Call `GC::Profiler.clear` only in one place

Previously, both InfluxSampler and RubySampler were relying on the
`GC::Profiler.total_time` data which is the sum over the list
of captured GC events. Also, both samplers asynchronously called
`GC::Profiler.clear` which led to incorrect metric data because
each sampler has the wrong assumption it is the only object who calls
`GC::Profiler.clear` and thus could rely on the gathered results between
such calls.

We should ensure that `GC::Profiler.total_time` is called only in one
place making it possible to rely on accumulated data between such wipes.

Also, we need to track the amount of profiler reports we lost.

changelogs/unreleased/63571-fix-gc-profiler-data-being-wiped.yml

+---
+title: Fix GC::Profiler metrics fetching
+merge_request: 31331
+author:
+type: fixed

config/initializers/peek.rb

@@ -9,5 +9,10 @@ Peek.into Peek::Views::ActiveRecord
 Peek.into Peek::Views::Gitaly
 Peek.into Peek::Views::RedisDetailed
 Peek.into Peek::Views::Rugged
-Peek.into Peek::Views::GC
+
+# `Peek::Views::GC` is currently disabled in production, as it runs with every request
+# even if PerformanceBar is inactive and clears `GC::Profiler` reports we need for metrics.
+# Check https://gitlab.com/gitlab-org/gitlab-ce/issues/65455
+Peek.into Peek::Views::GC if Rails.env.development?
+
 Peek.into Peek::Views::Tracing if Labkit::Tracing.tracing_url_enabled?

lib/gitlab/metrics/samplers/influx_sampler.rb

@@ -15,19 +15,14 @@ module Gitlab
           @last_step = nil
           @metrics = []
-
-          @last_minor_gc = Delta.new(GC.stat[:minor_gc_count])
-          @last_major_gc = Delta.new(GC.stat[:major_gc_count])
         end
         def sample
           sample_memory_usage
           sample_file_descriptors
-          sample_gc
           flush
         ensure
-          GC::Profiler.clear
           @metrics.clear
         end
@@ -43,23 +38,6 @@ module Gitlab
           add_metric('file_descriptors', value: System.file_descriptor_count)
         end
-        def sample_gc
-          time = GC::Profiler.total_time * 1000.0
-          stats = GC.stat.merge(total_time: time)
-
-          # We want the difference of GC runs compared to the last sample, not the
-          # total amount since the process started.
-          stats[:minor_gc_count] =
-            @last_minor_gc.compared_with(stats[:minor_gc_count])
-
-          stats[:major_gc_count] =
-            @last_major_gc.compared_with(stats[:major_gc_count])
-
-          stats[:count] = stats[:minor_gc_count] + stats[:major_gc_count]
-
-          add_metric('gc_statistics', stats)
-        end
-
         def add_metric(series, values, tags = {})
           prefix = sidekiq? ? 'sidekiq_' : 'rails_'

lib/gitlab/metrics/samplers/ruby_sampler.rb

@@ -6,7 +6,11 @@ module Gitlab
   module Metrics
     module Samplers
       class RubySampler < BaseSampler
+        GC_REPORT_BUCKETS = [0.001, 0.002, 0.005, 0.01, 0.05, 0.1, 0.5].freeze
+
         def initialize(interval)
+          GC::Profiler.clear
+
           metrics[:process_start_time_seconds].set(labels, Time.now.to_i)
           super
@@ -37,7 +41,7 @@ module Gitlab
             process_resident_memory_bytes:  ::Gitlab::Metrics.gauge(with_prefix(:process, :resident_memory_bytes), 'Memory used', labels),
             process_start_time_seconds:     ::Gitlab::Metrics.gauge(with_prefix(:process, :start_time_seconds), 'Process start time seconds'),
             sampler_duration:               ::Gitlab::Metrics.counter(with_prefix(:sampler, :duration_seconds_total), 'Sampler time', labels),
-            total_time:                     ::Gitlab::Metrics.counter(with_prefix(:gc, :duration_seconds_total), 'Total GC time', labels)
+            gc_duration_seconds:            ::Gitlab::Metrics.histogram(with_prefix(:gc, :duration_seconds), 'GC time', labels, GC_REPORT_BUCKETS)
           }
           GC.stat.keys.each do |key|
@@ -57,20 +61,27 @@ module Gitlab
           sample_gc
           metrics[:sampler_duration].increment(labels, System.monotonic_time - start_time)
-        ensure
-          GC::Profiler.clear
         end
         private
         def sample_gc
-          # Collect generic GC stats.
+          # Observe all GC samples
+          sample_gc_reports.each do |report|
+            metrics[:gc_duration_seconds].observe(labels, report[:GC_TIME])
+          end
+
+          # Collect generic GC stats
           GC.stat.each do |key, value|
             metrics[key].set(labels, value)
           end
-          # Collect the GC time since last sample in float seconds.
-          metrics[:total_time].increment(labels, GC::Profiler.total_time)
+        end
+        def sample_gc_reports
+          GC::Profiler.enable
+          GC::Profiler.raw_data
+        ensure
+          GC::Profiler.clear
         end
         def set_memory_usage_metrics

spec/lib/gitlab/metrics/samplers/influx_sampler_spec.rb

@@ -17,18 +17,10 @@ describe Gitlab::Metrics::Samplers::InfluxSampler do
     it 'samples various statistics' do
       expect(sampler).to receive(:sample_memory_usage)
       expect(sampler).to receive(:sample_file_descriptors)
-      expect(sampler).to receive(:sample_gc)
       expect(sampler).to receive(:flush)
       sampler.sample
     end
-
-    it 'clears any GC profiles' do
-      expect(sampler).to receive(:flush)
-      expect(GC::Profiler).to receive(:clear)
-
-      sampler.sample
-    end
   end
   describe '#flush' do
@@ -67,18 +59,6 @@ describe Gitlab::Metrics::Samplers::InfluxSampler do
     end
   end
-  describe '#sample_gc' do
-    it 'adds a metric containing garbage collection statistics' do
-      expect(GC::Profiler).to receive(:total_time).and_return(0.24)
-
-      expect(sampler).to receive(:add_metric)
-        .with(/gc_statistics/, an_instance_of(Hash))
-        .and_call_original
-
-      sampler.sample_gc
-    end
-  end
-
   describe '#add_metric' do
     it 'prefixes the series name for a Rails process' do
       expect(sampler).to receive(:sidekiq?).and_return(false)

spec/lib/gitlab/metrics/samplers/ruby_sampler_spec.rb

@@ -59,17 +59,29 @@ describe Gitlab::Metrics::Samplers::RubySampler do
     end
     it 'clears any GC profiles' do
-      expect(GC::Profiler).to receive(:clear)
+      expect(GC::Profiler).to receive(:clear).at_least(:once)
       sampler.sample
     end
   end
   describe '#sample_gc' do
-    it 'adds a metric containing garbage collection time statistics' do
-      expect(GC::Profiler).to receive(:total_time).and_return(0.24)
-      expect(sampler.metrics[:total_time]).to receive(:increment).with({}, 0.24)
+    let!(:sampler) { described_class.new(5) }
+    let(:gc_reports) { [{ GC_TIME: 0.1 }, { GC_TIME: 0.2 }, { GC_TIME: 0.3 }] }
+
+    it 're-enables GC::Profiler if needed' do
+      expect(GC::Profiler).to receive(:enable)
+
+      sampler.sample
+    end
+
+    it 'observes GC cycles time' do
+      expect(sampler).to receive(:sample_gc_reports).and_return(gc_reports)
+
+      expect(sampler.metrics[:gc_duration_seconds]).to receive(:observe).with({}, 0.1).ordered
+      expect(sampler.metrics[:gc_duration_seconds]).to receive(:observe).with({}, 0.2).ordered
+      expect(sampler.metrics[:gc_duration_seconds]).to receive(:observe).with({}, 0.3).ordered
       sampler.sample
     end