refactor(logs): make utilization monitor hot path lock-free

[UTXOnly]

What does this PR do?

Makes the logs TelemetryUtilizationMonitor hot path lock-free, addressing @gh123man's review feedback on #51973.

Targets UTXOnly/backpressure-t4 so it can be reviewed and merged into the backpressure PR.

Background

On main the monitor was lock-free: Start()/Stop() bracket every pipeline operation and run on a single goroutine, so no synchronization was needed. The backpressure PR added a 1 Hz sample() ticker on a separate goroutine, which forced a sync.Mutex onto the shared inUse/idle accumulators — and that mutex now sits on every Start()/Stop(), a CPU-sensitive hot path the reviewer profiled when first introducing these monitors.

Approach

Per the reviewer's suggestion (atomic accumulator + poll a published value):

• Hot path is now lock-free. Start/Stop do only atomic ops: Start publishes the in-use interval start; Stop adds the closed interval to a monotonic cumulativeBusyNanos counter. No EWMA or reporting work on the hot path anymore.
• The sampler is the sole owner of all derived state (avg, history, the saturation machine), so it needs no lock. Each tick it reads effectiveBusy(now) = cumulative + (started ? now − startInUse : 0) and differences it across the window. Because that's a point-in-time read of a monotonic function, a torn read against the hot path only shifts a sliver of time between adjacent windows and self-corrects on the next tick.
• avg is published via atomic.Float64, so the status page / any subscriber polls it without touching the hot path.

The registry (RWMutex) and rolling-history (Mutex) locks are unchanged — they guard sampler-write vs status-read, not the hot path.

Behavior note

The utilization ratio's denominator changes from inUse/(idle+inUse) (settled measured time) to windowBusy/wallClockElapsed. Equivalent in steady state, and arguably more correct for blocked-mid-operation detection.

Describe how you validated your changes

• dda inv test --targets=./comp/logs-library/metrics/ — 23/23 pass
• go test -race on the package — clean (hot path vs sampler vs status read)
• bazel test //comp/logs-library/metrics:metrics_test — pass
• Gazelle regenerated BUILD.bazel for the new go.uber.org/atomic dep

Still TODO before resolving the review thread: SMP load test comparing hot-path CPU before/after, which is the evidence the reviewer asked for.

[datadog-datadog-prod-us1-2]

✨ Fix all issues with BitsAI

⚠️ Warnings

🚦 1 Pipeline job failed

DataDog/datadog-agent | oracle: [21.3.0-xe]     

ℹ️ Info

🎯 Code Coverage (details)
• Patch Coverage: 81.08%
• Overall Coverage: 50.61% (-0.00%)

Useful? React with 👍 / 👎

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: a858055 | Docs | Datadog PR Page | Give us feedback!}

[dd-octo-sts]

Files inventory check summary

File checks results against ancestor 28445870:

Results for datadog-agent_7.81.0~devel.git.644.a858055.pipeline.117843316-1_amd64.deb:

No change detected

[dd-octo-sts]

Static quality checks

✅ Please find below the results from static quality gates
Comparison made with ancestor 2844587
📊 Static Quality Gates Dashboard
🔗 SQG Job

Successful checks

Info

	Quality gate	Change	Size (prev → curr → max)
✅	agent_deb_amd64	+139.22 KiB (0.02% increase, -2.08% of buffer)	751.652 → 751.788 → 758.200
✅	agent_deb_amd64_fips	+123.16 KiB (0.02% increase, -4.98% of buffer)	707.423 → 707.543 → 709.840
✅	agent_heroku_amd64	+60.0 KiB (0.02% increase, -1.19% of buffer)	310.320 → 310.379 → 315.230
✅	agent_rpm_amd64	+139.22 KiB (0.02% increase, -2.08% of buffer)	751.636 → 751.772 → 758.170
✅	agent_rpm_amd64_fips	+123.16 KiB (0.02% increase, -4.94% of buffer)	707.407 → 707.527 → 709.840
✅	agent_rpm_arm64	+111.44 KiB (0.01% increase, -4.15% of buffer)	727.039 → 727.148 → 729.660
✅	agent_rpm_arm64_fips	+119.47 KiB (0.02% increase, -5.07% of buffer)	686.558 → 686.674 → 688.860
✅	agent_suse_amd64	+139.22 KiB (0.02% increase, -2.08% of buffer)	751.636 → 751.772 → 758.170
✅	agent_suse_amd64_fips	+123.16 KiB (0.02% increase, -4.94% of buffer)	707.407 → 707.527 → 709.840
✅	agent_suse_arm64	+111.44 KiB (0.01% increase, -4.15% of buffer)	727.039 → 727.148 → 729.660
✅	agent_suse_arm64_fips	+119.47 KiB (0.02% increase, -5.07% of buffer)	686.558 → 686.674 → 688.860
✅	docker_agent_amd64	+139.22 KiB (0.02% increase, -5.02% of buffer)	811.080 → 811.216 → 813.790
✅	docker_agent_arm64	+111.44 KiB (0.01% increase, -3.09% of buffer)	811.512 → 811.621 → 815.030
✅	docker_agent_jmx_amd64	+139.22 KiB (0.01% increase, -5.38% of buffer)	1002.021 → 1002.157 → 1004.550
✅	docker_agent_jmx_arm64	+111.44 KiB (0.01% increase, -3.02% of buffer)	991.106 → 991.214 → 994.710
✅	docker_cluster_agent_amd64	+56.03 KiB (0.03% increase, -7.64% of buffer)	209.753 → 209.808 → 210.470
✅	docker_cluster_agent_arm64	+64.0 KiB (0.03% increase, -47.16% of buffer)	222.847 → 222.910 → 222.980
✅	docker_dogstatsd_amd64	+20.0 KiB (0.05% increase, -28.42% of buffer)	39.841 → 39.861 → 39.910
✅	dogstatsd_deb_amd64	+16.0 KiB (0.05% increase, -2.41% of buffer)	30.502 → 30.518 → 31.150
✅	dogstatsd_deb_arm64	+16.0 KiB (0.05% increase, -1.51% of buffer)	28.499 → 28.514 → 29.530
✅	dogstatsd_rpm_amd64	+16.0 KiB (0.05% increase, -2.41% of buffer)	30.502 → 30.518 → 31.150
✅	dogstatsd_suse_amd64	+16.0 KiB (0.05% increase, -2.41% of buffer)	30.502 → 30.518 → 31.150
✅	iot_agent_deb_amd64	+52.0 KiB (0.11% increase, -8.21% of buffer)	45.762 → 45.812 → 46.380
✅	iot_agent_deb_arm64	+52.0 KiB (0.12% increase, -4.07% of buffer)	42.472 → 42.523 → 43.720
✅	iot_agent_deb_armhf	+56.0 KiB (0.13% increase, -7.93% of buffer)	43.270 → 43.325 → 43.960
✅	iot_agent_rpm_amd64	+52.0 KiB (0.11% increase, -8.22% of buffer)	45.762 → 45.813 → 46.380
✅	iot_agent_suse_amd64	+52.0 KiB (0.11% increase, -8.20% of buffer)	45.761 → 45.812 → 46.380

5 successful checks with minimal change (< 2 KiB)

	Quality gate	Current Size
✅	docker_cws_instrumentation_amd64	7.447 MiB
✅	docker_cws_instrumentation_arm64	6.877 MiB
✅	docker_dogstatsd_arm64	37.946 MiB
✅	docker_host_profiler_amd64	304.525 MiB
✅	docker_host_profiler_arm64	315.638 MiB

[cit-pr-commenter-54b7da]

Regression Detector

Regression Detector Results

Metrics dashboard
Target profiles
Run ID: f89ed0e8-cbaa-47cb-9e13-c713a28835f9

Baseline: 2844587
Comparison: a858055
Diff

Optimization Goals: ✅ No significant changes detected

Fine details of change detection per experiment

perf	experiment	goal	Δ mean %	Δ mean % CI	trials	links
➖	quality_gate_idle	memory utilization	+0.16	[+0.11, +0.21]	1	Logs bounds checks dashboard
➖	quality_gate_metrics_logs	memory utilization	-0.34	[-0.59, -0.09]	1	Logs bounds checks dashboard
➖	quality_gate_idle_all_features	memory utilization	-0.34	[-0.38, -0.31]	1	Logs bounds checks dashboard
➖	quality_gate_logs	% cpu utilization	-2.63	[-3.67, -1.60]	1	Logs bounds checks dashboard

Bounds Checks: ✅ Passed

perf	experiment	bounds_check_name	replicates_passed	observed_value	links
✅	quality_gate_idle	intake_connections	10/10	3 ≤ 4	bounds checks dashboard
✅	quality_gate_idle	memory_usage	10/10	144.90MiB ≤ 147MiB	bounds checks dashboard
✅	quality_gate_idle	total_bytes_received	10/10	735.25KiB ≤ 819.20KiB	bounds checks dashboard
✅	quality_gate_idle_all_features	intake_connections	10/10	3 ≤ 4	bounds checks dashboard
✅	quality_gate_idle_all_features	memory_usage	10/10	486.78MiB ≤ 495MiB	bounds checks dashboard
✅	quality_gate_idle_all_features	total_bytes_received	10/10	1.12MiB ≤ 1.25MiB	bounds checks dashboard
✅	quality_gate_logs	intake_connections	10/10	4 ≤ 6	bounds checks dashboard
✅	quality_gate_logs	memory_usage	10/10	183.26MiB ≤ 195MiB	bounds checks dashboard
✅	quality_gate_logs	missed_bytes	10/10	0B = 0B	bounds checks dashboard
✅	quality_gate_logs	total_bytes_received	10/10	264.32MiB ≤ 292MiB	bounds checks dashboard
✅	quality_gate_metrics_logs	cpu_usage	10/10	339.75 ≤ 2000	bounds checks dashboard
✅	quality_gate_metrics_logs	intake_connections	10/10	3 ≤ 6	bounds checks dashboard
✅	quality_gate_metrics_logs	memory_usage	10/10	386.23MiB ≤ 430MiB	bounds checks dashboard
✅	quality_gate_metrics_logs	missed_bytes	10/10	0B = 0B	bounds checks dashboard
✅	quality_gate_metrics_logs	total_bytes_received	10/10	0.94GiB ≤ 1.04GiB	bounds checks dashboard

Explanation

Confidence level: 90.00%
Effect size tolerance: |Δ mean %| ≥ 5.00%

Performance changes are noted in the perf column of each table:

• ✅ = significantly better comparison variant performance
• ❌ = significantly worse comparison variant performance
• ➖ = no significant change in performance

A regression test is an A/B test of target performance in a repeatable rig, where "performance" is measured as "comparison variant minus baseline variant" for an optimization goal (e.g., ingress throughput). Due to intrinsic variability in measuring that goal, we can only estimate its mean value for each experiment; we report uncertainty in that value as a 90.00% confidence interval denoted "Δ mean % CI".

For each experiment, we decide whether a change in performance is a "regression" -- a change worth investigating further -- if all of the following criteria are true:

1. Its estimated |Δ mean %| ≥ 5.00%, indicating the change is big enough to merit a closer look.

2. Its 90.00% confidence interval "Δ mean % CI" does not contain zero, indicating that if our statistical model is accurate, there is at least a 90.00% chance there is a difference in performance between baseline and comparison variants.

3. Its configuration does not mark it "erratic".

Replicate Execution Details

We run multiple replicates for each experiment/variant. However, we allow replicates to be automatically retried if there are any failures, up to 8 times, at which point the replicate is marked dead and we are unable to run analysis for the entire experiment. We call each of these attempts at running replicates a replicate execution. This section lists all replicate executions that failed due to the target crashing or being oom killed.

Note: In the below tables we bucket failures by experiment, variant, and failure type. For each of these buckets we list out the replicate indexes that failed with an annotation signifying how many times said replicate failed with the given failure mode. In the below example the baseline variant of the experiment named experiment_with_failures had two replicates that failed by oom kills. Replicate 0, which failed 8 executions, and replicate 1 which failed 6 executions, all with the same failure mode.

Experiment	Variant	Replicates	Failure	Logs	Debug Dashboard
experiment_with_failures	baseline	0 (x8) 1 (x6)	Oom killed		Debug Dashboard

The debug dashboard links will take you to a debugging dashboard specifically designed to investigate replicate execution failures.

❌ Retried Profiling Replicate Execution Failures (ddprof)

Note: Profiling replicas may still be executing. See the debug dashboard for up to date status.

Experiment	Variant	Replicates	Failure	Debug Dashboard
quality_gate_idle	comparison	10	Oom killed	Debug Dashboard
quality_gate_idle_all_features	baseline	10	Oom killed	Debug Dashboard
quality_gate_idle_all_features	comparison	10	Oom killed	Debug Dashboard
quality_gate_logs	baseline	10	Oom killed	Debug Dashboard
quality_gate_metrics_logs	baseline	10	Oom killed	Debug Dashboard
quality_gate_metrics_logs	comparison	10	Oom killed	Debug Dashboard

CI Pass/Fail Decision

✅ Passed. All Quality Gates passed.

• quality_gate_idle, bounds check intake_connections: 10/10 replicas passed. Gate passed.
• quality_gate_idle, bounds check total_bytes_received: 10/10 replicas passed. Gate passed.
• quality_gate_idle, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_idle_all_features, bounds check intake_connections: 10/10 replicas passed. Gate passed.
• quality_gate_idle_all_features, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_idle_all_features, bounds check total_bytes_received: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check cpu_usage: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check total_bytes_received: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check intake_connections: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check missed_bytes: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check missed_bytes: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check total_bytes_received: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check intake_connections: 10/10 replicas passed. Gate passed.