Re: [PATCH v6 2/2] x86,fs/resctrl: Keep mbm_assign_mode in default mode at boot

[Babu Moger <babu.moger@amd.com>]

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@amd.com>
>> Cc: stable@vger.kernel.org
>> 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.
> 
> 	...
> 	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@intel.com>
> 
Thank you.
Babu