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RFC-0091 - zx_system_get_event ZX_SYSTEM_EVENT_IMMINENT_OUT_OF_MEMORY

RFC-0091: zx_system_get_event ZX_SYSTEM_EVENT_IMMINENT_OUT_OF_MEMORY
StatusAccepted
Areas
  • Zircon
Description

Proposes adding a new event - `ZX_SYSTEM_EVENT_IMMINENT_OUT_OF_MEMORY`, to the `zx_system_get_event()` syscall.

Issues
Gerrit change
Authors
Reviewers
Date submitted (year-month-day)2021-04-12
Date reviewed (year-month-day)2021-05-04

Summary

This RFC proposes adding a new event type - ZX_SYSTEM_EVENT_IMMINENT_OUT_OF_MEMORY, to the zx_system_get_event() syscall. The new kernel event will be signaled prior to ZX_SYSTEM_EVENT_OUT_OF_MEMORY, to facilitate capturing memory diagnostics close to the point of OOM.

Motivation

ZX_SYSTEM_EVENT_OUT_OF_MEMORY is signaled when the system has almost run out of free physical memory to be able to fulfill memory allocation requests. The driver_manager process waits on this OOM event, and when it is signaled asks fshost to shut down filesystems, so that the device can be rebooted as cleanly as possible. There exists no good mechanism to capture meaningful memory state at this point though, which is critical in order to root cause the OOM.

Simply waiting on the OOM event, and trying to write out a memory report at the point of the OOM is not feasible, because that would race with filesystems shutting down. So instead of relying on ZX_SYSTEM_EVENT_OUT_OF_MEMORY, this RFC proposes exposing a new event ZX_SYSTEM_EVENT_IMMINENT_OUT_OF_MEMORY which will be signaled as close to the OOM as possible, while trying to provide some headroom before the OOM so that a memory report can be reliably written out. For example, if ZX_SYSTEM_EVENT_OUT_OF_MEMORY is signaled when the system free memory level drops to 50MB, ZX_SYSTEM_EVENT_IMMINENT_OUT_OF_MEMORY can be signaled at 60MB of free memory.

There is also a desire to perform orderly shutdown on OOM in the long term, which would involve a higher level userspace component (not driver_manager) waiting on the ZX_SYSTEM_EVENT_OUT_OF_MEMORY event and shutting down certain key components as well as filesystems. That is a larger scope project, at which point we will have to revisit the OOM event signaling and reporting code paths. As such the proposal in this RFC can be seen as an intermediate step, requiring minimal changes to the existing design. The memory_monitor process already waits on the Normal, Warning, and Critical events and generates memory reports when required, so it can be easily extended to wait on an Imminent-OOM event. This event might be removed in the future if report generation is handled by orderly shutdown on OOM, or we might decide to still keep it around if the report provides additional value.

Design

The kernel memory reclamation system signals memory pressure events based on three memory watermarks - Warning, Critical, and OOM. This logic can easily be extended to include a new Imminent-OOM watermark between Critical and OOM. The delta between the Imminent-OOM and OOM watermarks can be configured by a kernel commandline option.

To maintain consistency with the existing memory watermarks, which can also be configured via the kernel commandline, this delta will be an absolute amount of memory in MB. That also means that a single value will likely not be feasible for all platforms, and will require tuning, which is a property shared by existing memory watermarks as well. Also, memory report generation itself will require a certain amount of memory, which should also be taken into account when picking a value for this delta.

As with other kernel events supported by zx_system_get_event(), a handle to the root job will be required. Unlike ZX_SYSTEM_EVENT_OUT_OF_MEMORY, ZX_SYSTEM_EVENT_IMMINENT_OUT_OF_MEMORY will not require the ZX_RIGHT_MANAGE_PROCESS right. This makes the new event similar to ZX_SYSTEM_EVENT_MEMORY_PRESSURE_NORMAL, ZX_SYSTEM_EVENT_MEMORY_PRESSURE_WARNING and ZX_SYSTEM_EVENT_MEMORY_PRESSURE_CRITICAL, which also do not require the ZX_RIGHT_MANAGE_PROCESS right. In other words, Normal, Warning, Critical, and Imminent-OOM events can use the RootJobForInspect protocol, while the OOM event requires the RootJob protocol.

Implementation in the kernel will require minimal changes to the existing memory reclamation logic. We are simply adding another watermark to the list of watermarks passed to pmm_init_reclamation(). The only aspect where the Imminent-OOM level differs from the other memory pressure levels, is that it will not trigger any memory reclamation in the kernel (eviction of pager backed memory and discardable memory). This level is intended purely as a diagnostic level; triggering memory reclamation at this point would alter the memory state the event is meant to capture in the first place. As a result, this new event will not have a corresponding signal in the fuchsia.memorypressure protocol, which is used to drive memory reclamation in userspace.

Implementation

The zx_system_get_event() syscall can be extended in a single CL; waiters on existing kernel events will not be affected in any way with this change. Waiters on the new event can later adopt it in a separate CL.

Performance

The new event will not affect the performance of waiters of the existing event. The new event is also expected to have similar performance characteristics as the existing ones, both for retrieving the event handle and for waiting on it. Kernel performance for signaling the event will also be similar to the existing ones - when the free memory level changes, the PMM system finds the closest memory watermark in order to infer the memory pressure level; the proposal is to simply add a new watermark without changing the rest of the existing logic.

Ergonomics

None.

Backwards Compatibility

None.

Security considerations

None.

Privacy considerations

None.

Testing

Zircon core-tests will be written that retrieve and wait on ZX_SYSTEM_EVENT_IMMINENT_OUT_OF_MEMORY. Unit tests to verify client side behavior will also be written when the event is adopted.

Documentation

The syscall documentation for zx_system_get_event() will need to be updated. An entry will also be added to the kernel concepts documentation explaining the motivation behind the various memory pressure levels, the response that is expected at each, and how they tie in with the rest of the system.

Drawbacks, alternatives, and unknowns

Drawback: Memory reports might not be precise

Since the event is signaled prior to OOM, the memory state captured will not coincide exactly with the OOM. However, as long as the delta from the OOM watermark is small enough (a few tens of MB), that should still prove useful in diagnosing the OOM.

We might see cases where a report is generated without an OOM following it, i.e. if the device is somehow able to recover after the Imminent-OOM event. Even though there will not be an OOM event associated with such a report, it is useful memory data to have nevertheless, as it still points to increased memory usage that is worth investigating.

It is also possible for the waiter of the event to not be able to generate reports in the case of rapid memory allocation spikes. Such cases will still have to be diagnosed by reproducing the issue locally while gathering memory metrics in parallel. Reports should still be able to capture memory diagnostics for a majority of OOMs, since very rapid allocation spikes are rare.

Alternative 1: Using existing events instead of a new one

ZX_SYSTEM_EVENT_OUT_OF_MEMORY cannot be used to reliably write out a memory report. This is because the OOM event is used to drive filesystem shutdown, so we might not be able to write out the report in time.

This interaction could be changed by moving the OOM signal handling out from driver_manager to another userspace component that can drive a more orderly shutdown - first writing out a report, and then shutting down filesystems. This approach would need some synchronization to make sure the report is successfully written out before filesystems are shut down. It might also require us to relax the OOM watermark and the reboot delay interval after sending out the OOM signal, since we would now be performing more work when the OOM event is signaled. This change to how the OOM signal is handled ties in with the larger effort of orderly shutdown on OOM (briefly described in the Motivation section), which is a larger scope effort that might be undertaken in the future.

The other existing event that is signaled prior to OOM is ZX_SYSTEM_EVENT_MEMORY_PRESSURE_CRITICAL, which we do use to write memory reports. However, the Critical memory watermark is far enough from the OOM watermark that these reports can miss important memory state changes closer to the OOM.

Alternative 2: Richer API for more diagnostic events

Instead of exposing a specialized event close to OOM, we could have an alternate API where the user can specify multiple custom diagnostic memory levels to be notified at. This allows for a more flexible API which is easier to extend in the future. However, no concrete use cases exist at this time indicating that such an API would be useful.

The other memory pressure levels where it might be useful to capture diagnostic information are Warning and Critical. For these levels, we can directly use the kernel events signaling the level change. We already do this for the Critical level. The OOM level is special in this respect, because it is not feasible to capture diagnostics exactly at the point of the OOM. Capturing state at the point of the OOM is what we would want in the ideal case. Since that is not possible, we need another signal close enough to the OOM to be able to capture relevant state, but not so close that we run into a similar problem and are not able to write a report.

Prior art and references

The zx_system_get_event docs describe how the kernel memory events work. The fuchsia.memorypressure docs describe the userspace memory pressure signals that are built on top of the kernel events.