS_USER);
>>>>  			perf_evsel__set_sample_bit(evsel, STACK_USER);
>>>> -			attr->sample_regs_user |= PERF_REGS_MASK;
>>>> +			if (opts->sample_user_regs) {
>>>
>>> Where are you checking that opts->sample_user_regs doesn't have either
>>> IP or SP?
>>
>> Sure. The the intention was to avoid such a complication, merge two 
>> masks and provide explicit warning that the resulting mask is extended.
> 
> s/is/may be/g
>  
>> If you still see the checking and auto detection of the exact mask 
>> extension as essential it can be implemented.
> 
> perf, tracing, systems internals, etc are super complicated, full of
> details, the more precise we can make the messages, the better.
>  
>>> So, __perf_evsel__config_callchain its the routine that sets up the
>>> attr->sample_regs_user when callchains are asked for, and what was it
>>> doing? Asking for _all_ user regs, right?
>>>
>>> I.e. what you're saying is that when --callgraph-dwarf is asked for,
>>> then only IP and BP are needed, and we should stop doing that, so that
>>> would be a first patch, if that is the case. I.e. a patch that doesn't
>>> even mention opts->sample_user_regs.
>>>
>>> Then, a second patch would fix the opt->sample_user_regs request clash
>>> with --callgraph dwarf, i.e. it would do something like:
>>>
>>> 	      if ((opts->sample_regs_user & DWARF_REGS_MASK) != DWARF_REGS_MASK) {
>>> 	      		char * ip = (opts->sample_regs_user & (1ULL << PERF_REG_IP)) ? NULL : "IP",
>>> 	      		     * sp = (opts->sample_regs_user & (1ULL << PERF_REG_SP)) ? NULL : "SP",
>>> 			     * all = (!ip && !sp) ?  "s" : "";
>>>
>>> 			pr_warning("WARNING: specified --user-regs register set doesn't include register%s "
>>> 				   "needed by also specified --call-graph=dwarf, auto adding %s%s%s register%s.\n",
>>> 				   all, ip, all : ", " : "", sp, all);
>>> 		}
>>>
>>> This if and only if all the registers that are needed to do DWARF
>>> unwinding are just IP and BP, which doesn't look like its true, since
>>> when no --user_regs is set (i.e. opts->user_regs is not set) then we
>>> continue asking for PERF_REGS_MASK...
>>>
>>> Can you check where I'm missing something?
>>
>> 1.  -g call-graph dwarf,K                         full_regs
>> 2.  --user-regs=user_regs                         user_regs
>> 3.  -g call-graph dwarf,K --user-regs=user_regs	  user_regs + dwarf_regs
>>
>> The default behavior stays the same for cases 1, 2 above.
>> For case 3 register set becomes the one asked using --user_regs option.
>> If the option value misses IP or SP or the both then they are explicitly
>> added to the option value and a warning message mentioning the exact 
>> added registers is provided.
>  
>>> Jiri DWARF unwind uses just IP and SP? Looking at
>>> tools/perf/util/unwind-libunwind-local.c's access_reg() I don't think
>>> so, right?
>  
>> If you ask me, AFAIK, DWARF unwind rules sometimes can refer additional 
>> general purpose registers for frames boundaries calculation.
> 
> :-) So that DWARF_REGS is misleading, should be something like
> DWARF_MINIMAL_REGS, as we may need other registers, so the original code
> was correct, right?

Right. Actually came to the same conclusion with the same naming for IP,SP mask :)

> 
> After all if the user asks for both --call-graph dwarf and --user-regs,
> then probably we should require --force? I.e. the message then would be:
> 
> "
> WARNING: The use of --call-graph=dwarf may require all the user
> registers, specifying a subset with --user-regs may render DWARF
> unwinding unreliable, please use --force if you're sure that the subset
> specified via --user-regs is enough for your specific use case.
> "
> 
> And then plain refuse, if the user _really_ wants it, then we have
> --force/-f for those cases.
> 
> Does this sound better?

If --user-regs is specified jointly with --call-graph dwarf option then
--user-regs already serves as the --force and, IMHO, a warning does the best.

The ideal solution, I could imagine, is to also dynamically calculate regs 
set extension and provide it in the warning, but it is only for two registers.

So, if --call-graph dwarf --user-regs=A,B,C are specified jointly then
"
WARNING: The use of --call-graph=dwarf may require all the user registers, 
specifying a subset with --user-regs may render DWARF unwinding unreliable,
so the minimal registers set (IP, SP) is explicitly forced.
"
The message is precise and it would fit the majority of use cases.
Final decision is up to you.

~Alexey

> 
> - Arnaldo
>