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

[Adrian Hunter]

On 19/09/23 00:48, Ian Rogers wrote:
> On Mon, Sep 18, 2023 at 1:14 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> 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@xxxxxxxxxx>
>>> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
>>> ---
>>>  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);
 
 	/*



> Software events like page faults have a period, but the
> dummy/side-band events appear never to check the period - but I could
> be wrong. We can add the patch above but perhaps without a fixes tag.
> 
> Thanks,
> Ian
> 
>> Nevertheless:
>>
>> Acked-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
>>