Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event
From: Namhyung Kim
Date: Thu Sep 21 2023 - 15:27:12 EST
Hi,
On Mon, Sep 18, 2023 at 11:00 PM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>
> 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>
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.