x86,fs/resctrl: Keep mbm_assign_mode in default mode at boot

The mbm_event mode assigns hardware MBM counters to RMID/event pairs.  It is
intended for deployments that need to manage counter assignment on platforms
where the number of monitoring groups exceeds the available hardware counters.
Hardware counters are scarce on these platforms, so mbm_event mode suits
workflows that monitor a subset of groups at a time and rotate assignments as
needed.

x86 resctrl enables the mbm_event mode by default on hardware that supports
it. This breaks the pqos tool¹, which assumes the historical default mode.
It creates 16 or more monitoring groups and uses two counters per group. On
a platform with 32 counters per domain, that exhausts the mbm_event counters
pool, causing additional groups to read "Unassigned".  pqos interprets
a non-numeric event read as zero bandwidth and reports 0 MB/s for those
groups:

  $ sudo pqos -m all:0-4
  CORE         IPC      MISSES     LLC[KB]   MBL[MB/s]   MBR[MB/s]
     0        0.69         16k       192.0         0.0         0.0
     1        0.40          3k         0.0         0.0         0.0
     2        0.44          1k        64.0         0.0         0.0
     3        0.39          1k        64.0         0.0         0.0
     4        0.43          1k         0.0         0.0         0.0

Leave mbm_assign_mode in default mode during initialization as decided in the
thread at the Link tag. Default mode uses one counter per monitoring group. On
existing AMD platforms, the default active counters pool is 64 and may be
larger on newer hardware.  The active counters pool is the number of counters
that the hardware can actively track in default mode.

Deployments within the active counters pool receive accurate bandwidth
measurements in the default mode. Users can create more monitoring groups (for
example, 4096 on current hardware) than there are active counters in the pool.
Beyond that pool, readings may be misleading or report as "Unavailable", with
no user-visible indication.

Default mode is still limited once the number of monitoring groups exceeds
that active counters pool. After hardware re-allocates a counter, a read
may return "Unavailable". pqos still treats "Unavailable" as zero.
Successive reads can be a count, "Unavailable", then another count. pqos
treats those jumps as wraparound and can report an inconsistent rate:

  $ sudo pqos -m all:0-4
  CORE     IPC      MISSES     LLC[KB]   MBL[MB/s]   MBR[MB/s]
  0        0.76         60k        32.0         0.0         0.0
  1        0.45          1k        32.0         0.0         0.0
  2        1.57        107k        64.0         0.0 17592186044184.9
  3        1.62        276k      4928.0         3.2         0.5

Users that need stable measurements beyond the active counters pool should
use mbm_event mode and rotate assignments as needed. Enable it with:

  # echo mbm_event > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode

¹https://github.com/intel/intel-cmt-cat

  [ bp: Massage commit message. ]

Fixes: 0f1576e43adc ("x86/resctrl: Configure mbm_event mode if supported")
Closes: https://github.com/intel/intel-cmt-cat/issues/311
Signed-off-by: Babu Moger <babu.moger@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20260804191608.GDanI6eK2Q07w6pNcb@fat_crate.local

0 files changed, 0 insertions, 0 deletions