Runaway CPU investigation for dockerd: socket churn at mem_cgroup_handle_over_high

Prompt

## Plan Pass

You are planning a fixer patch before any edits happen.

Read the evidence bundle at `./evidence.json`. The prepared workspace is `./workspace` and it was acquired via `debian-source`. The original pre-edit snapshot is available at `./source` if you need to inspect it. For interpreter processes, plan from the script/application entrypoint evidence first and include the runtime only as a second investigation target unless the evidence proves a runtime bug.

Upstream-style expectation: before planning or editing, check for contribution/style docs (`CONTRIBUTING`, `HACKING`, `README-hacking`, `README.md`, `docs/`, `dev-docs/`) and scan the touched subsystem for local helpers. If the project has wrappers for file IO, path-relative IO, process spawning, memory allocation, logging, locking, or platform compatibility, prefer those wrappers over generic libc/std APIs. Do not invent a reproducer or user-visible failure that is not in the evidence bundle; if the evidence is profiler-only or indirect, describe it as a targeted mitigation or stop with a diagnosis instead of presenting a speculative patch as a confirmed bug fix. In the plan and final validation, name any such helper, convention, or evidence limit you found, or say that no relevant local helper was found. Treat this as a `docker.io` upstream patch, not just a Debian-local workaround. Inspect the relevant code, nearby callers, project contribution docs, and local helper/compat APIs, but do not edit files in this pass.

Return a short markdown plan with these exact sections:

## Problem
## Evidence Confidence
## Proposed Subject
## Patch Plan
## Risks
## Validation

Classify `## Evidence Confidence` as exactly one of `reproduced`, `observed`, or `inferred`. Use `inferred` only for a no-patch diagnosis/report plan unless you can name the extra evidence you will collect before editing; inferred source patches are blocked by Fixer because they are not pull-request-ready. For `observed` source-patch plans, plan to say in the final `## Issue Connection` that the failure was observed by Fixer and not independently reproduced. If the plan touches authentication, authorization, credentials, cryptography, sandboxing, permissions, or timing/throttling behavior, treat it as security-sensitive: do not plan a source patch unless validation can reproduce the behavior and analyze the security impact; otherwise plan a no-patch diagnosis/report. The plan must explain how the proposed code change addresses the observed issue evidence, call out any prior Fixer patch that should be improved or replaced, reject awkward control flow such as avoidable `goto` if there is a cleaner bounded alternative, name any local helper APIs or maintainer conventions the patch should follow, and keep the intended maintainer-facing explanation clear enough that someone unfamiliar with the local complaint wording can still follow the fix. In `## Validation`, name the reproducible configure/build/test entrypoint you will try from the workspace root before any focused leaf compile or smoke check, and include one bounded independent reproduction attempt for the collected failure signal when it is safe and cheap. Do not plan to claim `reproduced` unless that reproduction command or test can actually show the failure.

## Patch Pass

You are working on a bounded fixer proposal.

Read the evidence bundle at `./evidence.json`. The prepared workspace is `./workspace` and it was acquired via `debian-source`. Produce the smallest reasonable patch for the target repository, keep the change upstreamable, prefer the clearest control flow available, and do not keep avoidable `goto` when a simpler structure would read better. Before introducing new file, process, allocation, locking, networking, or platform APIs, inspect nearby code and project contribution docs for existing helpers or compatibility wrappers and use those local patterns unless you can explain why they do not fit. Validate from a reproducible workspace-root entrypoint before falling back to focused leaf commands; if a build or test cannot run, report the exact command, the exact blocker, and any narrower check you ran instead. During validation, also try one bounded independent reproduction of the collected failure signal when it is safe and cheap, such as a failing test, smoke command, perf/strace comparison, or before/after runtime check. Only use `reproduced` if that command or test actually reproduced the failure; otherwise keep `observed` and report the reproduction blocker. The final explanation must connect the observed issue evidence to the actual code change, not just paraphrase the diff. Write like a maintainer is going to read the patch mail cold: explain the bug in plain language, define subsystem-specific jargon the first time you need it, and make the causal story obvious. Explicitly classify evidence confidence as `reproduced`, `observed`, or `inferred`: `reproduced` means you reproduced the failure locally; `observed` means Fixer has direct crash/log/trace evidence but you did not independently reproduce it; `inferred` means the source patch is not pull-request-ready, so do not leave a source diff unless you first gather stronger observed/reproduced evidence; otherwise return a no-patch diagnosis/report. For any source-changing `observed` patch, say explicitly in `## Issue Connection` that the failure was observed by Fixer and not independently reproduced. Security-sensitive areas such as authentication, authorization, credentials, cryptography, sandboxing, permissions, and timing/throttling behavior need reproduced evidence plus explicit security-impact analysis before leaving a source diff; otherwise return a no-patch diagnosis/report for human review. If you introduce non-obvious state translation, index remapping, or backend split logic, add a short source comment that explains the invariant being preserved.

Start by explaining the likely root cause from the collected perf, strace, and /proc evidence. If you cannot land a safe patch, leave a diagnosis that is strong enough for an upstream bug report.

Upstream-style expectation: before planning or editing, check for contribution/style docs (`CONTRIBUTING`, `HACKING`, `README-hacking`, `README.md`, `docs/`, `dev-docs/`) and scan the touched subsystem for local helpers. If the project has wrappers for file IO, path-relative IO, process spawning, memory allocation, logging, locking, or platform compatibility, prefer those wrappers over generic libc/std APIs. Do not invent a reproducer or user-visible failure that is not in the evidence bundle; if the evidence is profiler-only or indirect, describe it as a targeted mitigation or stop with a diagnosis instead of presenting a speculative patch as a confirmed bug fix. In the plan and final validation, name any such helper, convention, or evidence limit you found, or say that no relevant local helper was found. Treat this as a `docker.io` upstream patch, not just a Debian-local workaround. 

Keep the change narrowly scoped and summarize validation clearly.

In every authoring pass, your final response must start with `Subject: <single-line git commit subject>` and then include these markdown sections exactly:

## Commit Message
A short upstream-friendly explanation of what changed and why. Write it in plain language that a maintainer can follow without local complaint context. If you use subsystem jargon, define it immediately.

## Evidence Confidence
Exactly one word: `reproduced`, `observed`, or `inferred`. Use `reproduced` only when you reproduced the failure locally with a command or test, and include that command/test in `## Validation`. Use `observed` when Fixer has direct crash/log/trace evidence but you did not independently reproduce it. If `## Git Add Paths` lists source files for an `observed` patch, `## Issue Connection` must explicitly say the failure was observed by Fixer and not independently reproduced. Security-sensitive source changes touching authentication, authorization, credentials, cryptography, sandboxing, permissions, or timing/throttling behavior require reproduced evidence and explicit security-impact analysis; otherwise list `None` under `## Git Add Paths` and write a no-patch diagnosis/report. Use `inferred` for profiler/strace/indirect evidence; inferred responses may be no-patch diagnoses or reports, but inferred source patches are not pull-request-ready until stronger evidence is gathered.

## Issue Connection
Write this as maintainer-facing patch mail, not as local Fixer notes. Cover four things explicitly in readable sentences: the user-visible symptom or the exact collected signal, the code-level cause or the cautious inference from evidence, the specific change you made, and the expected effect. Do not invent a reproducer, command line, crash, or user-visible failure that is not present in the evidence bundle. If the evidence is direct-but-not-reproduced, say it was observed by Fixer and not independently reproduced. If the evidence is indirect and you did not gather stronger evidence, do not leave a source diff; write a no-patch diagnosis/report instead. Include an explicit effect sentence such as `The expected effect is ...`, `This should reduce ...`, or `This prevents ...` for source patches. If the logic is non-obvious in code, mention that you added a short explanatory comment.

## Git Add Paths
List the repo-relative paths that belong in the final patch, one per line. Use `None` only when you intentionally made no source changes. Include intentionally new files, and do not list generated build artifacts.

## Validation
List the checks you ran, or say clearly that you could not run them. Include the independent reproduction command/test and result when `## Evidence Confidence` is `reproduced`; if reproduction was attempted but blocked, name the exact blocker and keep confidence at `observed` or `inferred`.

Before editing, read the plan at `./plan-output.txt` and follow it unless the code proves part of it wrong. If you change course, say so explicitly in the final write-up instead of silently drifting from the plan.

## Review Pass 1

You are reviewing a freshly generated fixer patch.

Read the evidence bundle at `./evidence.json`. The prepared workspace is `./workspace` and it was acquired via `debian-source`. Review the first patch pass. The original pre-edit snapshot is available at `./source` for diffing.

Upstream-style expectation: before planning or editing, check for contribution/style docs (`CONTRIBUTING`, `HACKING`, `README-hacking`, `README.md`, `docs/`, `dev-docs/`) and scan the touched subsystem for local helpers. If the project has wrappers for file IO, path-relative IO, process spawning, memory allocation, logging, locking, or platform compatibility, prefer those wrappers over generic libc/std APIs. Do not invent a reproducer or user-visible failure that is not in the evidence bundle; if the evidence is profiler-only or indirect, describe it as a targeted mitigation or stop with a diagnosis instead of presenting a speculative patch as a confirmed bug fix. In the plan and final validation, name any such helper, convention, or evidence limit you found, or say that no relevant local helper was found. Treat this as a `docker.io` upstream patch, not just a Debian-local workaround. The latest author response is at `./patch-output.txt`. Inspect the current code and changed paths like a strict code reviewer. Focus on correctness, regressions, maintainability, awkward control flow such as avoidable `goto`, missing validation, weak or non-gittable commit message text, and explanations that fail to connect the observed issue evidence to the code change. Also review the maintainer experience: the patch mail should be easy to accept upstream, the user-visible bug should be explained in plain language, subsystem-specific jargon should be defined when first used, and any non-obvious state translation or index remapping in code should have a short explanatory comment. Reject patches that omit `## Evidence Confidence`, use anything other than `reproduced`, `observed`, or `inferred`, or sound more certain than that evidence level allows. Reject patches that claim `reproduced` unless `## Validation` names the concrete reproduction command or test and its result. Reject source patches in security-sensitive areas such as authentication, authorization, credentials, cryptography, sandboxing, permissions, and timing/throttling behavior unless the author reproduced the behavior and explicitly analyzed the security impact of the change; observed-only evidence should become a diagnosis/report, not a PR-ready source diff. Reject patches that invent a reproducer, command line, crash, or user-visible failure not present in the evidence bundle; indirect profiler or strace evidence may justify a cautious mitigation, but the author must say it is indirect instead of presenting it as a confirmed upstream bug. Reject patches that introduce generic libc/std APIs when nearby code or project docs provide a local compat/helper API for the same job, unless the author explicitly justifies the exception. Reject validation that only reports a leaf object/syntax build when a project-level configure/build/test entrypoint exists and was not attempted; if a full build cannot run, the author must show the exact failed command and blocker.

Do not apply code changes in this pass.

Return a short markdown review report. The first non-empty line must be exactly one of:

RESULT: ok
RESULT: fix-needed

If you choose `RESULT: fix-needed`, add a `## Findings` section with concrete, actionable items.

Response

## Plan Pass

## Problem
Fixer observed `dockerd` sleeping with many threads blocked in futex waits, low CPU (`0.1%`), a kernel hot path in `mem_cgroup_handle_over_high`, and an automated `socket-churn` classification. The available backtraces are unsymbolized, the raw investigation files are not present in this workspace, and no concrete Docker command, API request, or user-visible failure is provided.

## Evidence Confidence
observed

## Proposed Subject
No source patch yet: diagnose and report the indirect `dockerd` profiler signal until symbolized/source-level evidence identifies a Moby bug. A source patch would only be planned after collecting extra evidence that maps the observed futex/socket/cgroup activity to a specific daemon loop or retry path.

## Patch Plan
Do not edit files in the next pass from this evidence alone. The current bundle supports an observed performance/profile anomaly, not a pull-request-ready bug fix.

Start from the `dockerd` entrypoint evidence: `engine/cmd/dockerd/docker.go` -> `newDaemonCLI`/`runDaemon` in `engine/cmd/dockerd/daemon.go` -> containerd initialization/client creation in `engine/daemon/daemon.go` and `engine/libcontainerd/supervisor/remote_daemon.go`.

Follow Moby conventions found in `engine/CONTRIBUTING.md` and `engine/TESTING.md`: changes need focused unit tests, `gofmt -s`, and no new `integration-cli` tests. Local helpers/conventions found include `containerd/log` via `log.G(ctx)`, `github.com/pkg/errors` wrapping, `pidfile.Write`, `homedir.StickRuntimeDirContents`, `user.MkdirAllAndChown`, containerd `dialer.ContextDialer`, and existing gRPC/containerd timeout/backoff setup. No relevant wrapper was found that directly addresses the profiler’s kernel `mem_cgroup_handle_over_high` signal.

No prior Fixer patch is listed in the evidence (`prior_fixer_patch: null`), so there is nothing to replace.

Avoid proposing awkward retry-control changes such as adding `goto` or ad hoc sleeps. Any future patch in containerd connection/retry timing would be security-sensitive timing/throttling behavior and must first reproduce the behavior and analyze impact.

## Risks
The hottest symbol is in the kernel cgroup memory path, and the process was sleeping with very low CPU. A Moby source change could easily be speculative or mask an environmental cgroup/kernel pressure issue.

Unsymbolized Go backtraces prevent reliable source attribution. The evidence labels `socket-churn`, but the shown strace excerpt is dominated by futex waits plus a small number of `epoll_pwait`/`waitid` calls.

## Validation
Before any focused check, use the project entrypoint from the workspace root: `cd engine && make test-unit`.

For a bounded independent reproduction attempt, first build a daemon binary, then run a short traced startup/validation command against an isolated temp root and exec-root, capturing only futex/socket/epoll/wait syscalls with `timeout` and `strace`. Do not claim reproduced unless that command shows the same failure signal. If a future source patch is justified, run focused package tests such as `cd engine && TESTDIRS='./cmd/dockerd ./daemon ./libcontainerd/supervisor' make test-unit`, plus any new regression test package.

## Patch Pass

Subject: No patch: diagnose dockerd profiler signal

## Commit Message
No source change is proposed. The collected signal most likely points to memory-cgroup pressure around `dockerd`: the hot path is the kernel `mem_cgroup_handle_over_high` handler, while `/proc` and strace excerpts show a sleeping process with many futex waits rather than a symbolized Moby loop. Without a reproducer, symbolized Go frames, or accessible raw artifacts, a Docker source patch would be speculative.

## Evidence Confidence
inferred

## Issue Connection
Fixer collected profiler and trace evidence for `/usr/sbin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock`: low CPU, sleeping state, many futex waits, and a kernel hot path in `mem_cgroup_handle_over_high`. That kernel routine is involved when a cgroup exceeds its configured memory high threshold, so the cautious inference is memory-cgroup pressure or kernel/runtime waiting, not a confirmed daemon retry loop.

I followed the saved plan in `plan-output.txt`: no source patch was left because the evidence is profiler/strace-only, unsymbolized, and the raw investigation directory is absent from this workspace. I inspected the daemon/containerd startup area and found existing local conventions and helpers: `log.G(ctx)`, `github.com/pkg/errors` wrapping, `pidfile.Write`, `homedir.StickRuntimeDirContents`, `user.MkdirAllAndChown`, containerd `dialer.ContextDialer`, and existing gRPC timeout/backoff setup. No relevant helper or Moby code path was identified that directly explains the kernel memory-cgroup hot path.

No prior Fixer patch was listed. I also did not change retry/backoff or other timing behavior, because that would be timing/throttling-sensitive and would need reproduced evidence plus security-impact analysis first.

The expected effect is no runtime change; this leaves a maintainer-ready diagnosis instead of an upstream patch that could mask an environmental cgroup/kernel issue.

## Git Add Paths
None

## Validation
`cd engine && make test-unit` failed before tests ran. Exact blocker: Docker build setup tried to create `/root/.docker` and failed with `ERROR: mkdir /root/.docker: read-only file system`; make exited at `Makefile:207: build`.

Independent reproduction attempt:
`cd engine && timeout 15s strace -f -e trace=futex,epoll_pwait,waitid,socket,connect ./bundles/binary-daemon/dockerd -H unix:///tmp/fixer-dockerd.sock --data-root /tmp/fixer-docker-root --exec-root /tmp/fixer-docker-exec --pidfile /tmp/fixer-dockerd.pid --containerd=/run/containerd/containerd.sock --debug`

That did not reproduce the signal. Exact blockers: `strace` reported `PTRACE_TRACEME: Operation not permitted`, and `./bundles/binary-daemon/dockerd` was absent because the build did not complete.

I also checked the raw artifact path from the evidence; `/var/lib/fixer/investigations/2026-05-06T10-55-05.762950439+00-00-dockerd-b732bcc6a3ff` is not present. `git diff --stat` and `git status --short` showed no source changes.

## Review Pass 1

RESULT: ok

No findings. The pass leaves no source diff, correctly treats the profiler/strace evidence as indirect, uses an allowed `## Evidence Confidence` value, reports the project-level validation attempt and blocker, and avoids inventing a confirmed Moby bug or unsafe timing/throttling patch.