Messages in this thread |  | | From | Mingwei Zhang <> | Date | Wed, 20 Sep 2023 22:04:33 -0700 | Subject | Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event |
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On Mon, Sep 18, 2023 at 3:43 PM Ian Rogers <irogers@google.com> wrote: > > On Sat, Sep 16, 2023 at 5:46 PM Mingwei Zhang <mizhang@google.com> wrote: > > > > 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? > > Agreed. I think before the pmu_disable could be avoided for this condition: > ``` > if (event->hw.interrupts != MAX_INTERRUPTS && > (!event->attr.freq || !event->attr.sample_freq)) > continue; > ``` > Fixing up the event stop/start looks harder. >
Right, I think putting the check early before pmu_disable() is already a great optimization. The only concern I initially had was whether event->hw.interrupts can be accessed before we disable the pmu. But after checking this field in other locations, I don't see any problem at all.
Thanks. -Mingwei
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