Interaction problems between Intel TSX and Intel performance registers

  • 7023734
  • 21-Feb-2019
  • 08-Nov-2019

Environment

SUSE Linux Enterprise Server 15
SUSE Linux Enterprise Server 12
SUSE Linux Enterprise Server 11

Situation

Intel reported that after updating the CPU Microcode on a set of modern Intel processors using Intel® Transactional Synchronization Extensions (TSX) extensions, the Performance Measurement Register 3 is not usable due to technical limitations.

This issue could lead to incorrect performance measurements when register 3 is in use.
It does not affect systems or processes where the registers are not in use.

Affected processors families :
Intel Skylake
Intel Kaby Lake
Intel Coffee Lake
Intel Whiskey Lake

Resolution

Solution will be in a new microcode update.

In the interim, Intel's guidance is to avoid usage of the Performance Counter 3 (so only use counters 0 to 2).

SUSE is implementing this guidance and restriction in the perf framework in the Linux Kernel and will release kernel updates containing this restriction.

There is a new kernel boot option added that re-enables all 4 performance counters:
enable_all_counters=on/off

If this option is enabled, usage of TSX will be force-disabled during performance measurement.
The number of available performance counters is exported to userspace over sysfs.

Cause

CPU Microcode update on a specific set of modern Intel processors.

Additional Information

Also there are performance monitor flags that can be used to adjust behaviour :
  • Add a "force_rtm_abort" attribute per perf event attribute to allow user programs to opt in to use counter 3 and disable TSX while the perf event is active.
  • Also add a "allow_rtm" attribute to allow programs to make sure TSX is enabled during the measurement (e.g. if they want to measure TSX itself)

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