Re: [PATCH V3 0/5] bugs fix for large PEBS mmap read and rdpmc read

[Stephane Eranian]

On Tue, Jan 30, 2018 at 7:25 AM, Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>
>
> On 1/30/2018 10:04 AM, Jiri Olsa wrote:
>>
>> On Tue, Jan 30, 2018 at 09:59:15AM -0500, Liang, Kan wrote:
>>>
>>>
>>>
>>> On 1/30/2018 8:39 AM, Jiri Olsa wrote:
>>>>
>>>> On Tue, Jan 30, 2018 at 01:16:39AM -0800, Stephane Eranian wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> On Mon, Jan 29, 2018 at 8:29 AM, <kan.liang@xxxxxxxxxxxxxxx> wrote:
>>>>>>
>>>>>>
>>>>>> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
>>>>>>
>>>>>> ------
>>>>>>
>>>>>> Changes since V2:
>>>>>>    - Refined the changelog
>>>>>>    - Introduced specific read function for large PEBS.
>>>>>>      The previous generic PEBS read function is confusing.
>>>>>>      Disabled PMU in pmu::read() path for large PEBS.
>>>>>>      Handled the corner case when reload_times == 0.
>>>>>>    - Modified the parameter of intel_pmu_save_and_restart_reload()
>>>>>>      Discarded local64_cmpxchg
>>>>>>    - Added fixes tag
>>>>>>    - Added WARN to handle reload_times == 0 || reload_val == 0
>>>>>>
>>>>>> Changes since V1:
>>>>>>    - Check PERF_X86_EVENT_AUTO_RELOAD before call
>>>>>>      intel_pmu_save_and_restore()
>>>>>
>>>>>
>>>>> It is not yet clear to me why PERF_SAMPLE_PERIOD is not allowed
>>>>> with large PEBS. Large PEBS requires fixed period. So the kernel could
>>>>> make up the period from the event and store it in the sampling buffer.
>>>>>
>>>>> I tried using large PEBS recently, and despite trying different option
>>>>> combination of perf record, I was not able to get it to work.
>>>>>
>>>>> $ perf record  -c 1  -e cpu/event=0xd1,umask=0x10,period=19936/pp
>>>>> --no-timestamp --no-period -a -C 0
>>>>>
>>>>> But I was able to make this work with a much older kernel.
>>>>>
>>>>> Another annoyance I ran into is with perf record requiring -c period
>>>>> in order not to set
>>>>> PERF_SAMPLE_PERIOD in the event.
>>>>>
>>>>> If I do:
>>>>> perf record  -c 1  -e cpu/event=0xd1,umask=0x10,period=19936/pp
>>>>> --no-timestamp --no-period -a -C 0
>>>>>
>>>>> I get
>>>>>
>>>>> perf_event_attr:
>>>>>     type                             4
>>>>>     size                             112
>>>>>     config                           0x10d1
>>>>>     { sample_period, sample_freq }   199936
>>>>>     sample_type                      IP|TID|CPU
>>>>>
>>>>> But if I do:
>>>>> perf record  -e cpu/event=0xd1,umask=0x10,period=19936/pp
>>>>> --no-timestamp --no-period -a -C 0
>>>>>
>>>>> I get
>>>>>
>>>>> perf_event_attr:
>>>>>     type                             4
>>>>>     size                             112
>>>>>     config                           0x10d1
>>>>>     { sample_period, sample_freq }   199936
>>>>>     sample_type                      IP|TID|CPU|PERIOD
>>>>>
>>>>> Perf should check if all events have a period=, then it should not
>>>>> pass PERF_SAMPLE_PERIOD, even
>>>>> more so when only one event is defined.
>>>>>
>>>>> Also it does not seem to honor --no-period.
>>>>
>>>>
>>>> yep, there's a bug in period=x term handling
>>>> we did not re/set the sample_type based on that
>>>>
>>>> attached patch fixes that for me, also takes into account
>>>> the --no/-period options
>>>>
>>>> jirka
>>>>
>>>>
>>>> ---
>>>> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
>>>> index f251e824edac..907267206973 100644
>>>> --- a/tools/perf/builtin-record.c
>>>> +++ b/tools/perf/builtin-record.c
>>>> @@ -1566,7 +1566,8 @@ static struct option __record_options[] = {
>>>>         OPT_BOOLEAN_SET('T', "timestamp", &record.opts.sample_time,
>>>>                         &record.opts.sample_time_set,
>>>>                         "Record the sample timestamps"),
>>>> -       OPT_BOOLEAN('P', "period", &record.opts.period, "Record the
>>>> sample period"),
>>>> +       OPT_BOOLEAN_SET('P', "period", &record.opts.period,
>>>> &record.opts.period_set,
>>>> +                       "Record the sample period"),
>>>>         OPT_BOOLEAN('n', "no-samples", &record.opts.no_samples,
>>>>                     "don't sample"),
>>>>         OPT_BOOLEAN_SET('N', "no-buildid-cache",
>>>> &record.no_buildid_cache,
>>>> diff --git a/tools/perf/perf.h b/tools/perf/perf.h
>>>> index 2357f4ccc9c7..cfe46236a5e5 100644
>>>> --- a/tools/perf/perf.h
>>>> +++ b/tools/perf/perf.h
>>>> @@ -50,6 +50,7 @@ struct record_opts {
>>>>         bool         sample_time_set;
>>>>         bool         sample_cpu;
>>>>         bool         period;
>>>> +       bool         period_set;
>>>>         bool         running_time;
>>>>         bool         full_auxtrace;
>>>>         bool         auxtrace_snapshot_mode;
>>>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
>>>> index 66fa45198a11..ff359c9ece2e 100644
>>>> --- a/tools/perf/util/evsel.c
>>>> +++ b/tools/perf/util/evsel.c
>>>> @@ -745,12 +745,14 @@ static void apply_config_terms(struct perf_evsel
>>>> *evsel,
>>>>                         if (!(term->weak && opts->user_interval !=
>>>> ULLONG_MAX)) {
>>>>                                 attr->sample_period = term->val.period;
>>>>                                 attr->freq = 0;
>>>> +                               perf_evsel__reset_sample_bit(evsel,
>>>> PERIOD);
>>>>                         }
>>>>                         break;
>>>>                 case PERF_EVSEL__CONFIG_TERM_FREQ:
>>>>                         if (!(term->weak && opts->user_freq !=
>>>> UINT_MAX)) {
>>>>                                 attr->sample_freq = term->val.freq;
>>>>                                 attr->freq = 1;
>>>> +                               perf_evsel__set_sample_bit(evsel,
>>>> PERIOD);
>>>>                         }
>>>
>>>
>>> If we do so, some events could be in fixed mode without PERIOD set. Other
>>> events could be in freq mode with PERIOD set.
>>
>>
>> it also sets the attr->freq, so it's not in fixed mode
>>
>
> I mean the TERM_PERIOD. It probably enable the large PEBS.
>>>>                             attr->freq = 0;
>>>> +                           perf_evsel__reset_sample_bit(evsel, PERIOD);
>
> The events in fixed mode could enable large PEBS. Events in freq mode should
> not enable large PEBS.
> I think that could be a problem if some events try to enable large PEBS,
> while others not.
>
You only enable large PEBS if 100% of the events use fixed periods,
either via -c period
or because they all use individual period=p. The --no-period could
also be used to remove
the period for measurements where the period is not needed.

> Thanks,
> Kan
>
>