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

[Ian Rogers]

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.
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>
>