Re: [PATCH net-next 1/4] bpf: add helper bpf_perf_read_counter_time for perf event array map

On 9/1/17 9:53 AM, Yonghong Song wrote:

Hardware pmu counters are limited resources. When there are more
pmu based perf events opened than available counters, kernel will
multiplex these events so each event gets certain percentage
(but not 100%) of the pmu time. In case that multiplexing happens,
the number of samples or counter value will not reflect the
case compared to no multiplexing. This makes comparison between
different runs difficult.

Typically, the number of samples or counter value should be
normalized before comparing to other experiments. The typical
normalization is done like:
  normalized_num_samples = num_samples * time_enabled / time_running
  normalized_counter_value = counter_value * time_enabled / time_running
where time_enabled is the time enabled for event and time_running is
the time running for event since last normalization.

This patch adds helper bpf_perf_read_counter_time for kprobed based perf
event array map, to read perf counter and enabled/running time.
The enabled/running time is accumulated since the perf event open.
To achieve scaling factor between two bpf invocations, users
can can use cpu_id as the key (which is typical for perf array usage model)
to remember the previous value and do the calculation inside the
bpf program.

Signed-off-by: Yonghong Song <redacted>

...

+BPF_CALL_4(bpf_perf_read_counter_time, struct bpf_map *, map, u64, flags,
+	struct bpf_perf_counter_time *, buf, u32, size)
+{
+	struct perf_event *pe;
+	u64 now;
+	int err;
+
+	if (unlikely(size != sizeof(struct bpf_perf_counter_time)))
+		return -EINVAL;
+	err = get_map_perf_counter(map, flags, &buf->counter, &pe);
+	if (err)
+		return err;
+
+	calc_timer_values(pe, &now, &buf->time.enabled, &buf->time.running);
+	return 0;
+}

Peter,
I believe we're doing it correctly above.
It's a copy paste of the same logic as in total_time_enabled/running.
We cannot expose total_time_enabled/running to bpf, since they are
different counters. The above two are specific to bpf usage.
See commit log.

for the whole set:
Acked-by: Alexei Starovoitov <ast@kernel.org>