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Date:   Thu, 30 May 2019 21:15:47 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
Cc:     Jiri Olsa <jolsa@...hat.com>, Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] perf record: collect user registers set jointly with
 dwarf stacks


On 30.05.2019 21:04, Arnaldo Carvalho de Melo wrote:
> Em Thu, May 30, 2019 at 07:24:57PM +0300, Alexey Budankov escreveu:
>>
>> 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.
>> "
> 
> I think with this wording and the renaming of DWARF_REGS to
> DWARF_MINIMAL_REGS it should be enough.

Well, let's have it like this in v5.

Thanks,
Alexey

> 
> - Arnaldo
> 

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