Date:   Wed, 8 Dec 2021 11:14:34 -0600
From:   Kim Phillips <kim.phillips@....com>
To:     kajoljain <kjain@...ux.ibm.com>, Jiri Olsa <jolsa@...hat.com>
Cc:     Arnaldo Carvalho de Melo <acme@...hat.com>,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Ian Rogers <irogers@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Michael Petlan <mpetlan@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Robert Richter <robert.richter@....com>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 2/2] perf tools: Improve IBS error handling

On 12/8/21 12:33 AM, kajoljain wrote:
> On 11/30/21 3:39 AM, Kim Phillips wrote:
>> On 11/24/21 2:00 AM, kajoljain wrote:
>>> On 11/23/21 8:55 PM, Kim Phillips wrote:
>>>> On 11/23/21 2:40 AM, kajoljain wrote:
>>>>> On 10/8/21 12:47 AM, Kim Phillips wrote:
>>>>>> On 10/7/21 12:28 PM, Jiri Olsa wrote:
>>>>>>> On Mon, Oct 04, 2021 at 04:41:14PM -0500, Kim Phillips wrote:
>>>>>>>> ---
>>>>>>>>      tools/perf/util/evsel.c | 24 ++++++++++++++++++++++++
>>>>>>>>      1 file changed, 24 insertions(+)
>>>>>>>>
>>>>>>>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
>>>>>>>> index b915840690d4..f8a9cbd99314 100644
>>>>>>>> --- a/tools/perf/util/evsel.c
>>>>>>>> +++ b/tools/perf/util/evsel.c
>>>>>>>> @@ -2743,9 +2743,22 @@ static bool find_process(const char *name)
>>>>>>>>          return ret ? false : true;
>>>>>>>>      }
>>>>>>>>      +static bool is_amd(const char *arch, const char *cpuid)
>>>>>>>> +{
>>>>>>>> +    return arch && !strcmp("x86", arch) && cpuid &&
>>>>>>>> strstarts(cpuid,
>>>>>>>> "AuthenticAMD");
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> +static bool is_amd_ibs(struct evsel *evsel)
>>>>>>>> +{
>>>>>>>> +    return evsel->core.attr.precise_ip || !strncmp(evsel->pmu_name,
>>>>>>>> "ibs", 3);
>>>>>>>> +}
>>>>>>>> +
>>>>>>>>      int evsel__open_strerror(struct evsel *evsel, struct target
>>>>>>>> *target,
>>>>>>>>                   int err, char *msg, size_t size)
>>>>>>>>      {
>>>>>>>> +    struct perf_env *env = evsel__env(evsel);
>>>>>>>> +    const char *arch = perf_env__arch(env);
>>>>>>>> +    const char *cpuid = perf_env__cpuid(env);
>>>>>>>>          char sbuf[STRERR_BUFSIZE];
>>>>>>>>          int printed = 0, enforced = 0;
>>>>>>>>      @@ -2841,6 +2854,17 @@ int evsel__open_strerror(struct evsel
>>>>>>>> *evsel, struct target *target,
>>>>>>>>                  return scnprintf(msg, size, "wrong clockid (%d).",
>>>>>>>> clockid);
>>>>>>>>              if (perf_missing_features.aux_output)
>>>>>>>>                  return scnprintf(msg, size, "The 'aux_output'
>>>>>>>> feature
>>>>>>>> is not supported, update the kernel.");
>>>>>>>> +        if (is_amd(arch, cpuid)) {
>>>>>>>> +            if (is_amd_ibs(evsel)) {
>>>>>>>
>>>>>>> would single 'is_amd_ibs' call be better? checking on both amd and
>>>>>>> ibs
>>>>>>
>>>>>> Good suggestion. If you look at the later patch in the
>>>>>> BRS series, I have rewritten it to add the new
>>>>>> AMD PMU like so:
>>>>>>
>>>>>>     if (is_amd()) {
>>>>>>         if (is_amd_ibs()) {
>>>>>>             if (evsel->this)
>>>>>>                 return
>>>>>>             if (evsel->that)
>>>>>>                 return
>>>>>>         }
>>>>>> +    if (is_amd_brs()) {
>>>>>> +        if (evsel->this)
>>>>>> +            return
>>>>>> +        if (evsel->that)
>>>>>> +            return
>>>>>> +    }
>>>>>>     }
>>>>>
>>>>> Hi Kim,
>>>>>         From my point of view, it won't be a good idea of adding so many
>>>>> checks in common function definition itself.
>>>>> Can you just create a check to see if its amd machine and then add a
>>>>> function call which will handle all four conditions together?
>>>>>
>>>>> which is basically for:
>>>>>
>>>>> +        if (is_amd(arch, cpuid)) {
>>>>> +            if (is_amd_ibs(evsel)) {
>>>>> +                if (evsel->core.attr.exclude_kernel)
>>>>> +                    return scnprintf(msg, size,
>>>>> +    "AMD IBS can't exclude kernel events.  Try running at a higher
>>>>> privilege level.");
>>>>> +                if (!evsel->core.system_wide)
>>>>> +                    return scnprintf(msg, size,
>>>>> +    "AMD IBS may only be available in system-wide/per-cpu mode.  Try
>>>>> using
>>>>> -a, or -C and workload affinity");
>>>>> +            }
>>>>>
>>>>> and this:
>>>>>
>>>>> +            if (is_amd_brs(evsel)) {
>>>>> +                if (evsel->core.attr.freq)
>>>>> +                    return scnprintf(msg, size,
>>>>> +    "AMD Branch Sampling does not support frequency mode sampling,
>>>>> must
>>>>> pass a fixed sampling period via -c option or
>>>>> cpu/branch-brs,period=xxxx/.");
>>>>> +                /* another reason is that the period is too small */
>>>>> +                return scnprintf(msg, size,
>>>>> +    "AMD Branch Sampling does not support sampling period smaller than
>>>>> what is reported in /sys/devices/cpu/caps/branches.");
>>>>> +            }
>>>>
>>>> IIRC, I tried something like that but carrying the
>>>>
>>>>
>>>> struct target *target, int err, char *msg, size_t size
>>>>
>>>> parameters made things worse.
>>>>
>>>>> So, incase we are in amd machine,  common function evsel__open_strerror
>>>>> will call function may be something like amd_evesel_open_strerror_check
>>>>> which will look for both ibs and brs conditions and return
>>>>> corresponding
>>>>> error statement.
>>>>
>>>> The vast majority of decisions made by evsel__open_strerror are
>>>> going to be common across most arch/uarches.  AMD has only these
>>>> two pesky exceptions to the rule and therefore IMO it's ok
>>>> to have them inline with the common function, since the decisions
>>>> are so deeply intertwined.  A new amd_evsel_open_strerror_check
>>>> sounds like it'd duplicate too much of the common function code
>>>> in order to handle the common error cases.
>>>
>>> Hi Kim,
>>>      Sorry for the confusion, what I meant by adding new function is just
>>> to handle these corner error cases and not duplicating whole
>>> evsel__open_strerror code.
>>>
>>> Maybe something like below code, Its just prototype of code to show you
>>> the flow, you can refine it and check for any build or indentation
>>> issues using checkpatch.pl script.
>>>
>>> So basically, in common function we can just have 2 calls, first to
>>> check if we are in amd system and second to return corresponding error
>>> message, rather then adding whole chunk of if's which are specific to
>>> amd.
>>>
>>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
>>> index ac0127be0459..adefb162ae08 100644
>>> --- a/tools/perf/util/evsel.c
>>> +++ b/tools/perf/util/evsel.c
>>> @@ -2852,9 +2852,40 @@ static bool find_process(const char *name)
>>>           return ret ? false : true;
>>>    }
>>>
>>> +static bool is_amd(const char *arch, const char *cpuid)
>>> +{
>>> +       return arch && !strcmp("x86", arch) && cpuid && strstarts(cpuid,
>>> "AuthenticAMD");
>>> +}
>>> +
>>> +static int error_amd_ibs_brs(struct evsel *evsel, char *msg, size_t
>>> size)
>>> +{
>>> +       if (evsel->core.attr.precise_ip || !strncmp(evsel->pmu_name,
>>> "ibs", 3)) {
>>> +               if (evsel->core.attr.exclude_kernel)
>>> +                       return scnprintf(msg, size,
>>> +       "AMD IBS can't exclude kernel events.  Try running at a higher
>>> privilege level.");
>>> +               if (!evsel->core.system_wide)
>>> +                       return scnprintf(msg, size,
>>> +       "AMD IBS may only be available in system-wide/per-cpu mode.  Try
>>> using -a, or -C and workload affinity");
>>> +       }
>>> +
>>> +       if (((evsel->core.attr.config & 0xff) == 0xc4) &&
>>> (evsel->core.attr.sample_type & PERF_SAMPLE_BRANCH_STACK)) {
>>> +               if (evsel->core.attr.freq) {
>>> +                       return scnprintf(msg, size,
>>> +       "AMD Branch Sampling does not support frequency mode sampling,
>>> must pass a fixed sampling
>>> +          period via -c option or cpu/branch-brs,period=xxxx/.");
>>> +                /* another reason is that the period is too small */
>>> +               return scnprintf(msg, size,
>>> +       "AMD Branch Sampling does not support sampling period smaller
>>> than what is reported in /sys/devices/cpu/caps/branches.");
>>> +               }
>>> +       }
>>> +}
>>> +
>>>    int evsel__open_strerror(struct evsel *evsel, struct target *target,
>>>                            int err, char *msg, size_t size)
>>>    {
>>> +       struct perf_env *env = evsel__env(evsel);
>>> +       const char *arch = perf_env__arch(env);
>>> +       const char *cpuid = perf_env__cpuid(env);
>>>           char sbuf[STRERR_BUFSIZE];
>>>           int printed = 0, enforced = 0;
>>>
>>> @@ -2950,6 +2981,8 @@ int evsel__open_strerror(struct evsel *evsel,
>>> struct target *target,
>>>                           return scnprintf(msg, size, "wrong clockid
>>> (%d).", clockid);
>>>                   if (perf_missing_features.aux_output)
>>>                           return scnprintf(msg, size, "The 'aux_output'
>>> feature is not supported, update the kernel.");
>>> +               if (is_amd(arch, cpuid))
>>> +                       return error_amd_ibs_brs(evsel, msg, size);
>>>                   break;
>>>           case ENODATA:
>>>                   return scnprintf(msg, size, "Cannot collect data source
>>> with the load latency event alone. "
>>
>> That change will makes AMD machines fail to fall back to the default
>> "The sys_perf_event_open() syscall returned with..." error string
>> in case it's not those AMD IBS and BRS sub-conditions.
> 
> Yes right, as I mentioned before, the code I pointed was just a
> prototype to show you the flow, these corner cases can be handled on top
> of it.

Right but these corner cases disrupt the existing flow: adding
int ret; ret = foo(); if (ret) goto report_generic_einval doesn't
go with the flow.

>> Is having the AMD error code checking in the main evsel__open_strerror()
>> so bad?  Other arches and their PMU implementations may find error
>> conditions that they have in common with AMD's, therefore
>> opening up the code for opposite types of refactoring and
>> reuse than what is being requested here.  E.g., I've seen
>> other hardware configurations - not specific to one architecture -
>> that could also use this message:
>>
> 
>  From my understanding, adding too many checks in common function
> for a specific arch is not a good practice. Since you already adding

My point above is that other arches can come in and adopt
the same error conditions and text.

> multiple functions to get information like ,if current system is
> amd/ibs/brs. Can't we rather just add a single function and handled all
> these checks there?

That will remove the code from the common path.  Code that's
possible (and in some cases likely) that will be adopted
by other arches.

Thanks,

Kim

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