Date:   Mon, 25 Sep 2023 11:35:21 +0800
From:   Yang Jihong <yangjihong1@...wei.com>
To:     Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>
CC:     Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        <linux-perf-users@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event

Hello,

On 2023/9/22 23:05, Ian Rogers wrote:
> On Thu, Sep 21, 2023 at 10:36 PM Adrian Hunter <adrian.hunter@...el.com> wrote:
>>
>> On 21/09/23 22:26, Namhyung Kim wrote:
>>> Hi,
>>>
>>> On Mon, Sep 18, 2023 at 11:00 PM Adrian Hunter <adrian.hunter@...el.com> wrote:
>>>>
>>>> On 19/09/23 00:48, Ian Rogers wrote:
>>>>> On Mon, Sep 18, 2023 at 1:14 AM Adrian Hunter <adrian.hunter@...el.com> wrote:
>>>>>>
>>>>>> On 16/09/23 07:09, Ian Rogers wrote:
>>>>>>> Dummy events are created with an attribute where the period and freq
>>>>>>> are zero. evsel__config will then see the uninitialized values and
>>>>>>> initialize them in evsel__default_freq_period. As fequency mode is
>>>>>>> used by default the dummy event would be set to use frequency
>>>>>>> mode. However, this has no effect on the dummy event but does cause
>>>>>>> unnecessary timers/interrupts. Avoid this overhead by setting the
>>>>>>> period to 1 for dummy events.
>>>>>>>
>>>>>>> evlist__add_aux_dummy calls evlist__add_dummy then sets freq=0 and
>>>>>>> period=1. This isn't necessary after this change and so the setting is
>>>>>>> removed.
>>>>>>>
>>>>>>>  From Stephane:
>>>>>>>
>>>>>>> The dummy event is not counting anything. It is used to collect mmap
>>>>>>> records and avoid a race condition during the synthesize mmap phase of
>>>>>>> perf record. As such, it should not cause any overhead during active
>>>>>>> profiling. Yet, it did. Because of a bug the dummy event was
>>>>>>> programmed as a sampling event in frequency mode. Events in that mode
>>>>>>> incur more kernel overheads because on timer tick, the kernel has to
>>>>>>> look at the number of samples for each event and potentially adjust
>>>>>>> the sampling period to achieve the desired frequency. The dummy event
>>>>>>> was therefore adding a frequency event to task and ctx contexts we may
>>>>>>> otherwise not have any, e.g., perf record -a -e
>>>>>>> cpu/event=0x3c,period=10000000/. On each timer tick the
>>>>>>> perf_adjust_freq_unthr_context() is invoked and if ctx->nr_freq is
>>>>>>> non-zero, then the kernel will loop over ALL the events of the context
>>>>>>> looking for frequency mode ones. In doing, so it locks the context,
>>>>>>> and enable/disable the PMU of each hw event. If all the events of the
>>>>>>> context are in period mode, the kernel will have to traverse the list for
>>>>>>> nothing incurring overhead. The overhead is multiplied by a very large
>>>>>>> factor when this happens in a guest kernel. There is no need for the
>>>>>>> dummy event to be in frequency mode, it does not count anything and
>>>>>>> therefore should not cause extra overhead for no reason.
>>>>>>>
>>>>>>> Fixes: 5bae0250237f ("perf evlist: Introduce perf_evlist__new_dummy constructor")
>>>>>>> Reported-by: Stephane Eranian <eranian@...gle.com>
>>>>>>> Signed-off-by: Ian Rogers <irogers@...gle.com>
>>>
>>> I'll take the original patch first.
>>>
>>>
>>>>>>> ---
>>>>>>>   tools/perf/util/evlist.c | 5 +++--
>>>>>>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>>>>>>
>>>>>>> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
>>>>>>> index 25c3ebe2c2f5..e36da58522ef 100644
>>>>>>> --- a/tools/perf/util/evlist.c
>>>>>>> +++ b/tools/perf/util/evlist.c
>>>>>>> @@ -251,6 +251,9 @@ static struct evsel *evlist__dummy_event(struct evlist *evlist)
>>>>>>>                .type   = PERF_TYPE_SOFTWARE,
>>>>>>>                .config = PERF_COUNT_SW_DUMMY,
>>>>>>>                .size   = sizeof(attr), /* to capture ABI version */
>>>>>>> +             /* Avoid frequency mode for dummy events to avoid associated timers. */
>>>>>>> +             .freq = 0,
>>>>>>> +             .sample_period = 1,
>>>>>>>        };
>>>>>>>
>>>>>>>        return evsel__new_idx(&attr, evlist->core.nr_entries);
>>>>>>> @@ -277,8 +280,6 @@ struct evsel *evlist__add_aux_dummy(struct evlist *evlist, bool system_wide)
>>>>>>>        evsel->core.attr.exclude_kernel = 1;
>>>>>>>        evsel->core.attr.exclude_guest = 1;
>>>>>>>        evsel->core.attr.exclude_hv = 1;
>>>>>>> -     evsel->core.attr.freq = 0;
>>>>>>> -     evsel->core.attr.sample_period = 1;
>>>>>>>        evsel->core.system_wide = system_wide;
>>>>>>>        evsel->no_aux_samples = true;
>>>>>>>        evsel->name = strdup("dummy:u");
>>>>>>
>>>>>> Note that evsel__config() will put it back to freq if -F is used.
>>>>>
>>>>> Right, I was looking for a minimal fix in part for the sake of back
>>>>> porting. For the -F we could do:
>>>>>
>>>>> ```
>>>>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
>>>>> index d5363d23f5d3..806185a39e17 100644
>>>>> --- a/tools/perf/util/evsel.c
>>>>> +++ b/tools/perf/util/evsel.c
>>>>> @@ -1083,11 +1083,15 @@ void __weak arch__post_evsel_config(struct
>>>>> evsel *evsel __maybe_unused,
>>>>> static void evsel__set_default_freq_period(struct record_opts *opts,
>>>>>                                            struct perf_event_attr *attr)
>>>>> {
>>>>> -       if (opts->freq) {
>>>>> +       bool is_dummy = attr->type == PERF_TYPE_SOFTWARE &&
>>>>> +               attr->config == PERF_COUNT_SW_DUMMY;
>>>>> +
>>>>> +       if (opts->freq && !is_dummy) {
>>>>>                 attr->freq = 1;
>>>>>                 attr->sample_freq = opts->freq;
>>>>>         } else {
>>>>> -               attr->sample_period = opts->default_interval;
>>>>> +               attr->freq = 0;
>>>>> +               attr->sample_period = is_dummy ? 1 : opts->default_interval;
>>>>>         }
>>>>> }
>>>>> ```
>>>>>
>>>>> But this felt like it could potentially have other side-effects.
>>>>
>>>> Perhaps leave it alone, if the period has already been defined:
>>>>
>>>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
>>>> index d5363d23f5d3..ad3e12f5ec88 100644
>>>> --- a/tools/perf/util/evsel.c
>>>> +++ b/tools/perf/util/evsel.c
>>>> @@ -1166,7 +1166,8 @@ void evsel__config(struct evsel *evsel, struct record_opts *opts,
>>>>          if ((evsel->is_libpfm_event && !attr->sample_period) ||
>>>>              (!evsel->is_libpfm_event && (!attr->sample_period ||
>>>>                                           opts->user_freq != UINT_MAX ||
>>>> -                                        opts->user_interval != ULLONG_MAX)))
>>>> +                                        opts->user_interval != ULLONG_MAX) &&
>>>> +            !(is_dummy && attr->sample_period)))
>>>>                  evsel__set_default_freq_period(opts, attr);
>>>>
>>>>          /*
>>>
>>> Or simply like this?
>>>
>>>
>>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
>>> index d5363d23f5d3..6ce832ce62f1 100644
>>> --- a/tools/perf/util/evsel.c
>>> +++ b/tools/perf/util/evsel.c
>>> @@ -1169,6 +1169,9 @@ void evsel__config(struct evsel *evsel, struct
>>> record_opts *opts,
>>>                                           opts->user_interval != ULLONG_MAX)))
>>>                  evsel__set_default_freq_period(opts, attr);
>>>
>>> +       if (evsel__is_dummy_event(evsel))
>>> +               attr->freq = 0;
>>> +
>>>          /*
>>>           * If attr->freq was set (here or earlier), ask for period
>>>           * to be sampled.
>>
>> I thought there might be corner cases where it made a difference,
>> but I can't find any, so that should do.
>>
> 
> It seemed more intention revealing to do it at creation/initialization
> than on a later not obviously executed code path - I'm thinking of
> future me trying to understand the code. My priority is the clearing
> of the flag, so I'm easy.
> 
evsel__apply_config_terms() also sets freq. For example:

# perf record -vv -e dummy/freq=100/ true
<SNIP>
------------------------------------------------------------
perf_event_attr:
   type                             1 (PERF_TYPE_SOFTWARE)
   size                             136
   config                           0x9 (PERF_COUNT_SW_DUMMY)
   { sample_period, sample_freq }   100
   sample_type                      IP|TID|TIME|PERIOD
   read_format                      ID|LOST
   disabled                         1
   inherit                          1
   mmap                             1
   comm                             1
   freq                             1
   enable_on_exec                   1
   task                             1
   sample_id_all                    1
   exclude_guest                    1
   mmap2                            1
   comm_exec                        1
   ksymbol                          1
   bpf_event                        1
------------------------------------------------------------
<SNIP>

Therefore, do we need to perform special processing on dummy events in 
evsel__apply_config_terms?

Thanks,
Yang

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