Messages in this thread Patch in this message |  | | From | Ian Rogers <> | Date | Mon, 18 Sep 2023 14:48:55 -0700 | Subject | Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event |
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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. 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> >
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