Scrub Control

Copyright (c) 2024-2025 HiSilicon Limited.

Author:

Shiju Jose <shiju.jose@huawei.com>

License:

The GNU Free Documentation License, Version 1.2 without Invariant Sections, Front-Cover Texts nor Back-Cover Texts. (dual licensed under the GPL v2)

• Written for: 6.15

Introduction

Increasing DRAM size and cost have made memory subsystem reliability an important concern. These modules are used where potentially corrupted data could cause expensive or fatal issues. Memory errors are among the top hardware failures that cause server and workload crashes.

Memory scrubbing is a feature where an ECC (Error-Correcting Code) engine reads data from each memory media location, corrects if necessary and writes the corrected data back to the same memory media location.

DIMMs can be scrubbed at a configurable rate to detect uncorrected memory errors and attempt recovery from detected errors, providing the following benefits:

1. Proactively scrubbing DIMMs reduces the chance of a correctable error becoming uncorrectable.

2. When detected, uncorrected errors caught in unallocated memory pages are isolated and prevented from being allocated to an application or the OS.

3. This reduces the likelihood of software or hardware products encountering memory errors.

4. The additional data on failures in memory may be used to build up statistics that are later used to decide whether to use memory repair technologies such as Post Package Repair or Sparing.

There are 2 types of memory scrubbing:

1. Background (patrol) scrubbing while the DRAM is otherwise idle.

2. On-demand scrubbing for a specific address range or region of memory.

Several types of interfaces to hardware memory scrubbers have been identified, such as CXL memory device patrol scrub, CXL DDR5 ECS, ACPI RAS2 memory scrubbing, and ACPI NVDIMM ARS (Address Range Scrub).

The control mechanisms vary across different memory scrubbers. To enable standardized userspace tooling, there is a need to present these controls through a standardized ABI.

A generic memory EDAC scrub control allows users to manage underlying scrubbers in the system through a standardized sysfs control interface. It abstracts the management of various scrubbing functionalities into a unified set of functions.

Use cases of common scrub control feature

1. Several types of interfaces for hardware memory scrubbers have been identified, including the CXL memory device patrol scrub, CXL DDR5 ECS, ACPI RAS2 memory scrubbing features, ACPI NVDIMM ARS (Address Range Scrub), and software-based memory scrubbers.

   Of the identified interfaces to hardware memory scrubbers some support control over patrol (background) scrubbing (e.g., ACPI RAS2, CXL) and/or on-demand scrubbing (e.g., ACPI RAS2, ACPI ARS). However, the scrub control interfaces vary between memory scrubbers, highlighting the need for a standardized, generic sysfs scrub control interface that is accessible to userspace for administration and use by scripts/tools.

2. User-space scrub controls allow users to disable scrubbing if necessary, for example, to disable background patrol scrubbing or adjust the scrub rate for performance-aware operations where background activities need to be minimized or disabled.

3. User-space tools enable on-demand scrubbing for specific address ranges, provided that the scrubber supports this functionality.

4. User-space tools can also control memory DIMM scrubbing at a configurable scrub rate via sysfs scrub controls. This approach offers several benefits:

   4.1. Detects uncorrectable memory errors early, before user access to affected

   memory, helping facilitate recovery.

   4.2. Reduces the likelihood of correctable errors developing into uncorrectable

   errors.

5. Policy control for hotplugged memory is necessary because there may not be a system-wide BIOS or similar control to manage scrub settings for a CXL device added after boot. Determining these settings is a policy decision, balancing reliability against performance, so userspace should control it. Therefore, a unified interface is recommended for handling this function in a way that aligns with other similar interfaces, rather than creating a separate one.

Scrubbing features

CXL Memory Scrubbing features

CXL spec r3.1 [1] section 8.2.9.9.11.1 describes the memory device patrol scrub control feature. The device patrol scrub proactively locates and makes corrections to errors in regular cycle. The patrol scrub control allows the userspace request to change CXL patrol scrubber’s configurations.

The patrol scrub control allows the requester to specify the number of hours in which the patrol scrub cycles must be completed, provided that the requested scrub rate must be within the supported range of the scrub rate that the device is capable of. In the CXL driver, the number of seconds per scrub cycles, which user requests via sysfs, is rescaled to hours per scrub cycles.

In addition, they allow the host to disable the feature in case it interferes with performance-aware operations which require the background operations to be turned off.

Error Check Scrub (ECS)

CXL spec r3.1 [1] section 8.2.9.9.11.2 describes Error Check Scrub (ECS) - a feature defined in the JEDEC DDR5 SDRAM Specification (JESD79-5) and allowing DRAM to internally read, correct single-bit errors, and write back corrected data bits to the DRAM array while providing transparency to error counts.

The DDR5 device contains number of memory media Field Replaceable Units (FRU) per device. The DDR5 ECS feature and thus the ECS control driver supports configuring the ECS parameters per FRU.

ACPI RAS2 Hardware-based Memory Scrubbing

ACPI spec 6.5 [2] section 5.2.21 ACPI RAS2 describes an ACPI RAS2 table which provides interfaces for platform RAS features and supports independent RAS controls and capabilities for a given RAS feature for multiple instances of the same component in a given system.

Memory RAS features apply to RAS capabilities, controls and operations that are specific to memory. RAS2 PCC sub-spaces for memory-specific RAS features have a Feature Type of 0x00 (Memory).

The platform can use the hardware-based memory scrubbing feature to expose controls and capabilities associated with hardware-based memory scrub engines. The RAS2 memory scrubbing feature supports as per spec,

1. Independent memory scrubbing controls for each NUMA domain, identified using its proximity domain.

2. Provision for background (patrol) scrubbing of the entire memory system, as well as on-demand scrubbing for a specific region of memory.

ACPI Address Range Scrubbing (ARS)

ACPI spec 6.5 [2] section 9.19.7.2 describes Address Range Scrubbing (ARS). ARS allows the platform to communicate memory errors to system software. This capability allows system software to prevent accesses to addresses with uncorrectable errors in memory. ARS functions manage all NVDIMMs present in the system. Only one scrub can be in progress system wide at any given time.

The following functions are supported as per the specification:

1. Query ARS Capabilities for a given address range, indicates platform supports the ACPI NVDIMM Root Device Unconsumed Error Notification.

2. Start ARS triggers an Address Range Scrub for the given memory range. Address scrubbing can be done for volatile or persistent memory, or both.

3. Query ARS Status command allows software to get the status of ARS, including the progress of ARS and ARS error record.

4. Clear Uncorrectable Error.

5. Translate SPA

6. ARS Error Inject etc.

The kernel supports an existing control for ARS and ARS is currently not supported in EDAC.

[1] (1,2)

https://computeexpresslink.org/cxl-specification/

[2] (1,2)

https://uefi.org/specs/ACPI/6.5/

Comparison of various scrubbing features

Name	ACPI RAS2	CXL patrol scrub	CXL ECS	ARS
On-demand Scrubbing	Supported	No	No	Supported
Background scrubbing	Supported	Supported	Supported	No
Mode of scrubbing	Scrub ctrl per NUMA domain.	per device	per memory media	Unknown
Query scrub capabilities	Supported	Supported	Supported	Supported
Setting address range	Supported	No	No	Supported
Setting scrub rate	Supported	Supported	No	No
Unit for scrub rate	Not Defined	in hours	No	No
Scrub status/ Completion	Supported on-demand scrubbing only	No	No	Supported
UC error reporting	Exception	CXL general media/DRAM event/media scan?	CXL general media/DRAM event/media scan?	ACPI UCE notify and query ARS status
Support for EDAC control	Supported	Supported	Supported	No

The File System

The control attributes of a registered scrubber instance could be accessed in:

/sys/bus/edac/devices/<dev-name>/scrubX/

sysfs

Sysfs files are documented in ABI file testing/sysfs-edac-scrub

ABI file testing/sysfs-edac-ecs