Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event

[Adrian Hunter <adrian.hunter@intel.com>]

On 21/09/23 22:26, Namhyung Kim wrote:
> Hi,
> 
> On Mon, Sep 18, 2023 at 11:00 PM Adrian Hunter <adrian.hunter@intel.com> wrote:
>>
>> On 19/09/23 00:48, Ian Rogers wrote:
>>> On Mon, Sep 18, 2023 at 1:14 AM Adrian Hunter <adrian.hunter@intel.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@google.com>
>>>>> Signed-off-by: Ian Rogers <irogers@google.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.