Messages in this thread Patch in this message |  | | From | Namhyung Kim <> | Date | Thu, 21 Sep 2023 12:26:55 -0700 | Subject | Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event |
| |
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.
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