Messages in this thread | | | From | Namhyung Kim <> | Date | Fri, 29 Sep 2023 23:06:48 -0700 | Subject | Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event |
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Hello,
On Mon, Sep 25, 2023 at 10:37 AM Stephane Eranian <eranian@google.com> wrote: > > On Sun, Sep 24, 2023 at 8:35 PM Yang Jihong <yangjihong1@huawei.com> wrote: > > > > Hello, > > > > On 2023/9/22 23:05, Ian Rogers wrote: > > > 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.
It's in the perf-tools-next.
> > >>> > > >>> > > >>>>>>> --- > > >>>>>>> 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.
Yeah I meant to keep the initialization and to prevent updating it for -F later. I can add a comment in this code like
/* * The kernel counts events with freq=1 separately to update * the sample frequency in the timer tick handler. Do not * confuse the kernel since the dummy event never fires and * no need to update the frequency. */ if (evsel__is_dummy_event(evsel)) attr->freq = 0;
ok?
> > > > > evsel__apply_config_terms() also sets freq. For example: > > > > # perf record -vv -e dummy/freq=100/ true > > This example is NOT relevant to the issue. > Of course, if you want to explicitly use the dummy event with > frequency mode, you should be allowed to do so.
Agreed.
> The problem we are solving here is different. We are preventing perf > record internal use of the dummy event > from using frequency mode for no good reason as any frequency mode > event adds additional overhead. > The perf record internal dummy event is used for one goal: to capture > MMAP records to avoid a race condition > between synthesize phase and processes being created and not captured > by synthesize. In that mode, it acts > as an aggregator of all MMAP records during the entire run of the > tool. This does not require any frequency mode. > > > > <SNIP> > > ------------------------------------------------------------ > > perf_event_attr: > > type 1 (PERF_TYPE_SOFTWARE) > > size 136 > > config 0x9 (PERF_COUNT_SW_DUMMY) > > { sample_period, sample_freq } 100 > > sample_type IP|TID|TIME|PERIOD > > read_format ID|LOST > > disabled 1 > > inherit 1 > > mmap 1 > > comm 1 > > freq 1 > > enable_on_exec 1 > > task 1 > > sample_id_all 1 > > exclude_guest 1 > > mmap2 1 > > comm_exec 1 > > ksymbol 1 > > bpf_event 1 > > ------------------------------------------------------------ > > <SNIP> > > > > Therefore, do we need to perform special processing on dummy events in > > evsel__apply_config_terms?
As Stephane said, I don't worry about the case as it's explicitly requested by the user.
Thanks, Namhyung
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