Messages in this thread | | | From | Ian Rogers <> | Date | Fri, 22 Sep 2023 08:05:21 -0700 | Subject | Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event |
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On Thu, Sep 21, 2023 at 10:36 PM Adrian Hunter <adrian.hunter@intel.com> wrote: > > On 21/09/23 22:26, Namhyung Kim wrote: > > 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. > > I thought there might be corner cases where it made a difference, > but I can't find any, so that should do. >
It seemed more intention revealing to do it at creation/initialization than on a later not obviously executed code path - I'm thinking of future me trying to understand the code. My priority is the clearing of the flag, so I'm easy.
Thanks, Ian
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