Re: [PATCH 00/11] x86,fs/resctrl: Improve resctrl quality and consistency

Reinette Chatre
2026-03-17T18:10:00Z

Hi Ben,

On 3/17/26 3:25 AM, Ben Horgan wrote:
> On 3/16/26 18:18, Reinette Chatre wrote:
>> On 3/16/26 10:44 AM, Ben Horgan wrote:
>>> On 3/2/26 18:46, Reinette Chatre wrote:
>> ...
>>> One related issue I've just noticed is that when ABMC and mbm_assign_on_mkdir are
>>> enabled the creation of MON/CTRL_MON directories may succeed but an error message
>>> is written to last_cmd_status. E.g.
>>>
>>> /sys/fs/resctrl# mkdir mon_groups/new5
>>> /sys/fs/resctrl# cat info/last_cmd_status
>>> Failed to allocate counter for mbm_total_bytes in domain 2
>>>
>>> The failure is ignored, as expected, in rdt_assign_cntrs() but the last_cmd_status
>>> is never cleared. I think this could be fixed by:
>>>
>>> diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
>>> index 62edb464410a..396f17ed72c6 100644
>>> --- a/fs/resctrl/monitor.c
>>> +++ b/fs/resctrl/monitor.c
>>> @@ -1260,6 +1260,8 @@ void rdtgroup_assign_cntrs(struct rdtgroup *rdtgrp)
>>> if (resctrl_is_mon_event_enabled(QOS_L3_MBM_LOCAL_EVENT_ID))
>>> rdtgroup_assign_cntr_event(NULL, rdtgrp,
>>> &mon_event_all[QOS_L3_MBM_LOCAL_EVENT_ID]);
>>> +
>>> + rdt_last_cmd_clear();
>>> }
>>>
>>> Is this right thing to do? Let me know if you want a proper patch.
>>
>> Letting group be created without any counters assigned while writing the error
>> to last_cmd_status is the intended behavior. If the last_cmd_status buffer is cleared
>> at this point then user space will never have the opportunity to see the message that
>> contains the details.
>>
>> It did not seem appropriate to let resource group creation fail when no counters
>> are available. I see that the documentation is not clear on this. What do you think
>> of an update to documentation instead? Would something like below help clarify behavior?
>>
>> diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
>> index ba609f8d4de5..20dc58d281cf 100644
>> --- a/Documentation/filesystems/resctrl.rst
>> +++ b/Documentation/filesystems/resctrl.rst
>> @@ -478,6 +478,12 @@ with the following files:
>> # cat /sys/fs/resctrl/info/L3_MON/mbm_assign_on_mkdir
>> 0
>>
>> + 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. Consult last_cmd_status for details during
>> + such scenario.
>> +
>
> This does the improve the situation but the multiple domain failure behaviour depends
> on the order the domains are iterated through. This is stable as the list is sorted but
> does seem a bit complicated.
> I.e. if you have two domains, with ids 2 and 3, with no counters remaining on domain 2 but
> some on domain 3 then rdtgroup_assign_cntr_event() will fail early and the counter won't
> be assigned for domain 3 but the last_cmd_status will only be about domain 2. The user
> either needs to know a failure at one domain means all higher domains will not be
> considered for that counter or look at the new mbm_L3_assignments to understand what's happened.
> In this case we have:
>
> /sys/fs/resctrl# cat info/L3_MON/mbm_assign_on_mkdir
> 1
> /sys/fs/resctrl# cat info/L3_MON/available_mbm_cntrs
> 2=0;3=1
> /sys/fs/resctrl# mkdir mon_groups/new
> /sys/fs/resctrl# cat info/last_cmd_status
> Failed to allocate counter for mbm_total_bytes in domain 2
> /sys/fs/resctrl# cat mon_groups/new/mbm_L3_assignments
> mbm_total_bytes:2=_;3=_

Good point.

>
> Would it be better for each domain to be considered even if a previous failure occurred or
> is this now a fixed behaviour? For illustration:

I do not believe this needs to be fixed.

>
> diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
> index 62edb464410a..8e061bce9742 100644
> --- a/fs/resctrl/monitor.c
> +++ b/fs/resctrl/monitor.c
> @@ -1248,18 +1248,25 @@ static int rdtgroup_assign_cntr_event(struct rdt_l3_mon_domain *d, struct rdtgro
> void rdtgroup_assign_cntrs(struct rdtgroup *rdtgrp)
> {
> struct rdt_resource *r = resctrl_arch_get_resource(RDT_RESOURCE_L3);
> + struct rdt_l3_mon_domain *d;
>
> if (!r->mon_capable || !resctrl_arch_mbm_cntr_assign_enabled(r) ||
> !r->mon.mbm_assign_on_mkdir)
> return;
>
> - if (resctrl_is_mon_event_enabled(QOS_L3_MBM_TOTAL_EVENT_ID))
> - rdtgroup_assign_cntr_event(NULL, rdtgrp,
> - &mon_event_all[QOS_L3_MBM_TOTAL_EVENT_ID]);
> + if (resctrl_is_mon_event_enabled(QOS_L3_MBM_TOTAL_EVENT_ID)) {
> + list_for_each_entry(d, &r->mon_domains, hdr.list) {
> + rdtgroup_assign_cntr_event(d, rdtgrp,
> + &mon_event_all[QOS_L3_MBM_TOTAL_EVENT_ID]);
> + }
> + }
>
> - if (resctrl_is_mon_event_enabled(QOS_L3_MBM_LOCAL_EVENT_ID))
> - rdtgroup_assign_cntr_event(NULL, rdtgrp,
> - &mon_event_all[QOS_L3_MBM_LOCAL_EVENT_ID]);
> + if (resctrl_is_mon_event_enabled(QOS_L3_MBM_LOCAL_EVENT_ID)) {
> + list_for_each_entry(d, &r->mon_domains, hdr.list) {
> + rdtgroup_assign_cntr_event(d, rdtgrp,
> + &mon_event_all[QOS_L3_MBM_LOCAL_EVENT_ID]);
> + }
> + }
> }

With a solution like this last_cmd_status could potentially contain multiple lines, one
line per domain that failed. last_cmd_status is 512 bytes so if this is a system with
many domains there is a risk of overflow and user space not seeing all failures.
That may be ok?

I think this can be simplified within rdt_assign_cntr_event() though. Consider:

diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
index 49f3f6b846b2..a6a791a15e30 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -1209,12 +1209,13 @@ 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. Ignore errors when attempting to assign a counter to all domains
+ * since only some domains may have counters available and goal is to assign
+ * counters where possible. Only caller providing @d of NULL is
+ * rdtgroup_assign_cntrs() that ignores errors.
*
* Return:
- * 0 on success, < 0 on failure.
+ * 0 on success when @d is specified, 0 always when @d is NULL, < 0 on failure.
*/
static int rdtgroup_assign_cntr_event(struct rdt_l3_mon_domain *d, struct rdtgroup *rdtgrp,
struct mon_evt *mevt)
@@ -1223,11 +1224,8 @@ static int rdtgroup_assign_cntr_event(struct rdt_l3_mon_domain *d, struct rdtgro
int ret = 0;

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;
- }
+ list_for_each_entry(d, &r->mon_domains, hdr.list)
+ rdtgroup_alloc_assign_cntr(r, d, rdtgrp, mevt);
} else {
ret = rdtgroup_alloc_assign_cntr(r, d, rdtgrp, mevt);
}

Reinette