Do not adjust pprof sample time by dividing by CPU count. (#1348)

Summary: Previously we divided the sample time duration by CPU count,
i.e. such that total time would match wall clock time. Here we stop that
practice because it is more correct to report the time spent on core
(even if the total time reported is longer than wall clock time).

Type of change: /kind bug fix

Test Plan: Tests will be merged with #1336.

---------

Signed-off-by: Pete Stevenson <jps@pixielabs.ai>

Author: Pete Stevenson

src/shared/pprof/pprof.cc

@@ -24,8 +24,7 @@
 namespace px {
 namespace shared {
 
-PProfProfile CreatePProfProfile(const uint32_t num_cpus, const uint32_t period_ms,
-                                const absl::flat_hash_map<std::string, uint64_t>& histo) {
+PProfProfile CreatePProfProfile(const uint32_t period_ms, const PProfHisto& histo) {
   // Info on the pprof proto format:
   // https://github.com/google/pprof/blob/main/proto/profile.proto
 
@@ -68,7 +67,7 @@ PProfProfile CreatePProfProfile(const uint32_t num_cpus, const uint32_t period_m
     auto sample = profile.add_sample();
 
     // That sample will record its count and time in nanos.
-    const uint64_t nanos = count * period_ns / num_cpus;
+    const uint64_t nanos = count * period_ns;
     sample->add_value(count);
     sample->add_value(nanos);
 

src/shared/pprof/pprof.h

@@ -28,10 +28,10 @@ namespace px {
 namespace shared {
 
 using PProfProfile = ::perftools::profiles::Profile;
+using PProfHisto = absl::flat_hash_map<std::string, uint64_t>;
 
 // https://github.com/google/pprof/blob/main/proto/profile.proto
-PProfProfile CreatePProfProfile(const uint32_t num_cpus, const uint32_t period_ms,
-                                const absl::flat_hash_map<std::string, uint64_t>& histo);
+PProfProfile CreatePProfProfile(const uint32_t period_ms, const PProfHisto& histo);
 
 }  // namespace shared
 }  // namespace px

src/stirling/binaries/stirling_profiler.cc

@@ -44,9 +44,8 @@ class Profiler : public UnitConnector<PerfProfileConnector> {
     PX_RETURN_IF_ERROR(BuildHistogram());
 
     // Create the pprof profile.
-    const uint32_t num_cpus = get_nprocs_conf();
     const uint32_t period_ms = FLAGS_stirling_profiler_stack_trace_sample_period_ms;
-    const auto pprof_pb = px::shared::CreatePProfProfile(num_cpus, period_ms, histo_);
+    const auto pprof_pb = px::shared::CreatePProfProfile(period_ms, histo_);
 
     // Write the pprof profile to disk.
     std::fstream outfile(FLAGS_pprof_pb_file, std::ios::out | std::ios::trunc | std::ios::binary);