Messages in this thread Patch in this message |  | | Date | Tue, 19 Sep 2023 08:59:10 +0300 | Subject | Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event | From | Adrian Hunter <> |
| |
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> >>> --- >>> 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@intel.com> >>
|  |