[PATCH v1] perf evlist: Avoid frequency mode for the dummy event

[Ian Rogers]

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@xxxxxxxxxx>
Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
---
 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