Re: [PATCH v5 26/41] arm_mpam: resctrl: Add monitor initialisation and domain boilerplate

Ben Horgan
2026-02-26T10:26:39Z

Hi Zeng,

On 2/26/26 03:47, Zeng Heng wrote:
> Hi Ben,
>
> On 2026/2/25 1:57, Ben Horgan wrote:
>> Add the boilerplate that tells resctrl about the mpam monitors that are
>> available. resctrl expects all (non-telemetry) monitors to be on the
>> L3 and
>> so advertise them there and invent an L3 resctrl resource if required.
>> The
>> L3 cache itself has to exist as the cache ids are used as the domain
>> ids.
>>
>> Bring the resctrl monitor domains online and offline based on the cpus
>> they contain.
>>
>> Support for specific monitor types is left to later.
>>
>> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
>> ---
>> New patch but mostly moved from the existing patches to
>> separate the monitors from the controls and the boilerplate
>> from the specific counters.
>> Use l3->mon_capable in resctrl_arch_mon_capable() as
>> resctrl_enable_mon_event() now returns a bool.
>> ---
>>   drivers/resctrl/mpam_internal.h |   7 ++
>>   drivers/resctrl/mpam_resctrl.c  | 142 +++++++++++++++++++++++++++++---
>>   2 files changed, 139 insertions(+), 10 deletions(-)
>>
>
> [...]
>
>> @@ -922,6 +1000,20 @@ mpam_resctrl_alloc_domain(unsigned int cpu,
>> struct mpam_resctrl_res *res)
>>       } else {
>>           pr_debug("Skipped control domain online - no controls\n");
>>       }
>> +
>> +    if (resctrl_arch_mon_capable()) {
>> +        mon_d = &dom->resctrl_mon_dom;
>> +        mpam_resctrl_domain_hdr_init(cpu, any_mon_comp, r->rid,
>> &mon_d->hdr);
>> +        mon_d->hdr.type = RESCTRL_MON_DOMAIN;
>> +        err = resctrl_online_mon_domain(r, &mon_d->hdr);
>> +        if (err)
>> +            goto offline_ctrl_domain;
>> +
>> +        mpam_resctrl_domain_insert(&r->mon_domains, &mon_d->hdr);
>> +    } else {
>> +        pr_debug("Skipped monitor domain online - no monitors\n");
>> +    }
>> +
>>       return dom;
>>
>
> I noticed that resctrl_arch_mon_capable() only performs checks for L3
> monitoring functionality. This leads to an issue on platforms that
> include L2 monitoring capabilities, where the code incorrectly enters
> this branch and triggers the following warning by
> mpam_resctrl_domain_insert():
>
> [   22.867070] ------------[ cut here ]------------
> [   22.867073] WARNING: drivers/resctrl/mpam_resctrl.c:1495 at
> mpam_resctrl_domain_insert+0x74/0x80, CPU#2: cpuhp/2/25
> [   29.376035] Modules linked in:
> [   29.379080] CPU: 2 UID: 0 PID: 25 Comm: cpuhp/2 Not tainted 7.0.0-
> rc1-g4288ec146462 #30 PREEMPT
> [   29.387853] Hardware name: To Be Filled By O.E.M. 183.0/To Be Filled
> By O.E.M., BIOS 183.0 02/12/2026
> [   29.397058] pstate: 61400009 (nZCv daif +PAN -UAO -TCO +DIT -SSBS
> BTYPE=--)
> [   29.404007] pc : mpam_resctrl_domain_insert+0x74/0x80
> [   29.409048] lr : mpam_resctrl_domain_insert+0x34/0x80
> [   29.414088] sp : ffff8000876abc60
>  ...
> [   29.488625] Call trace:
> [   29.491060]  mpam_resctrl_domain_insert+0x74/0x80 (P)
> [   29.496100]  mpam_resctrl_online_cpu+0x2b4/0x428
> [   29.500706]  mpam_cpu_online+0x274/0x298
> [   29.504618]  cpuhp_invoke_callback+0x104/0x20c
> [   29.509052]  cpuhp_thread_fun+0xa4/0x17c
> [   29.512963]  smpboot_thread_fn+0x220/0x24c
> [   29.517048]  kthread+0x120/0x12c
> [   29.520265]  ret_from_fork+0x10/0x20
> [   29.523830] ---[ end trace 0000000000000000 ]---

Thanks for reporting this bug. It looks to be because resctrl_arch_mon_capable() is telling us if
there is any mon capable resource when really what we want to know is if this resource is mon capable.
The pattern occurs in a few places. Does this diff help?

diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
index 694ea8548a05..19b306017845 100644
--- a/drivers/resctrl/mpam_resctrl.c
+++ b/drivers/resctrl/mpam_resctrl.c
@@ -1543,7 +1543,7 @@ mpam_resctrl_alloc_domain(unsigned int cpu, struct mpam_resctrl_res *res)
if (!dom)
return ERR_PTR(-ENOMEM);

- if (resctrl_arch_alloc_capable()) {
+ if (r->alloc_capable) {
dom->ctrl_comp = ctrl_comp;

ctrl_d = &dom->resctrl_ctrl_dom;
@@ -1558,7 +1558,7 @@ mpam_resctrl_alloc_domain(unsigned int cpu, struct mpam_resctrl_res *res)
pr_debug("Skipped control domain online - no controls\n");
}

- if (resctrl_arch_mon_capable()) {
+ if (r->mon_capable) {
struct mpam_component *any_mon_comp;
struct mpam_resctrl_mon *mon;
enum resctrl_event_id eventid;
@@ -1603,7 +1603,7 @@ mpam_resctrl_alloc_domain(unsigned int cpu, struct mpam_resctrl_res *res)
return dom;

offline_ctrl_domain:
- if (resctrl_arch_alloc_capable()) {
+ if (r->alloc_capable) {
mpam_resctrl_offline_domain_hdr(cpu, &ctrl_d->hdr);
resctrl_offline_ctrl_domain(r, ctrl_d);
}
@@ -1671,6 +1671,7 @@ int mpam_resctrl_online_cpu(unsigned int cpu)
guard(mutex)(&domain_list_lock);
for_each_mpam_resctrl_control(res, rid) {
struct mpam_resctrl_dom *dom;
+ struct rdt_resource *r = &res->resctrl_res;

if (!res->class)
continue; // dummy_resource;
@@ -1679,12 +1680,12 @@ int mpam_resctrl_online_cpu(unsigned int cpu)
if (!dom) {
dom = mpam_resctrl_alloc_domain(cpu, res);
} else {
- if (resctrl_arch_alloc_capable()) {
+ if (r->alloc_capable) {
struct rdt_ctrl_domain *ctrl_d = &dom->resctrl_ctrl_dom;

mpam_resctrl_online_domain_hdr(cpu, &ctrl_d->hdr);
}
- if (resctrl_arch_mon_capable()) {
+ if (r->mon_capable) {
struct rdt_l3_mon_domain *mon_d = &dom->resctrl_mon_dom;

mpam_resctrl_online_domain_hdr(cpu, &mon_d->hdr);
@@ -1712,6 +1713,7 @@ void mpam_resctrl_offline_cpu(unsigned int cpu)
struct rdt_l3_mon_domain *mon_d;
struct rdt_ctrl_domain *ctrl_d;
bool ctrl_dom_empty, mon_dom_empty;
+ struct rdt_resource *r = &res->resctrl_res;

if (!res->class)
continue; // dummy resource
@@ -1720,7 +1722,7 @@ void mpam_resctrl_offline_cpu(unsigned int cpu)
if (WARN_ON_ONCE(!dom))
continue;

- if (resctrl_arch_alloc_capable()) {
+ if (r->alloc_capable) {
ctrl_d = &dom->resctrl_ctrl_dom;
ctrl_dom_empty = mpam_resctrl_offline_domain_hdr(cpu, &ctrl_d->hdr);
if (ctrl_dom_empty)
@@ -1729,7 +1731,7 @@ void mpam_resctrl_offline_cpu(unsigned int cpu)
ctrl_dom_empty = true;
}

- if (resctrl_arch_mon_capable()) {
+ if (r->mon_capable) {
mon_d = &dom->resctrl_mon_dom;
mon_dom_empty = mpam_resctrl_offline_domain_hdr(cpu, &mon_d->hdr);
if (mon_dom_empty)

>
>
> To preserve the existing public interface of resctrl_arch_mon_capable(),
> please consider the following approach:
>
> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/
> mpam_resctrl.c
> index 694ea8548a05..b06a89494ff0 100644
> --- a/drivers/resctrl/mpam_resctrl.c
> +++ b/drivers/resctrl/mpam_resctrl.c
> @@ -1563,6 +1563,10 @@ mpam_resctrl_alloc_domain(unsigned int cpu,
> struct mpam_resctrl_res *res)
>         if (resctrl_arch_mon_capable()) {
>                 struct mpam_component *any_mon_comp;
>                 struct mpam_resctrl_mon *mon;
>                 enum resctrl_event_id eventid;
>
> +               /* TODO: Only supports L3 monitor type currently. */
> +               if (r->rid != RDT_RESOURCE_L3)
> +                       return dom;
>
>
>
> Best regards,
> Zeng Heng

Thanks,

Ben