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

Reinette Chatre
2026-03-18T16:36:39Z

Hi Ben,

On 3/18/26 4:59 AM, Ben Horgan wrote:
> On 3/17/26 18:09, Reinette Chatre wrote:
>> 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?
>
> Probably but maybe we can do better. I wonder that given that we know we are trying to allocate
> counters in all domains the information in last_cmd_status could summarize which
> domains failed. 512 bytes isn't that large so a hex encoded bit map or something else information
> dense would be needed. This does make last_cmd_status a bit less human readable so may not
> be the way to go.

Indeed, at this point it becomes difficult to convey all the failures. I expect a bit more
from user space during such scenarios though. Specifically, a user space needing to work in
constrained environment would be better off disabling mbm_assign_on_mkdir and manage counters
itself or ensure there are enough counters available before creating a new monitor group.

What resctrl could do in such scenario is to at least convey that some messages were
dropped. Consider, for example:

diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 5da305bd36c9..ea77fa6a38f7 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -973,10 +973,13 @@ static int rdt_last_cmd_status_show(struct kernfs_open_file *of,

mutex_lock(&rdtgroup_mutex);
len = seq_buf_used(&last_cmd_status);
- if (len)
+ if (len) {
seq_printf(seq, "%.*s", len, last_cmd_status_buf);
- else
+ if (seq_buf_has_overflowed(&last_cmd_status))
+ seq_puts(seq, "[truncated]\n");
+ } else {
seq_puts(seq, "ok\n");
+ }
mutex_unlock(&rdtgroup_mutex);
return 0;
}

>
>>
>> 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);
>> }
>
> That does the job but is it simpler to delete rdtgroup_assign_cntr_event()
> and call rdtgroup_alloc_assign_cntr() directly from rdtgroup_assign_cntrs()
> and rdtgroup_modify_assign_state() rather than special casing it further?

Yes, I agree, that would be much simpler.

Reinette