[PATCH v5 0/7] x86,fs/resctrl: Pave the way for MPAM counter assignment

[PATCH v5 0/7] x86,fs/resctrl: Pave the way for MPAM counter assignment

[Ben Horgan]

Some small changes from v4 addressing the comments there. Changelogs in patches.

Description from a previous version's cover letter:

A little bit of preparatory work to get ready for MPAM counter
assignment. Resctrl gained support last year for counter assignment for AMD
machines supporting ABMC. Tighten a few things up, that weren't needed for
AMD, so that the MPAM driver can emulate ABMC and hence support counter
assignment.

Based on v7.1-rc1

v1:
https://lore.kernel.org/lkml/20260225201905.3568624-1-ben.horgan@arm.com/
v2:
https://lore.kernel.org/lkml/20260313174524.3482767-1-ben.horgan@arm.com/
v3:
https://lore.kernel.org/lkml/20260319162225.378485-1-ben.horgan@arm.com/
v4:
https://lore.kernel.org/lkml/20260326172551.1553871-1-ben.horgan@arm.com/

Ben Horgan (7):
  fs/resctrl: Tidy up the error path in resctrl_mkdir_event_configs()
  x86,fs/resctrl: Create 'event_filter' files read only if they're not
    configurable
  fs/resctrl: Disallow the software controller when MBM counters are
    assignable
  fs/resctrl: Add monitor property 'mbm_cntr_assign_fixed'
  fs/resctrl: Continue counter allocation after failure
  fs/resctrl: Document that automatic counter assignment is best effort
  fs/resctrl: Document tasks file behaviour for task id 0 and idle tasks

 Documentation/filesystems/resctrl.rst | 22 ++++++++++++++------
 arch/x86/kernel/cpu/resctrl/monitor.c |  1 +
 fs/resctrl/internal.h                 |  2 ++
 fs/resctrl/monitor.c                  | 30 ++++++++++++++++++++-------
 fs/resctrl/rdtgroup.c                 | 25 +++++++++++++++-------
 include/linux/resctrl.h               | 18 +++++++++-------
 6 files changed, 70 insertions(+), 28 deletions(-)

-- 
2.43.0

[PATCH v5 1/7] fs/resctrl: Tidy up the error path in resctrl_mkdir_event_configs()

[Ben Horgan]

The error path in resctrl_mkdir_event_configs() is unnecessarily
complicated. Simplify it to just return directly on error.

Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
 fs/resctrl/rdtgroup.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 5dfdaa6f9d8f..fb6999178fe2 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -2333,20 +2333,19 @@ static int resctrl_mkdir_event_configs(struct rdt_resource *r, struct kernfs_nod
 		kn_subdir2 = kernfs_create_dir(kn_subdir, mevt->name, kn_subdir->mode, mevt);
 		if (IS_ERR(kn_subdir2)) {
 			ret = PTR_ERR(kn_subdir2);
-			goto out;
+			return ret;
 		}
 
 		ret = rdtgroup_kn_set_ugid(kn_subdir2);
 		if (ret)
-			goto out;
+			return ret;
 
 		ret = rdtgroup_add_files(kn_subdir2, RFTYPE_ASSIGN_CONFIG);
 		if (ret)
-			break;
+			return ret;
 	}
 
-out:
-	return ret;
+	return 0;
 }
 
 static int rdtgroup_mkdir_info_resdir(void *priv, char *name,
-- 
2.43.0

Re: [PATCH v5 1/7] fs/resctrl: Tidy up the error path in resctrl_mkdir_event_configs()

[Reinette Chatre]

Hi Ben,

On 4/28/26 6:04 AM, Ben Horgan wrote:
> The error path in resctrl_mkdir_event_configs() is unnecessarily
> complicated. Simplify it to just return directly on error.
> 
> Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>

resctrl's MAINTAINERS entry recently received a "Subsystem Profile"
entry to make it clear that resctrl follows the rules in the tip
tree handbook. This is required since resctrl fs patches flow
via the tip tree.

For a smooth merge we should thus aim to follow the tip rules and
for commit tag ordering this means the "Reviewed-by" tag follows
the "Signed-off-by" tag. For reference, "Ordering of commit tags"
in Documentation/process/maintainer-tip.rst

Reinette

[PATCH v5 2/7] x86,fs/resctrl: Create 'event_filter' files read only if they're not configurable

[Ben Horgan]

When the counter assignment mode is mbm_event resctrl assumes the MBM
events are configurable and exposes the 'event_filter' files. These files
live at info/L3_MON/event_configs/<event>/event_filter and are used to
display and set the event configuration.

The MPAM architecture has support for configuring the memory bandwidth
utilization (MBWU) counters to only count reads or only count
writes. However, In MPAM, this event filtering support is optional in the
hardware (and not yet implemented in the MPAM driver) but MBM counter
assignment is always possible for MPAM MBWU counters.

In order to support mbm_event mode with MPAM, create the 'event_filter'
files read only if the event configuration can't be changed. A user can
still chmod the file and so also return early with an error from
event_filter_write().

Introduce a new monitor property, mbm_cntr_configurable, to indicate
whether or not assignable MBM counters are configurable. On x86, set this
to true whenever mbm_cntr_assignable is true to keep existing behaviour.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
Changes since v2:
Use property, mbm_cntr_configurable, rather than arch hook
Change the event_filter mode to read only in res_common_files[]
Add resctrl_file_mode_init() and use in resctrl_l3_mon_resource_init()
set mbm_cntr_configurable for x86 ABMC and mention in commit message

Changes since v4:
Add documentation
make -> create in commit message
---
 Documentation/filesystems/resctrl.rst | 11 +++++------
 arch/x86/kernel/cpu/resctrl/monitor.c |  1 +
 fs/resctrl/internal.h                 |  2 ++
 fs/resctrl/monitor.c                  |  7 +++++++
 fs/resctrl/rdtgroup.c                 | 11 ++++++++++-
 include/linux/resctrl.h               | 16 +++++++++-------
 6 files changed, 34 insertions(+), 14 deletions(-)

diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
index b003bed339fd..2898a51e6f4b 100644
--- a/Documentation/filesystems/resctrl.rst
+++ b/Documentation/filesystems/resctrl.rst
@@ -427,9 +427,9 @@ with the following files:
 
 	Two MBM events are supported by default: mbm_local_bytes and mbm_total_bytes.
 	Each MBM event's sub-directory contains a file named "event_filter" that is
-	used to view and modify which memory transactions the MBM event is configured
-	with. The file is accessible only when "mbm_event" counter assignment mode is
-	enabled.
+	used to view and (if writable) modify which memory transactions the MBM event
+	is configured with. The file is accessible only when "mbm_event" counter
+	assignment mode is enabled.
 
 	List of memory transaction types supported:
 
@@ -454,9 +454,8 @@ with the following files:
 	  # cat /sys/fs/resctrl/info/L3_MON/event_configs/mbm_local_bytes/event_filter
 	  local_reads,local_non_temporal_writes,local_reads_slow_memory
 
-	Modify the event configuration by writing to the "event_filter" file within
-	the "event_configs" directory. The read/write "event_filter" file contains the
-	configuration of the event that reflects which memory transactions are counted by it.
+	The memory transactions the MBM event is configured with can be changed
+	if "event_filter" is writable.
 
 	For example::
 
diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
index 9bd87bae4983..794a6fb175e4 100644
--- a/arch/x86/kernel/cpu/resctrl/monitor.c
+++ b/arch/x86/kernel/cpu/resctrl/monitor.c
@@ -454,6 +454,7 @@ int __init rdt_get_l3_mon_config(struct rdt_resource *r)
 	    (rdt_cpu_has(X86_FEATURE_CQM_MBM_TOTAL) ||
 	     rdt_cpu_has(X86_FEATURE_CQM_MBM_LOCAL))) {
 		r->mon.mbm_cntr_assignable = true;
+		r->mon.mbm_cntr_configurable = true;
 		cpuid_count(0x80000020, 5, &eax, &ebx, &ecx, &edx);
 		r->mon.num_mbm_cntrs = (ebx & GENMASK(15, 0)) + 1;
 		hw_res->mbm_cntr_assign_enabled = true;
diff --git a/fs/resctrl/internal.h b/fs/resctrl/internal.h
index 1a9b29119f88..48af75b9dc85 100644
--- a/fs/resctrl/internal.h
+++ b/fs/resctrl/internal.h
@@ -408,6 +408,8 @@ void __check_limbo(struct rdt_l3_mon_domain *d, bool force_free);
 
 void resctrl_file_fflags_init(const char *config, unsigned long fflags);
 
+void resctrl_file_mode_init(const char *config, umode_t mode);
+
 void rdt_staged_configs_clear(void);
 
 bool closid_allocated(unsigned int closid);
diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
index 9fd901c78dc6..916f7a9d5658 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -1422,6 +1422,11 @@ ssize_t event_filter_write(struct kernfs_open_file *of, char *buf, size_t nbytes
 		ret = -EINVAL;
 		goto out_unlock;
 	}
+	if (!r->mon.mbm_cntr_configurable) {
+		rdt_last_cmd_puts("event_filter is not configurable\n");
+		ret = -EPERM;
+		goto out_unlock;
+	}
 
 	ret = resctrl_parse_mem_transactions(buf, &evt_cfg);
 	if (!ret && mevt->evt_cfg != evt_cfg) {
@@ -1886,6 +1891,8 @@ int resctrl_l3_mon_resource_init(void)
 		resctrl_file_fflags_init("available_mbm_cntrs",
 					 RFTYPE_MON_INFO | RFTYPE_RES_CACHE);
 		resctrl_file_fflags_init("event_filter", RFTYPE_ASSIGN_CONFIG);
+		if (r->mon.mbm_cntr_configurable)
+			resctrl_file_mode_init("event_filter", 0644);
 		resctrl_file_fflags_init("mbm_assign_on_mkdir", RFTYPE_MON_INFO |
 					 RFTYPE_RES_CACHE);
 		resctrl_file_fflags_init("mbm_L3_assignments", RFTYPE_MON_BASE);
diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index fb6999178fe2..b382a348dd79 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -2022,7 +2022,7 @@ static struct rftype res_common_files[] = {
 	},
 	{
 		.name		= "event_filter",
-		.mode		= 0644,
+		.mode		= 0444,
 		.kf_ops		= &rdtgroup_kf_single_ops,
 		.seq_show	= event_filter_show,
 		.write		= event_filter_write,
@@ -2215,6 +2215,15 @@ void resctrl_file_fflags_init(const char *config, unsigned long fflags)
 		rft->fflags = fflags;
 }
 
+void resctrl_file_mode_init(const char *config, umode_t mode)
+{
+	struct rftype *rft;
+
+	rft = rdtgroup_get_rftype_by_name(config);
+	if (rft)
+		rft->mode = mode;
+}
+
 /**
  * rdtgroup_kn_mode_restrict - Restrict user access to named resctrl file
  * @r: The resource group with which the file is associated.
diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
index 006e57fd7ca5..06e8c72e8660 100644
--- a/include/linux/resctrl.h
+++ b/include/linux/resctrl.h
@@ -286,13 +286,14 @@ enum resctrl_schema_fmt {
 
 /**
  * struct resctrl_mon - Monitoring related data of a resctrl resource.
- * @num_rmid:		Number of RMIDs available.
- * @mbm_cfg_mask:	Memory transactions that can be tracked when bandwidth
- *			monitoring events can be configured.
- * @num_mbm_cntrs:	Number of assignable counters.
- * @mbm_cntr_assignable:Is system capable of supporting counter assignment?
- * @mbm_assign_on_mkdir:True if counters should automatically be assigned to MBM
- *			events of monitor groups created via mkdir.
+ * @num_rmid:			Number of RMIDs available.
+ * @mbm_cfg_mask:		Memory transactions that can be tracked when
+ *				bandwidth monitoring events can be configured.
+ * @num_mbm_cntrs:		Number of assignable counters.
+ * @mbm_cntr_assignable:	Is system capable of supporting counter assignment?
+ * @mbm_assign_on_mkdir:	True if counters should automatically be assigned to MBM
+ *				events of monitor groups created via mkdir.
+ * @mbm_cntr_configurable:	True if assignable counters are configurable.
  */
 struct resctrl_mon {
 	u32			num_rmid;
@@ -300,6 +301,7 @@ struct resctrl_mon {
 	int			num_mbm_cntrs;
 	bool			mbm_cntr_assignable;
 	bool			mbm_assign_on_mkdir;
+	bool			mbm_cntr_configurable;
 };
 
 /**
-- 
2.43.0

Re: [PATCH v5 2/7] x86,fs/resctrl: Create 'event_filter' files read only if they're not configurable

[Reinette Chatre]

Hi Ben,

On 4/28/26 6:04 AM, Ben Horgan wrote:
> When the counter assignment mode is mbm_event resctrl assumes the MBM
> events are configurable and exposes the 'event_filter' files. These files
> live at info/L3_MON/event_configs/<event>/event_filter and are used to
> display and set the event configuration.
> 
> The MPAM architecture has support for configuring the memory bandwidth
> utilization (MBWU) counters to only count reads or only count
> writes. However, In MPAM, this event filtering support is optional in the

nit: In -> in

> hardware (and not yet implemented in the MPAM driver) but MBM counter
> assignment is always possible for MPAM MBWU counters.
> 
> In order to support mbm_event mode with MPAM, create the 'event_filter'
> files read only if the event configuration can't be changed. A user can
> still chmod the file and so also return early with an error from
> event_filter_write().
> 
> Introduce a new monitor property, mbm_cntr_configurable, to indicate
> whether or not assignable MBM counters are configurable. On x86, set this
> to true whenever mbm_cntr_assignable is true to keep existing behaviour.
> 
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
> ---

Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>

Reinette

[PATCH v5 3/7] fs/resctrl: Disallow the software controller when MBM counters are assignable

[Ben Horgan]

The software controller requires that there is one MBM counter per monitor
group that is assigned to the event backing the software controller, as per
mba_MBps_event. When mbm_event mode is in use, it is not guaranteed that
any particular event will have an assigned counter.

Currently, only AMD systems support counter assignment, but the MBA delay
is non-linear and so the software controller is never supported anyway. On
MPAM systems, the MBA delay is linear and so the software controller could
be supported. The MPAM driver, unless a need arises, will not support the
'default' mbm_assign_mode and will always use the 'mbm_event' mode for
memory bandwidth monitoring.

Rather than develop a way to guarantee the counter assignment requirements
needed by the software controller, take the pragmatic approach. Don't allow
the software controller to be used at the same time as 'mbm_event' mode. As
MPAM is the only relevant architecture and it will use 'mbm_event' mode
whenever there are assignable MBM counters, for simplicity's sake, don't
allow the software controller when the MBM counters are assignable.

Implement this by failing the mount if the user requests the software
controller, the mba_MBps option, and the MBM counters are assignable.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
Changes since v4:
Remove "for each MBA control" from commit message
Update last_cmd_status message
---
 fs/resctrl/rdtgroup.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index b382a348dd79..be84bb210e3a 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -2530,7 +2530,8 @@ static bool supports_mba_mbps(void)
 
 	return (resctrl_is_mbm_enabled() &&
 		r->alloc_capable && is_mba_linear() &&
-		r->ctrl_scope == rmbm->mon_scope);
+		r->ctrl_scope == rmbm->mon_scope &&
+		!rmbm->mon.mbm_cntr_assignable);
 }
 
 /*
@@ -2945,7 +2946,7 @@ static int rdt_parse_param(struct fs_context *fc, struct fs_parameter *param)
 		ctx->enable_cdpl2 = true;
 		return 0;
 	case Opt_mba_mbps:
-		msg = "mba_MBps requires MBM and linear scale MBA at L3 scope";
+		msg = "mba_MBps requires MBM (mbm_event mode not supported) and linear scale MBA at L3 scope";
 		if (!supports_mba_mbps())
 			return invalfc(fc, msg);
 		ctx->enable_mba_mbps = true;
-- 
2.43.0

Re: [PATCH v5 3/7] fs/resctrl: Disallow the software controller when MBM counters are assignable

[Reinette Chatre]

Hi Ben,

On 4/28/26 6:04 AM, Ben Horgan wrote:

> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
> index b382a348dd79..be84bb210e3a 100644
> --- a/fs/resctrl/rdtgroup.c
> +++ b/fs/resctrl/rdtgroup.c
> @@ -2530,7 +2530,8 @@ static bool supports_mba_mbps(void)
>  
>  	return (resctrl_is_mbm_enabled() &&
>  		r->alloc_capable && is_mba_linear() &&
> -		r->ctrl_scope == rmbm->mon_scope);
> +		r->ctrl_scope == rmbm->mon_scope &&
> +		!rmbm->mon.mbm_cntr_assignable);
>  }

For better or worse supports_mba_mbps() has a function description that documents
the above tests. Updating the tests but not the accompanying description creates
an inconsistency. How about something like below?

diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 36d21652616e..b456a67b2c24 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -2536,10 +2536,12 @@ static void mba_sc_domain_destroy(struct rdt_resource *r,
 }
 
 /*
- * MBA software controller is supported only if
- * MBM is supported and MBA is in linear scale,
- * and the MBM monitor scope is the same as MBA
- * control scope.
+ * MBA software controller is supported only if MBM is supported and MBA is in
+ * linear scale, and the MBM monitor scope is the same as MBA control scope.
+ *
+ * Software controller cannot be supported when the MBM counters are assignable.
+ * There is no guarantee that MBM counters are assigned to the event backing
+ * the software controller in all monitoring domains of all monitoring groups.
  */
 static bool supports_mba_mbps(void)
 {

>  
>  /*
> @@ -2945,7 +2946,7 @@ static int rdt_parse_param(struct fs_context *fc, struct fs_parameter *param)
>  		ctx->enable_cdpl2 = true;
>  		return 0;
>  	case Opt_mba_mbps:
> -		msg = "mba_MBps requires MBM and linear scale MBA at L3 scope";
> +		msg = "mba_MBps requires MBM (mbm_event mode not supported) and linear scale MBA at L3 scope";
>  		if (!supports_mba_mbps())
>  			return invalfc(fc, msg);
>  		ctx->enable_mba_mbps = true;

Reinette

Re: [PATCH v5 3/7] fs/resctrl: Disallow the software controller when MBM counters are assignable

[Ben Horgan]

Hi Reinette,

On 5/4/26 17:01, Reinette Chatre wrote:
> Hi Ben,
> 
> On 4/28/26 6:04 AM, Ben Horgan wrote:
> 
>> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
>> index b382a348dd79..be84bb210e3a 100644
>> --- a/fs/resctrl/rdtgroup.c
>> +++ b/fs/resctrl/rdtgroup.c
>> @@ -2530,7 +2530,8 @@ static bool supports_mba_mbps(void)
>>  
>>  	return (resctrl_is_mbm_enabled() &&
>>  		r->alloc_capable && is_mba_linear() &&
>> -		r->ctrl_scope == rmbm->mon_scope);
>> +		r->ctrl_scope == rmbm->mon_scope &&
>> +		!rmbm->mon.mbm_cntr_assignable);
>>  }
> 
> For better or worse supports_mba_mbps() has a function description that documents
> the above tests. Updating the tests but not the accompanying description creates
> an inconsistency. How about something like below?
> 
> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
> index 36d21652616e..b456a67b2c24 100644
> --- a/fs/resctrl/rdtgroup.c
> +++ b/fs/resctrl/rdtgroup.c
> @@ -2536,10 +2536,12 @@ static void mba_sc_domain_destroy(struct rdt_resource *r,
>  }
>  
>  /*
> - * MBA software controller is supported only if
> - * MBM is supported and MBA is in linear scale,
> - * and the MBM monitor scope is the same as MBA
> - * control scope.
> + * MBA software controller is supported only if MBM is supported and MBA is in
> + * linear scale, and the MBM monitor scope is the same as MBA control scope.
> + *
> + * Software controller cannot be supported when the MBM counters are assignable.
> + * There is no guarantee that MBM counters are assigned to the event backing
> + * the software controller in all monitoring domains of all monitoring groups.

Yes, that makes sense to me. I'll use that.

Thanks,

Ben

>   */
>  static bool supports_mba_mbps(void)
>  {
> 
>>  
>>  /*
>> @@ -2945,7 +2946,7 @@ static int rdt_parse_param(struct fs_context *fc, struct fs_parameter *param)
>>  		ctx->enable_cdpl2 = true;
>>  		return 0;
>>  	case Opt_mba_mbps:
>> -		msg = "mba_MBps requires MBM and linear scale MBA at L3 scope";
>> +		msg = "mba_MBps requires MBM (mbm_event mode not supported) and linear scale MBA at L3 scope";
>>  		if (!supports_mba_mbps())
>>  			return invalfc(fc, msg);
>>  		ctx->enable_mba_mbps = true;
> 
> Reinette

Re: [PATCH v5 3/7] fs/resctrl: Disallow the software controller when MBM counters are assignable

[Ben Horgan]

On 5/5/26 10:34, Ben Horgan wrote:
> Hi Reinette,
> 
> On 5/4/26 17:01, Reinette Chatre wrote:
>> Hi Ben,
>>
>> On 4/28/26 6:04 AM, Ben Horgan wrote:
>>
>>> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
>>> index b382a348dd79..be84bb210e3a 100644
>>> --- a/fs/resctrl/rdtgroup.c
>>> +++ b/fs/resctrl/rdtgroup.c
>>> @@ -2530,7 +2530,8 @@ static bool supports_mba_mbps(void)
>>>  
>>>  	return (resctrl_is_mbm_enabled() &&
>>>  		r->alloc_capable && is_mba_linear() &&
>>> -		r->ctrl_scope == rmbm->mon_scope);
>>> +		r->ctrl_scope == rmbm->mon_scope &&
>>> +		!rmbm->mon.mbm_cntr_assignable);
>>>  }
>>
>> For better or worse supports_mba_mbps() has a function description that documents
>> the above tests. Updating the tests but not the accompanying description creates
>> an inconsistency. How about something like below?
>>
>> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
>> index 36d21652616e..b456a67b2c24 100644
>> --- a/fs/resctrl/rdtgroup.c
>> +++ b/fs/resctrl/rdtgroup.c
>> @@ -2536,10 +2536,12 @@ static void mba_sc_domain_destroy(struct rdt_resource *r,
>>  }
>>  
>>  /*
>> - * MBA software controller is supported only if
>> - * MBM is supported and MBA is in linear scale,
>> - * and the MBM monitor scope is the same as MBA
>> - * control scope.
>> + * MBA software controller is supported only if MBM is supported and MBA is in
>> + * linear scale, and the MBM monitor scope is the same as MBA control scope.
>> + *
>> + * Software controller cannot be supported when the MBM counters are assignable.
>> + * There is no guarantee that MBM counters are assigned to the event backing
>> + * the software controller in all monitoring domains of all monitoring groups.
> 
> Yes, that makes sense to me. I'll use that.

I've add 'The' to the start of the first two sentences as that makes it read a bit better to me.

That ends up as:

/*
 * The MBA software controller is supported only if MBM is supported and MBA is
 * in linear scale, and the MBM monitor scope is the same as MBA control scope.
 *
 * The software controller cannot be supported when the MBM counters are
 * assignable.  There is no guarantee that MBM counters are assigned to the
 * event backing the software controller in all monitoring domains of all
 * monitoring groups.
 */

Thanks,

Ben

> 
> Thanks,
> 
> Ben
> 
>>   */
>>  static bool supports_mba_mbps(void)
>>  {
>>
>>>  
>>>  /*
>>> @@ -2945,7 +2946,7 @@ static int rdt_parse_param(struct fs_context *fc, struct fs_parameter *param)
>>>  		ctx->enable_cdpl2 = true;
>>>  		return 0;
>>>  	case Opt_mba_mbps:
>>> -		msg = "mba_MBps requires MBM and linear scale MBA at L3 scope";
>>> +		msg = "mba_MBps requires MBM (mbm_event mode not supported) and linear scale MBA at L3 scope";
>>>  		if (!supports_mba_mbps())
>>>  			return invalfc(fc, msg);
>>>  		ctx->enable_mba_mbps = true;
>>
>> Reinette
> 

Re: [PATCH v5 3/7] fs/resctrl: Disallow the software controller when MBM counters are assignable

[Reinette Chatre]

Hi Ben,

On 5/5/26 3:20 AM, Ben Horgan wrote:
> 
> /*
>  * The MBA software controller is supported only if MBM is supported and MBA is
>  * in linear scale, and the MBM monitor scope is the same as MBA control scope.
>  *
>  * The software controller cannot be supported when the MBM counters are
>  * assignable.  There is no guarantee that MBM counters are assigned to the
>  * event backing the software controller in all monitoring domains of all
>  * monitoring groups.
>  */
> 

Thank you very much. For this patch with above added:

| Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>

Reinette

Re: [PATCH v5 3/7] fs/resctrl: Disallow the software controller when MBM counters are assignable

[Ben Horgan]

Hi Reinette,

On 5/5/26 16:21, Reinette Chatre wrote:
> Hi Ben,
> 
> On 5/5/26 3:20 AM, Ben Horgan wrote:
>>
>> /*
>>  * The MBA software controller is supported only if MBM is supported and MBA is
>>  * in linear scale, and the MBM monitor scope is the same as MBA control scope.
>>  *
>>  * The software controller cannot be supported when the MBM counters are
>>  * assignable.  There is no guarantee that MBM counters are assigned to the
>>  * event backing the software controller in all monitoring domains of all
>>  * monitoring groups.
>>  */
>>
> 
> Thank you very much. For this patch with above added:
> 
> | Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>

Thanks for all the reviews. I'll send out a v6 today.

Ben

> 
> Reinette

[PATCH v5 4/7] fs/resctrl: Add monitor property 'mbm_cntr_assign_fixed'

[Ben Horgan]

Commit

  3b497c3f4f04 ("fs/resctrl: Introduce the interface to display monitoring modes")

introduced CONFIG_RESCTRL_ASSIGN_FIXED but left adding the Kconfig
entry until it was necessary. The counter assignment mode is fixed in
MPAM, even when there are assignable counters, and so addressing this
is needed to support MPAM.

To avoid the burden of another Kconfig entry, replace
CONFIG_RESCTRL_ASSIGN_FIXED with a new property in 'struct resctrl_mon',
'mbm_cntr_assign_fixed' to be set by the architecture.

Do not request the architecture to change the counter assignment mode if it
does not support doing so. Provide insight to user space about why such a
request fails.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
Changes since v2:
Change to a resctrl_mon property rather than a arch hook
Update the commit message to mention the property

Changes since v4:
fix -> fixed in comment
commit message drop x86
In commit message add "to be set by the architecture" and update
last paragraph.
---
 fs/resctrl/monitor.c    | 8 +++++++-
 include/linux/resctrl.h | 2 ++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
index 916f7a9d5658..5fbcc64e50ce 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -1456,7 +1456,7 @@ int resctrl_mbm_assign_mode_show(struct kernfs_open_file *of,
 		else
 			seq_puts(s, "[default]\n");
 
-		if (!IS_ENABLED(CONFIG_RESCTRL_ASSIGN_FIXED)) {
+		if (!r->mon.mbm_cntr_assign_fixed) {
 			if (enabled)
 				seq_puts(s, "default\n");
 			else
@@ -1507,6 +1507,12 @@ ssize_t resctrl_mbm_assign_mode_write(struct kernfs_open_file *of, char *buf,
 	}
 
 	if (enable != resctrl_arch_mbm_cntr_assign_enabled(r)) {
+		if (r->mon.mbm_cntr_assign_fixed) {
+			ret = -EINVAL;
+			rdt_last_cmd_puts("Counter assignment mode is not configurable\n");
+			goto out_unlock;
+		}
+
 		ret = resctrl_arch_mbm_cntr_assign_set(r, enable);
 		if (ret)
 			goto out_unlock;
diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
index 06e8c72e8660..73ff522448a0 100644
--- a/include/linux/resctrl.h
+++ b/include/linux/resctrl.h
@@ -294,6 +294,7 @@ enum resctrl_schema_fmt {
  * @mbm_assign_on_mkdir:	True if counters should automatically be assigned to MBM
  *				events of monitor groups created via mkdir.
  * @mbm_cntr_configurable:	True if assignable counters are configurable.
+ * @mbm_cntr_assign_fixed:	True if the counter assignment mode is fixed.
  */
 struct resctrl_mon {
 	u32			num_rmid;
@@ -302,6 +303,7 @@ struct resctrl_mon {
 	bool			mbm_cntr_assignable;
 	bool			mbm_assign_on_mkdir;
 	bool			mbm_cntr_configurable;
+	bool			mbm_cntr_assign_fixed;
 };
 
 /**
-- 
2.43.0

Re: [PATCH v5 4/7] fs/resctrl: Add monitor property 'mbm_cntr_assign_fixed'

[Reinette Chatre]

Hi Ben,

On 4/28/26 6:04 AM, Ben Horgan wrote:
> Commit
> 
>   3b497c3f4f04 ("fs/resctrl: Introduce the interface to display monitoring modes")
> 
> introduced CONFIG_RESCTRL_ASSIGN_FIXED but left adding the Kconfig
> entry until it was necessary. The counter assignment mode is fixed in
> MPAM, even when there are assignable counters, and so addressing this
> is needed to support MPAM.
> 
> To avoid the burden of another Kconfig entry, replace
> CONFIG_RESCTRL_ASSIGN_FIXED with a new property in 'struct resctrl_mon',
> 'mbm_cntr_assign_fixed' to be set by the architecture.
> 
> Do not request the architecture to change the counter assignment mode if it
> does not support doing so. Provide insight to user space about why such a
> request fails.
> 
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
> ---

Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>

Reinette

[PATCH v5 5/7] fs/resctrl: Continue counter allocation after failure

[Ben Horgan]

In mbm_event mode, with mbm_assign_on_mkdir set to 1, when a user creates a
new CTRL_MON or MON group resctrl attempts to allocate counters for each of
the supported MBM events on each resctrl domain. As counters are limited,
such allocation may fail and when it does counter allocations for the
remaining domains are skipped even if the domains have available counters.

Since a counter allocation failure may result in counter allocation being
skipped on other domains the user needs to view the resource group's
mbm_L3_assignments files to get an accurate view of counter assignment in a
new resource group and then manually create counters in the skipped domains
with available counters.

Writes to mbm_L3_assignments using the wildcard format, <event>:*=e, also
skip counter allocation in other domains after a counter allocation
failure.

When handling a request to create counters in all domains it is unnecessary
for a counter allocation in one domain to prevent counter allocation in
other domains. Always attempt to allocate all the counters requested.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
Changes since v4:
Update commit message (Reinette)
---
 fs/resctrl/monitor.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
index 5fbcc64e50ce..0e6a389a16bf 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -1211,9 +1211,10 @@ static int rdtgroup_alloc_assign_cntr(struct rdt_resource *r, struct rdt_l3_mon_
  * NULL; otherwise, assign the counter to the specified domain @d.
  *
  * If all counters in a domain are already in use, rdtgroup_alloc_assign_cntr()
- * will fail. The assignment process will abort at the first failure encountered
- * during domain traversal, which may result in the event being only partially
- * assigned.
+ * will fail. When attempting to assign counters to all domains, carry on trying
+ * to assign counters after a failure since only some domains may have counters
+ * and the goal is to assign counters where possible. If any counter assignment
+ * fails, return the error from the last failing assignment.
  *
  * Return:
  * 0 on success, < 0 on failure.
@@ -1226,9 +1227,11 @@ static int rdtgroup_assign_cntr_event(struct rdt_l3_mon_domain *d, struct rdtgro
 
 	if (!d) {
 		list_for_each_entry(d, &r->mon_domains, hdr.list) {
-			ret = rdtgroup_alloc_assign_cntr(r, d, rdtgrp, mevt);
-			if (ret)
-				return ret;
+			int err;
+
+			err = rdtgroup_alloc_assign_cntr(r, d, rdtgrp, mevt);
+			if (err)
+				ret = err;
 		}
 	} else {
 		ret = rdtgroup_alloc_assign_cntr(r, d, rdtgrp, mevt);
-- 
2.43.0

Re: [PATCH v5 5/7] fs/resctrl: Continue counter allocation after failure

[Reinette Chatre]

Hi Ben,

On 4/28/26 6:04 AM, Ben Horgan wrote:
> In mbm_event mode, with mbm_assign_on_mkdir set to 1, when a user creates a
> new CTRL_MON or MON group resctrl attempts to allocate counters for each of
> the supported MBM events on each resctrl domain. As counters are limited,
> such allocation may fail and when it does counter allocations for the
> remaining domains are skipped even if the domains have available counters.
> 
> Since a counter allocation failure may result in counter allocation being
> skipped on other domains the user needs to view the resource group's
> mbm_L3_assignments files to get an accurate view of counter assignment in a

nit: files -> file

> new resource group and then manually create counters in the skipped domains
> with available counters.
> 
> Writes to mbm_L3_assignments using the wildcard format, <event>:*=e, also
> skip counter allocation in other domains after a counter allocation
> failure.
> 
> When handling a request to create counters in all domains it is unnecessary
> for a counter allocation in one domain to prevent counter allocation in
> other domains. Always attempt to allocate all the counters requested.
> 
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
> ---

Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>

Reinette

[PATCH v5 6/7] fs/resctrl: Document that automatic counter assignment is best effort

[Ben Horgan]

When using automatic counter assignment it's useful for a user to know
which counters they can expect to be assigned on group creation.

Document that automatic counter assignment is best effort and how to
discover any assignment failures.

Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
Changes since v4:
Point user to mbm_L3_assignments rather than last_cmd_status (Reinette)
Add "in all domains" (Reinette)
---
 Documentation/filesystems/resctrl.rst | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
index 2898a51e6f4b..b388e9193896 100644
--- a/Documentation/filesystems/resctrl.rst
+++ b/Documentation/filesystems/resctrl.rst
@@ -479,6 +479,12 @@ with the following files:
 	"1":
 		Auto assignment is enabled.
 
+	Automatic counter assignment is done with best effort. If auto
+	assignment is enabled but there are not enough available counters then
+	monitor group creation could succeed while one or more events belonging
+	to the group may not have a counter assigned in all domains. Consult
+	mbm_L3_assignments for counter assignment states of the new groups.
+
 	Example::
 
 	  # echo 0 > /sys/fs/resctrl/info/L3_MON/mbm_assign_on_mkdir
-- 
2.43.0

Re: [PATCH v5 6/7] fs/resctrl: Document that automatic counter assignment is best effort

[Reinette Chatre]

Hi Ben,

On 4/28/26 6:04 AM, Ben Horgan wrote:
> When using automatic counter assignment it's useful for a user to know
> which counters they can expect to be assigned on group creation.
> 
> Document that automatic counter assignment is best effort and how to
> discover any assignment failures.
> 
> Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
> ---

Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>

Reinette

[PATCH v5 7/7] fs/resctrl: Document tasks file behaviour for task id 0 and idle tasks

[Ben Horgan]

When 0 is written to the tasks file it is interpreted as the current task
in rdtgroup_move_task(). Each CPU's idle task has task_struct::pid set to 0
and, on x86, task_struct::closid to RESCTRL_RESERVED_CLOSID and
task_struct::rmid to RESCTRL_RESERVED_RMID. Equivalently, on MPAM
platforms, thread_info::mpam_partid_pmg is encoded with PARTID and PMG set
to RESCTRL_RESERVED_CLOSID and RESCTRL_RESERVED_RMID, respectively. As
there is no interface to change these from the default, the resctrl
configuration for the idle tasks is fixed and they always behave
equivalently to a task in the default tasks file and so take their
configuration from the cpus/cpus_list files.

On read of the tasks file, show_rdt_tasks() filters out any 0 PID. Hence, a
task id of 0 is never shown in the tasks file and the idle tasks are not
represented either.

Document the user visible behaviour.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
I have confirmed this experimentally on an MPAM platform.

Changes since v4:
Clean up the commit message (Reinette)
---
 Documentation/filesystems/resctrl.rst | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
index b388e9193896..e4b66af55ffb 100644
--- a/Documentation/filesystems/resctrl.rst
+++ b/Documentation/filesystems/resctrl.rst
@@ -575,6 +575,11 @@ All groups contain the following files:
 	then the task must already belong to the CTRL_MON parent of this
 	group. The task is removed from any previous MON group.
 
+	When writing to this file, a task id of 0 is interpreted as the
+	task id of the currently running task. On reading the file, a task
+	id of 0 will never be shown and there is no representation of the
+	idle tasks. Instead, a CPU's idle task is always considered as a
+	member of the group owning the CPU.
 
 "cpus":
 	Reading this file shows a bitmask of the logical CPUs owned by
-- 
2.43.0

Re: [PATCH v5 7/7] fs/resctrl: Document tasks file behaviour for task id 0 and idle tasks

[Reinette Chatre]

Hi Ben,

On 4/28/26 6:04 AM, Ben Horgan wrote:
> When 0 is written to the tasks file it is interpreted as the current task
> in rdtgroup_move_task(). Each CPU's idle task has task_struct::pid set to 0
> and, on x86, task_struct::closid to RESCTRL_RESERVED_CLOSID and
> task_struct::rmid to RESCTRL_RESERVED_RMID. Equivalently, on MPAM
> platforms, thread_info::mpam_partid_pmg is encoded with PARTID and PMG set
> to RESCTRL_RESERVED_CLOSID and RESCTRL_RESERVED_RMID, respectively. As
> there is no interface to change these from the default, the resctrl
> configuration for the idle tasks is fixed and they always behave
> equivalently to a task in the default tasks file and so take their
> configuration from the cpus/cpus_list files.
> 
> On read of the tasks file, show_rdt_tasks() filters out any 0 PID. Hence, a
> task id of 0 is never shown in the tasks file and the idle tasks are not
> represented either.
> 
> Document the user visible behaviour.
> 
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
> ---

Thank you very much for adding this.

Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>

Reinette