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

Ben Horgan
2026-03-18T17:10:56Z

Hi Reinette,

On 3/18/26 16:35, Reinette Chatre wrote:
> 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;
> }

Adding a truncation indication makes sense to me. Would it be good to reserve space in the
last_cmd_status_buf[] to ensure this can always be displayed? It looks like space could be
made by interacting with seq->size directly but I'm not sure if there is a cleaner way
to do it.

Thanks,

Ben

>
>
>>
>>>
>>> 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
>