24.7. Configurable sysfs parameters for the x86-64 machine check code

Machine checks report internal hardware error conditions detected by the CPU. Uncorrected errors typically cause a machine check (often with panic), corrected ones cause a machine check log entry.

Machine checks are organized in banks (normally associated with a hardware subsystem) and subevents in a bank. The exact meaning of the banks and subevent is CPU specific.

mcelog knows how to decode them.

When you see the “Machine check errors logged” message in the system log then mcelog should run to collect and decode machine check entries from /dev/mcelog. Normally mcelog should be run regularly from a cronjob.

Each CPU has a directory in /sys/devices/system/machinecheck/machinecheckN (N = CPU number).

The directory contains some configurable entries:

bankNctl

(N bank number)

64bit Hex bitmask enabling/disabling specific subevents for bank N When a bit in the bitmask is zero then the respective subevent will not be reported. By default all events are enabled. Note that BIOS maintain another mask to disable specific events per bank. This is not visible here

The following entries appear for each CPU, but they are truly shared between all CPUs.

check_interval

How often to poll for corrected machine check errors, in seconds (Note output is hexadecimal). Default 5 minutes. When the poller finds MCEs it triggers an exponential speedup (poll more often) on the polling interval. When the poller stops finding MCEs, it triggers an exponential backoff (poll less often) on the polling interval. The check_interval variable is both the initial and maximum polling interval. 0 means no polling for corrected machine check errors (but some corrected errors might be still reported in other ways)

tolerant

Tolerance level. When a machine check exception occurs for a non corrected machine check the kernel can take different actions. Since machine check exceptions can happen any time it is sometimes risky for the kernel to kill a process because it defies normal kernel locking rules. The tolerance level configures how hard the kernel tries to recover even at some risk of deadlock. Higher tolerant values trade potentially better uptime with the risk of a crash or even corruption (for tolerant >= 3).

0: always panic on uncorrected errors, log corrected errors 1: panic or SIGBUS on uncorrected errors, log corrected errors 2: SIGBUS or log uncorrected errors, log corrected errors 3: never panic or SIGBUS, log all errors (for testing only)

Default: 1

Note this only makes a difference if the CPU allows recovery from a machine check exception. Current x86 CPUs generally do not.

trigger

Program to run when a machine check event is detected. This is an alternative to running mcelog regularly from cron and allows to detect events faster.

monarch_timeout

How long to wait for the other CPUs to machine check too on a exception. 0 to disable waiting for other CPUs. Unit: us

TBD document entries for AMD threshold interrupt configuration

For more details about the x86 machine check architecture see the Intel and AMD architecture manuals from their developer websites.

For more details about the architecture see http://one.firstfloor.org/~andi/mce.pdf