Add support stackprof in GitLab profiler

Stackprof runs faster and makes it easy to generate a quick profile
and flamegraph for a given endpoint. This commit adds support for
Stackprof in `Gitlab::Profiler.profile` via the `sampling_mode: true`
parameter.

By default, the sampling data will be written to a temporary file, but
`profiler_options` can be used to specify the destination and other
options.

Changelog: added

doc/development/profiling.md

@@ -91,6 +91,73 @@ printer = RubyProf::CallStackPrinter.new(result)
 printer.print(File.open('/tmp/profile.html', 'w'))
 ```
 
+### Stackprof support
+
+By default, `Gitlab::Profiler.profile` uses a tracing profiler called [`ruby-prof`](https://ruby-prof.github.io/). However, sampling profilers
+[run faster and use less memory](https://jvns.ca/blog/2017/12/17/how-do-ruby---python-profilers-work-/), so they might be preferred.
+
+You can switch to [Stackprof](https://github.com/tmm1/stackprof) (a sampling profiler) to generate a profile by passing `sampling_mode: true`.
+Pass in a `profiler_options` hash to configure the output file (`out`) of the sampling data. For example:
+
+```ruby
+Gitlab::Profiler.profile('/gitlab-org/gitlab-test', user: User.first, sampling_mode: true, profiler_options: { out: 'tmp/profile.dump' })
+```
+
+You can get a summary of where time was spent by running Stackprof against the sampling data. For example:
+
+```shell
+stackprof tmp/profile.dump
+```
+
+Example sampling data:
+
+```plaintext
+==================================
+  Mode: wall(1000)
+  Samples: 8745 (6.92% miss rate)
+  GC: 1399 (16.00%)
+==================================
+     TOTAL    (pct)     SAMPLES    (pct)     FRAME
+      1022  (11.7%)        1022  (11.7%)     Sprockets::PathUtils#stat
+       957  (10.9%)         957  (10.9%)     (marking)
+       493   (5.6%)         493   (5.6%)     Sprockets::PathUtils#entries
+       576   (6.6%)         471   (5.4%)     Mustermann::AST::Translator#decorator_for
+       439   (5.0%)         439   (5.0%)     (sweeping)
+       630   (7.2%)         241   (2.8%)     Sprockets::Cache::FileStore#get
+       208   (2.4%)         208   (2.4%)     ActiveSupport::FileUpdateChecker#watched
+       206   (2.4%)         206   (2.4%)     Digest::Instance#file
+       544   (6.2%)         176   (2.0%)     Sprockets::Cache::FileStore#safe_open
+       176   (2.0%)         176   (2.0%)     ActiveSupport::FileUpdateChecker#max_mtime
+       268   (3.1%)         147   (1.7%)     ActiveRecord::ConnectionAdapters::PostgreSQLAdapter#exec_no_cache
+       140   (1.6%)         140   (1.6%)     ActiveSupport::BacktraceCleaner#add_gem_filter
+       116   (1.3%)         116   (1.3%)     Bootsnap::CompileCache::ISeq.storage_to_output
+       160   (1.8%)         113   (1.3%)     Gem::Version#<=>
+       109   (1.2%)         109   (1.2%)     block in <main>
+       108   (1.2%)         108   (1.2%)     Gem::Version.new
+       131   (1.5%)         105   (1.2%)     Sprockets::EncodingUtils#unmarshaled_deflated
+      1166  (13.3%)          82   (0.9%)     Mustermann::RegexpBased#initialize
+        82   (0.9%)          78   (0.9%)     FileUtils.touch
+        72   (0.8%)          72   (0.8%)     Sprockets::Manifest.compile_match_filter
+        71   (0.8%)          70   (0.8%)     Grape::Router#compile!
+        91   (1.0%)          65   (0.7%)     ActiveRecord::ConnectionAdapters::PostgreSQL::DatabaseStatements#query
+        93   (1.1%)          64   (0.7%)     ActionDispatch::Journey::Path::Pattern::AnchoredRegexp#accept
+        59   (0.7%)          59   (0.7%)     Mustermann::AST::Translator.dispatch_table
+        62   (0.7%)          59   (0.7%)     Rails::BacktraceCleaner#initialize
+      2492  (28.5%)          49   (0.6%)     Sprockets::PathUtils#stat_directory
+       242   (2.8%)          49   (0.6%)     Gitlab::Instrumentation::RedisBase.add_call_details
+        47   (0.5%)          47   (0.5%)     URI::RFC2396_Parser#escape
+        46   (0.5%)          46   (0.5%)     #<Class:0x00000001090c2e70>#__setobj__
+        44   (0.5%)          44   (0.5%)     Sprockets::Base#normalize_logical_path
+```
+
+You can also generate flamegraphs:
+
+```shell
+stackprof --d3-flamegraph tmp/profile.dump > flamegraph.html
+```
+
+See [the Stackprof documentation](https://github.com/tmm1/stackprof) for more details.
+
 ## Speedscope flamegraphs
 
 You can generate a flamegraph for a particular URL by selecting a flamegraph sampling mode button in the performance bar or by adding the `performance_bar=flamegraph` parameter to the request.

lib/gitlab/profiler.rb

@@ -28,7 +28,7 @@ module Gitlab
     ].freeze
 
     # Takes a URL to profile (can be a fully-qualified URL, or an absolute path)
-    # and returns the ruby-prof profile result. Formatting that result is the
+    # and returns the profiler result. Formatting that result is the
     # caller's responsibility. Requests are GET requests unless post_data is
     # passed.
     #
@@ -43,7 +43,13 @@ module Gitlab
     #
     # - private_token: instead of providing a user instance, the token can be
     #   given as a string. Takes precedence over the user option.
-    def self.profile(url, logger: nil, post_data: nil, user: nil, private_token: nil)
+    #
+    # - sampling_mode: When true, uses a sampling profiler (StackProf) instead of a tracing profiler (RubyProf).
+    #
+    # - profiler_options: A keyword Hash of arguments passed to the profiler. Defaults by profiler type:
+    #     RubyProf - {}
+    #     StackProf - { mode: :wall, out: <some temporary file>, interval: 1000, raw: true }
+    def self.profile(url, logger: nil, post_data: nil, user: nil, private_token: nil, sampling_mode: false, profiler_options: {})
       app = ActionDispatch::Integration::Session.new(Rails.application)
       verb = :get
       headers = {}
@@ -75,7 +81,9 @@ module Gitlab
 
         with_custom_logger(logger) do
           with_user(user) do
-            RubyProf.profile { app.public_send(verb, url, params: post_data, headers: headers) } # rubocop:disable GitlabSecurity/PublicSend
+            with_profiler(sampling_mode, profiler_options) do
+              app.public_send(verb, url, params: post_data, headers: headers) # rubocop:disable GitlabSecurity/PublicSend
+            end
           end
         end
       end
@@ -172,5 +180,16 @@ module Gitlab
 
       RubyProf::FlatPrinter.new(result).print($stdout, default_options.merge(options))
     end
+
+    def self.with_profiler(sampling_mode, profiler_options)
+      if sampling_mode
+        require 'stackprof'
+        args = { mode: :wall, interval: 1000, raw: true }.merge!(profiler_options)
+        args[:out] ||= ::Tempfile.new(["profile-#{Time.now.to_i}-", ".dump"]).path
+        ::StackProf.run(**args) { yield }
+      else
+        RubyProf.profile(**profiler_options) { yield }
+      end
+    end
   end
 end

spec/lib/gitlab/profiler_spec.rb

@@ -58,6 +58,30 @@ RSpec.describe Gitlab::Profiler do
 
       described_class.profile('/', user: user, private_token: private_token)
     end
+
+    context 'with sampling profiler' do
+      it 'generates sampling data' do
+        user = double(:user)
+        temp_data = Tempfile.new
+
+        expect(described_class).to receive(:with_user).with(user).and_call_original
+        described_class.profile('/', user: user, sampling_mode: true, profiler_options: { out: temp_data.path })
+
+        expect(File.stat(temp_data).size).to be > 0
+        File.unlink(temp_data)
+      end
+
+      it 'saves sampling data with a randomly-generated filename' do
+        user = double(:user)
+
+        expect(described_class).to receive(:with_user).with(user).and_call_original
+        result = described_class.profile('/', user: user, sampling_mode: true)
+
+        expect(result).to be_a(File)
+        expect(File.stat(result.path).size).to be > 0
+        File.unlink(result.path)
+      end
+    end
   end
 
   describe '.create_custom_logger' do