Add atomic sidekiq scheduler

This retrieves jobs from Sidekiq scheduled sets using a Lua script. This
allows multiple processes to process the queue efficiently.

config/feature_flags/development/atomic_sidekiq_scheduler.yml

+---
+name: atomic_sidekiq_scheduler
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/72380
+rollout_issue_url:
+milestone: '14.5'
+type: development
+group: group::project management
+default_enabled: false

lib/gitlab/runtime.rb

@@ -84,9 +84,12 @@ module Gitlab
         if puma? && Puma.respond_to?(:cli_config)
           threads += Puma.cli_config.options[:max_threads]
         elsif sidekiq?
-          # An extra thread for the poller in Sidekiq Cron:
+          # 2 extra threads for the pollers in Sidekiq and Sidekiq Cron:
           # https://github.com/ondrejbartas/sidekiq-cron#under-the-hood
-          threads += Sidekiq.options[:concurrency] + 1
+          #
+          # These threads execute Sidekiq client middleware when jobs
+          # are enqueued and those can access DB / Redis.
+          threads += Sidekiq.options[:concurrency] + 2
         end
 
         if action_cable?

lib/gitlab/sidekiq_enq.rb

 # frozen_string_literal: true
 
-# This is a copy of https://github.com/mperham/sidekiq/blob/32c55e31659a1e6bd42f98334cca5eef2863de8d/lib/sidekiq/scheduled.rb#L11-L34
-#
-# It effectively reverts
-# https://github.com/mperham/sidekiq/commit/9b75467b33759888753191413eddbc15c37a219e
-# because we observe that the extra ZREMs caused by this change can lead to high
-# CPU usage on Redis at peak times:
-# https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1179
-#
 module Gitlab
   class SidekiqEnq
+    LUA_ZPOPBYSCORE = <<~EOS
+      local key, now = KEYS[1], ARGV[1]
+      local jobs = redis.call("zrangebyscore", key, "-inf", now, "limit", 0, 1)
+      if jobs[1] then
+        redis.call("zrem", key, jobs[1])
+        return jobs[1]
+      end
+    EOS
+
+    LUA_ZPOPBYSCORE_SHA = Digest::SHA1.hexdigest(LUA_ZPOPBYSCORE)
+
     def enqueue_jobs(now = Time.now.to_f.to_s, sorted_sets = Sidekiq::Scheduled::SETS)
+      if Feature.enabled?(:atomic_sidekiq_scheduler, default_enabled: :yaml)
+        atomic_find_jobs_and_enqueue(now, sorted_sets)
+      else
+        find_jobs_and_enqueue(now, sorted_sets)
+      end
+    end
+
+    private
+
+    # This is a copy of https://github.com/mperham/sidekiq/blob/32c55e31659a1e6bd42f98334cca5eef2863de8d/lib/sidekiq/scheduled.rb#L11-L34
+    #
+    # It effectively reverts
+    # https://github.com/mperham/sidekiq/commit/9b75467b33759888753191413eddbc15c37a219e
+    # because we observe that the extra ZREMs caused by this change can lead to high
+    # CPU usage on Redis at peak times:
+    # https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1179
+    #
+    def find_jobs_and_enqueue(now, sorted_sets)
       # A job's "score" in Redis is the time at which it should be processed.
       # Just check Redis for the set of jobs with a timestamp before now.
       Sidekiq.redis do |conn|
@@ -24,8 +45,7 @@ module Gitlab
           # We need to go through the list one at a time to reduce the risk of something
           # going wrong between the time jobs are popped from the scheduled queue and when
           # they are pushed onto a work queue and losing the jobs.
-          while (job = conn.zrangebyscore(sorted_set, "-inf", now, limit: [0, 1]).first)
-
+          while job = conn.zrangebyscore(sorted_set, "-inf", now, limit: [0, 1]).first
             # Pop item off the queue and add it to the work queue. If the job can't be popped from
             # the queue, it's because another process already popped it so we can move on to the
             # next one.
@@ -47,5 +67,38 @@ module Gitlab
         end
       end
     end
+
+    def atomic_find_jobs_and_enqueue(now, sorted_sets)
+      Sidekiq.redis do |conn|
+        sorted_sets.each do |sorted_set|
+          start_time = ::Gitlab::Metrics::System.monotonic_time
+          jobs = 0
+
+          Sidekiq.logger.info(message: 'Atomically enqueuing scheduled jobs', status: 'start', sorted_set: sorted_set)
+
+          while job = redis_eval_lua(conn, LUA_ZPOPBYSCORE, LUA_ZPOPBYSCORE_SHA, keys: [sorted_set], argv: [now])
+            jobs += 1
+            Sidekiq::Client.push(Sidekiq.load_json(job))
+          end
+
+          end_time = ::Gitlab::Metrics::System.monotonic_time
+          Sidekiq.logger.info(message: 'Atomically enqueuing scheduled jobs',
+                              status: 'done',
+                              sorted_set: sorted_set,
+                              jobs_count: jobs,
+                              duration_s: end_time - start_time)
+        end
+      end
+    end
+
+    def redis_eval_lua(conn, script, sha, keys: nil, argv: nil)
+      conn.evalsha(sha, keys: keys, argv: argv)
+    rescue ::Redis::CommandError => e
+      if e.message.start_with?('NOSCRIPT')
+        conn.eval(script, keys: keys, argv: argv)
+      else
+        raise
+      end
+    end
   end
 end

spec/lib/gitlab/runtime_spec.rb

@@ -108,7 +108,7 @@ RSpec.describe Gitlab::Runtime do
       allow(sidekiq_type).to receive(:options).and_return(concurrency: 2)
     end
 
-    it_behaves_like "valid runtime", :sidekiq, 4
+    it_behaves_like "valid runtime", :sidekiq, 5
   end
 
   context "console" do

spec/lib/gitlab/sidekiq_enq_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::SidekiqEnq, :clean_gitlab_redis_queues do
+  let(:retry_set) { Sidekiq::Scheduled::SETS.first }
+  let(:schedule_set) { Sidekiq::Scheduled::SETS.last }
+
+  around do |example|
+    freeze_time { example.run }
+  end
+
+  shared_examples 'finds jobs that are due and enqueues them' do
+    before do
+      Sidekiq.redis do |redis|
+        redis.zadd(retry_set, (Time.current - 1.day).to_f.to_s, '{"jid": 1}')
+        redis.zadd(retry_set, Time.current.to_f.to_s, '{"jid": 2}')
+        redis.zadd(retry_set, (Time.current + 1.day).to_f.to_s, '{"jid": 3}')
+
+        redis.zadd(schedule_set, (Time.current - 1.day).to_f.to_s, '{"jid": 4}')
+        redis.zadd(schedule_set, Time.current.to_f.to_s, '{"jid": 5}')
+        redis.zadd(schedule_set, (Time.current + 1.day).to_f.to_s, '{"jid": 6}')
+      end
+    end
+
+    it 'enqueues jobs that are due' do
+      expect(Sidekiq::Client).to receive(:push).with({ 'jid' => 1 })
+      expect(Sidekiq::Client).to receive(:push).with({ 'jid' => 2 })
+      expect(Sidekiq::Client).to receive(:push).with({ 'jid' => 4 })
+      expect(Sidekiq::Client).to receive(:push).with({ 'jid' => 5 })
+
+      Gitlab::SidekiqEnq.new.enqueue_jobs
+
+      Sidekiq.redis do |redis|
+        expect(redis.zscan_each(retry_set).map(&:first)).to contain_exactly('{"jid": 3}')
+        expect(redis.zscan_each(schedule_set).map(&:first)).to contain_exactly('{"jid": 6}')
+      end
+    end
+  end
+
+  context 'when atomic_sidekiq_scheduler is disabled' do
+    before do
+      stub_feature_flags(atomic_sidekiq_scheduler: false)
+    end
+
+    it_behaves_like 'finds jobs that are due and enqueues them'
+
+    context 'when ZRANGEBYSCORE returns a job that is already removed by another process' do
+      before do
+        Sidekiq.redis do |redis|
+          redis.zadd(schedule_set, Time.current.to_f.to_s, '{"jid": 1}')
+
+          allow(redis).to receive(:zrangebyscore).and_wrap_original do |m, *args, **kwargs|
+            m.call(*args, **kwargs).tap do |jobs|
+              redis.zrem(schedule_set, jobs.first) if args[0] == schedule_set && jobs.first
+            end
+          end
+        end
+      end
+
+      it 'calls ZREM but does not enqueue the job' do
+        Sidekiq.redis do |redis|
+          expect(redis).to receive(:zrem).with(schedule_set, '{"jid": 1}').twice.and_call_original
+        end
+        expect(Sidekiq::Client).not_to receive(:push)
+
+        Gitlab::SidekiqEnq.new.enqueue_jobs
+      end
+    end
+  end
+
+  context 'when atomic_sidekiq_scheduler is enabled' do
+    before do
+      stub_feature_flags(atomic_sidekiq_scheduler: true)
+    end
+
+    context 'when Lua script is not yet loaded' do
+      before do
+        Gitlab::Redis::Queues.with { |redis| redis.script(:flush) }
+      end
+
+      it_behaves_like 'finds jobs that are due and enqueues them'
+    end
+
+    context 'when Lua script is already loaded' do
+      before do
+        Gitlab::SidekiqEnq.new.enqueue_jobs
+      end
+
+      it_behaves_like 'finds jobs that are due and enqueues them'
+    end
+  end
+end