6.1-stable review patch. If anyone has any objections, please let me know.
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From: Ian Rogers irogers@google.com
[ Upstream commit f9cdeb58a9cf46c09b56f5f661ea8da24b6458c3 ]
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 Acked-by: Adrian Hunter adrian.hunter@intel.com Cc: Yang Jihong yangjihong1@huawei.com Cc: Kan Liang kan.liang@linux.intel.com Link: https://lore.kernel.org/r/20230916035640.1074422-1-irogers@google.com Signed-off-by: Namhyung Kim namhyung@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- tools/perf/util/evlist.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
--- a/tools/perf/util/evlist.c +++ b/tools/perf/util/evlist.c @@ -252,6 +252,9 @@ static struct evsel *evlist__dummy_event .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); @@ -278,8 +281,6 @@ struct evsel *evlist__add_aux_dummy(stru 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");