wslc: idle-terminate per-user session VMs when inactive by benhillis · Pull Request #40781 · microsoft/WSL

benhillis · 2026-06-11T19:50:18Z

Summary

Idle-terminates a per-user WSLC session's backing VM when it has been inactive, freeing memory while the session object (and its persistent storage) lives on. The VM is transparently recreated on the next operation.

Builds on #40770 (IWSLCVirtualMachineFactory).

Behavior

Only sessions with persistent storage (StoragePath set) idle-terminate.
An idle worker thread tears the VM down after a grace period (currently 30s) once there is no in-flight activity and no active container lock.
In-flight work holds an activity reference so the VM cannot be torn down mid-operation:
- VmLease wraps CLI/container operations.
- BeginContainerOperation hands clients an activity token (IFastRundown so a client crash reclaims it promptly).
- Long-lived root-namespace processes (e.g. plugin hosts) created via CreateRootNamespaceProcess hold a keep-alive token for their lifetime.
Activity bookkeeping (count + wake event) lives in a shared IdleState held via shared_ptr, decoupled from the session's lifetime, so a held token suppresses idle teardown without extending the session object's lifetime (preserving the explicit-reset-invalidates-held-processes invariant from Add WSLC (WSL Containers) feature #40366).

VM plugin hooks

Idle-terminate breaks the old "session == running VM" assumption (the VM is lazily created, idle-terminated, and transparently recreated while the session lives on), so plugins gain VM-level hooks that fire on every VM (re)start/teardown, alongside the existing per-session OnSessionCreated/OnSessionStopping hooks:

WSLPluginAPI_OnWslcVmStarted / WSLPluginAPI_OnWslcVmStopping (WslPluginApi.h) with WslcVmStarted/WslcVmStopping fields in WSLPluginHooksV1.
IWSLCPluginNotifier::OnVmStarted() / OnVmStopping() (new methods, no new IID — service and per-user session ship together).
OnWslcVmStarted fires after releasing m_lock in EnsureVmRunning (the sole caller holds an activity reference across the gap, so idle teardown can't race the VM down; firing under the lock could deadlock on a reentrant WSLCCreateProcess).
OnWslcVmStopping fires under the lock in TearDownVmLockHeld, with fire-once pairing via std::atomic<bool> m_vmStartNotified so a failed bring-up emits no spurious stopping.

Testing

New WSLCE2EVmIdleTests E2E suite (5 tests) including WSLCE2E_VmIdle_RootProcessKeepsVmAlive.
New PluginTests::WslcVmRestart asserting the hook sequence: Session created -> VM started -> VM stopping -> VM started -> VM stopping -> Session stopping.
WSLCTests::CreateRootNamespaceProcess still passes.
Full x64 Debug build clean.

Notes / follow-ups (deferred)

Grace period is a hardcoded constexpr; making it injectable would enable deterministic race tests.
No crash-path (client dies holding token) automated coverage yet.

Note

Draft for early review.

Copilot

Pull request overview

This PR adds on-demand creation and idle-termination of per-user WSLC session VMs (for sessions with persistent storage), so memory can be reclaimed while keeping the session object and storage intact. It also introduces VM-liveness/activity bookkeeping to prevent teardown during in-flight operations and adds new E2E coverage around VM lifecycle behavior.

Changes:

Implement lazy VM bring-up and idle shutdown in wslcsession via an idle worker, activity counting/tokens, and a VmLease used by VM-requiring operations.
Add client-side “operation keep-alive” usage in wslc.exe container operations to prevent VM teardown between OpenContainer and subsequent calls/streaming.
Add a new E2E test suite validating lazy start, idle stop, persistence across restarts, keep-alive for root-namespace processes, and teardown/recreate races.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/windows/wslc/e2e/WSLCE2EVmIdleTests.cpp	New E2E tests covering lazy VM start, idle stop, persistence, keep-alive, and race scenarios.
test/windows/wslc/e2e/WSLCE2EHelpers.h	Exposes the underlying `IWSLCSession*` for diagnostics/test-only calls.
src/windows/wslcsession/WSLCSession.h	Adds VM lifecycle state, idle worker/tokens/lease declarations, and new session methods.
src/windows/wslcsession/WSLCSession.cpp	Implements lazy VM creation, idle teardown, activity tokens, and VM diagnostics reporting.
src/windows/wslcsession/WSLCProcessControl.cpp	Preserves a real exit code when signaling container release, only synthesizing SIGKILL when needed.
src/windows/wslcsession/WSLCProcess.h	Stores a keep-alive token on root-namespace processes to keep the VM alive for their lifetime.
src/windows/wslcsession/WSLCContainer.cpp	Signals idle re-checks on terminal container transitions; holds a VM lease during delete.
src/windows/wslcsession/IORelay.h	Adds `IsRelayThread()` to safely avoid destroying the relay on its own thread.
src/windows/wslcsession/IORelay.cpp	Co-initializes the relay thread into the MTA; implements `IsRelayThread()`.
src/windows/wslc/services/SessionModel.h	Adds a helper to acquire/hold a keep-alive token for client-side container operations.
src/windows/wslc/services/ContainerService.cpp	Uses the keep-alive token across container operations (attach/start/stop/kill/delete/exec/etc.).
src/windows/service/inc/wslc.idl	Adds VM diagnostics type + new session methods for diagnostics and operation keep-alive.
src/windows/service/exe/WSLCSessionManager.cpp	Updates comments to reflect on-demand VM creation and recreation after idle termination.

Copilot

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Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Copilot

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Copilot

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Copilot

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benhillis

Reviewed VM-related comments - all have been addressed:

Comment on WSLCSession.h:84: Already correct - lines 77-78 say "IWSLCVirtualMachineFactory" and "lazily on first use"

Comment on WSLCSession.cpp:571: Fixed by AddRef/Release activity tracking - idle worker checks ActivityCount (line 779), and container proxies increment it on AddRef 1→2 transition. VM will not idle-terminate while clients hold container proxies. See lines 672-674 comment.

Comment on WSLCSession.cpp:380: Already has exception handling - IdleWorker() is wrapped in CATCH_LOG() at lines 375-379

Comment on Session.cpp:65: Already correct - wil::unique_threadpool_wait (line 56) calls WaitForThreadpoolWaitCallbacks in destructor automatically

Copilot

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Copilot

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Copilot

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copy the termination reason/details into the session-visible fields before SetEvent() so a waiter woken by the terminated event cannot observe the default Unknown reason. Restores the pre-refactor ordering. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

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Copilot reviewed 24 out of 24 changed files in this pull request and generated 1 comment.

…factories Add a test-only TriggerIdleTermination COM method to force VM idle teardown on demand, with three TAEF tests that hammer the VM idle-termination race paths. Refactor WSLCContainer construction to take WSLCSessionRuntime& instead of five individual runtime members. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

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Copilot reviewed 24 out of 24 changed files in this pull request and generated 1 comment.

The TriggerIdleTerminationRecoversRunningContainer test left the persisted container in the shared default session on early-exit or if the recovered handle's best-effort delete failed, making later ListContainers-based tests order-dependent. Add a scope_exit that force-deletes the container by name regardless of exit path. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

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Copilot reviewed 24 out of 24 changed files in this pull request and generated 1 comment.

…dle-terminate-vm # Conflicts: # src/windows/wslcsession/WSLCContainer.cpp # src/windows/wslcsession/WSLCSession.h

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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…dle-terminate-vm

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Copilot

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Copilot

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Copilot

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Copilot reviewed 24 out of 24 changed files in this pull request and generated no new comments.

Session-level WSLC plugin hooks (OnSessionCreated/OnSessionStopping) assumed session == running VM. With idle termination (PR #40781) the VM is created lazily, torn down when idle, and transparently recreated while the session persists, so plugins had no notification of actual VM lifecycle. Add VM-level hooks OnWslcVmStarted/OnWslcVmStopping that fire on every VM (re)start and teardown, in addition to the once-per-session hooks. Both are best-effort (errors logged and ignored). OnWslcVmStarted fires after releasing the runtime exclusive lock so a plugin may reentrantly call back into the session (e.g. WSLCCreateProcess) without deadlocking; OnWslcVmStopping fires under the lock during teardown, gated on a fire-once flag so a failed bring-up emits no spurious stopping. Adds a PluginTests::WslcVmRestart case that drives first start, forced idle teardown (TriggerIdleTermination), and lazy restart, asserting the hook sequence and proving reentrancy is deadlock-free. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: f1c36ca0-19d0-46a7-82b3-10f1e1ee3e4c

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benhillis · 2026-07-09T21:03:12Z

/azp run

azure-pipelines · 2026-07-09T21:03:25Z

Azure Pipelines successfully started running 1 pipeline(s).

OneBlue · 2026-07-10T20:40:18Z

+    m_dockerClient(runtime.Docker()),
+    m_eventTracker(runtime.Events()),
+    m_ioRelay(*runtime.Relay()),
+    m_containerEvents(runtime.Events().RegisterContainerStateUpdates(


m_containerEvents keeps a reference to the DockerEventTracker it was created with, but if that object goes away, it will operate on a dangling pointer.

The easiest way I can think of to solve this would be to keep the same event tracker object between different VM's, and have logic to "restart" it when a new VM starts (other wise, all containers & processes would need to re-register with the new event tracker)

OneBlue · 2026-07-10T20:55:02Z

+    // the lock, so the handler must not call back into the session.
+    if (m_vmStartNotified.exchange(false) && m_hooks.OnVmStopping)
+    {
+        m_hooks.OnVmStopping();


I think there's a potential deadlock here. If a plugin receives a "VM stopping" notification and runs an operation that can try to acquire a VM lease, we could get stuck on m_lock in VmLease's constructor

OneBlue · 2026-07-10T21:03:59Z

-    // Wait for dockerd to be ready before starting the event tracker.
-    THROW_WIN32_IF_MSG(
-        ERROR_TIMEOUT, !m_dockerdReadyEvent.wait(Settings->BootTimeoutMs), "Timed out waiting for dockerd to start");
+        m_containers.clear();


I don't think we can do this unfortunately, because this will break any COM references that callers might have to containers

OneBlue · 2026-07-10T21:09:28Z

+
+    // Tear down the VM (if running) and all VM-scoped state, capturing the termination reason.
+    // This mirrors the soft teardown used for idle shutdown, but here it is permanent.
+    TearDownVmLockHeld(/* CaptureTerminationReason */ true);


The locking is confusing me a bit here, because we're calling TearDownVmLockHeld, but we don't actually hold m_lock, only the session lock.

I think the locking still works because StopVmLockHeld uses an std::atomic to transition the VM to stopped, but I think we should make the locking a bit more clear here

OneBlue · 2026-07-10T21:12:05Z

+        ULONG Id{};
+        std::wstring DisplayName;
+        const std::atomic<bool>* Terminating{};
+        wil::unique_event* SessionTerminatingEvent{};


nit: We could use wil::shared_event to avoid having to deal with pointers here

OneBlue · 2026-07-10T21:13:10Z

+        return S_OK;
+    }
+
+    g_logfile << "WSLC VM stopping, session=" << Session->SessionId << std::endl;


I recommend launching a process inside the VM & removing a mount here, to validate that we handle these calls correctly

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benhillis force-pushed the user/benhill/wslc-idle-terminate-vm branch from c12d7e1 to fa2eb47 Compare June 12, 2026 01:28

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Comment thread src/windows/wslcsession/WSLCSessionRuntime.cpp

wslc: skip containerd VHD unmount when storage was never mounted

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benhillis mentioned this pull request Jul 7, 2026

vmmemwslc-cli remains running after all containers have exited #40961

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Ben Hillis and others added 2 commits July 9, 2026 11:32

Merge remote-tracking branch 'origin/master' into user/benhill/wslc-i…

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…dle-terminate-vm # Conflicts: # src/windows/wslcsession/WSLCContainer.cpp # src/windows/wslcsession/WSLCSession.h

wslc: list WSLCIdleState.h in wslcsession HEADERS

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Merge remote-tracking branch 'origin/master' into user/benhill/wslc-i…

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…dle-terminate-vm

wslc: clang-format Open() call site

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wslc: correct idle grace-period comment to reference IdleTimeoutSec

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wslc: avoid reserved identifier in Fork structured binding

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benhillis mentioned this pull request Jul 9, 2026

Add WSLC VM start/stop plugin hooks #41043

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Pair OnVmStopping only when OnVmStarted fired

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Summary

Behavior

VM plugin hooks

Testing

Notes / follow-ups (deferred)

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benhillis commented Jun 11, 2026 •

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