Messages in this thread |  | | From | Mingwei Zhang <> | Date | Sat, 16 Sep 2023 17:45:44 -0700 | Subject | Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event |
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On Fri, Sep 15, 2023 at 9:10 PM Ian Rogers <irogers@google.com> 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"); > -- > 2.42.0.459.ge4e396fd5e-goog >
Thank you very much for the change. I have one quick question about the PMU unthrottling logic. When I am looking into the function perf_adjust_freq_unthr_context(), I see the loop with PMU stop and start in each iteration. Is there a good way to avoid this PMU reset operation while quickly figuring out the event in frequency mode?
Thanks. -Mingwei
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