Re: [PATCH v6 2/2] x86,fs/resctrl: Keep mbm_assign_mode in default mode at boot
From: Babu Moger
Date: Mon Nov 02 2026 - 17:18:09 EST
Hi Reinette,
On 10/30/26 17:08, Reinette Chatre wrote:
Hi Babu,
On 10/30/26 1:28 PM, Babu Moger wrote:
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 tool [1], 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. 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
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@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://github.com/intel/intel-cmt-cat # [1]
---
To help future readers understand how we landed on this I think it may be useful
to add a link to the decision that resulted in this fix. For example:
Looks like Boris is taking care of adding the below text.
Thank you.
...
Leave mbm_assign_mode in default mode during initialization as decided in [2].
...
Link: https://lore.kernel.org/20260804191608.GDanI6eK2Q07w6pNcb@fat_crate.local # [2]
| Reviewed-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Babu