linux-kernel - Re: [PATCH v14 27/32] x86,fs/resctrl: Auto assign/unassign counters on mkdir

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Message-ID: <6485fc52-15cc-4a6f-93cd-72d953cf7858@amd.com>
Date: Tue, 1 Jul 2025 14:06:54 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, corbet@....net,
 tony.luck@...el.com, Dave.Martin@....com, james.morse@....com,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com
Cc: x86@...nel.org, hpa@...or.com, akpm@...ux-foundation.org,
 rostedt@...dmis.org, paulmck@...nel.org, thuth@...hat.com, ardb@...nel.org,
 gregkh@...uxfoundation.org, seanjc@...gle.com, thomas.lendacky@....com,
 pawan.kumar.gupta@...ux.intel.com, manali.shukla@....com,
 perry.yuan@....com, kai.huang@...el.com, peterz@...radead.org,
 xiaoyao.li@...el.com, kan.liang@...ux.intel.com, mario.limonciello@....com,
 xin3.li@...el.com, gautham.shenoy@....com, xin@...or.com,
 chang.seok.bae@...el.com, fenghuay@...dia.com, peternewman@...gle.com,
 maciej.wieczor-retman@...el.com, eranian@...gle.com,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v14 27/32] x86,fs/resctrl: Auto assign/unassign counters
 on mkdir

Hi Reinette,

On 6/25/25 18:25, Reinette Chatre wrote:
> Hi Babu,
> 
> On 6/13/25 2:05 PM, Babu Moger wrote:
>> Resctrl provides a user-configurable option mbm_assign_on_mkdir that
>> determines if a counter will automatically be assigned to an RMID, event
>> pair when its associated monitor group is created via mkdir.
>>
>> Enable mbm_assign_on_mkdir by default and automatically assign or unassignq
>> counters when a resctrl group is created or deleted.
> 
> This is a bit confusing since I do not think mbm_assign_on_mkdir has *anything*
> to do with unassign of counters. Counters are always (irrespective of mbm_assign_on_mkdir)
> unassigned when a resctrl group is deleted, no?

Yes. That is correct. Changed the text now.

> 
> The subject also does not seem accurate since there is no unassign on
> mkdir.

Changed the subject to:

x86,fs/resctrl: Auto assign counters on mkdir and clean up on group removal

> 
>>
>> By default, each group requires two counters: one for the MBM total event
>> and one for the MBM local event.
>>
>> If the counters are exhausted, the kernel will log the error message
>> "Unable to allocate counter in domain" in
>> /sys/fs/resctrl/info/last_cmd_status when a new group is created and the
>> counter assignment will fail. However, the creation of a group should not
>> fail due to assignment failures. Users have the flexibility to modify the
>> assignments at a later time.
>>
>> Signed-off-by: Babu Moger <babu.moger@....com>
>> ---
> 
> ...
> 
>> ---
>>  arch/x86/kernel/cpu/resctrl/monitor.c |  1 +
>>  fs/resctrl/rdtgroup.c                 | 71 ++++++++++++++++++++++++++-
>>  2 files changed, 70 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
>> index ee0aa741cf6c..053f516a8e67 100644
>> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
>> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
>> @@ -429,6 +429,7 @@ int __init rdt_get_mon_l3_config(struct rdt_resource *r)
>>  		r->mon.mbm_cntr_assignable = true;
>>  		cpuid_count(0x80000020, 5, &eax, &ebx, &ecx, &edx);
>>  		r->mon.num_mbm_cntrs = (ebx & GENMASK(15, 0)) + 1;
>> +		r->mon.mbm_assign_on_mkdir = true;
>>  	}
>>  
>>  	r->mon_capable = true;
>> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
>> index bf5fd46bd455..128a9db339f3 100644
>> --- a/fs/resctrl/rdtgroup.c
>> +++ b/fs/resctrl/rdtgroup.c
>> @@ -2945,6 +2945,55 @@ static void schemata_list_destroy(void)
>>  	}
>>  }
>>  
>> +/**
>> + * rdtgroup_assign_cntrs() - Assign counters to MBM events. Called when
>> + *			     a new group is created.
>> + * If "mbm_event" mode is enabled, counters are automatically assigned.
> 
> "counters are automatically assigned" -> "counters should be automatically assigned
> if the "mbm_assign_on_mkdir" is set"?

Sure.

> 
>> + * Each group can accommodate two counters: one for the total event and
>> + * one for the local event. Assignments may fail due to the limited number
>> + * of counters. However, it is not necessary to fail the group creation
>> + * and thus no failure is returned. Users have the option to modify the
>> + * counter assignments after the group has been created.
>> + */
>> +static void rdtgroup_assign_cntrs(struct rdtgroup *rdtgrp)
>> +{
>> +	struct rdt_resource *r = resctrl_arch_get_resource(RDT_RESOURCE_L3);
>> +
>> +	if (!r->mon_capable)
>> +		return;
>> +
>> +	if (resctrl_arch_mbm_cntr_assign_enabled(r) && !r->mon.mbm_assign_on_mkdir)
>> +		return;
> 
> This check is not clear to me. It looks to me as though counter assignment
> will be attempted if !resctrl_arch_mbm_cntr_assign_enabled(r)? Perhaps
> something like:
> 	if (!r->mon_capable || !resctrl_arch_mbm_cntr_assign_enabled(r) ||
> 	    !r->mon.mbm_assign_on_mkdir)
> 		return;
> 

Yes. Good catch.


>> +
>> +	if (resctrl_is_mon_event_enabled(QOS_L3_MBM_TOTAL_EVENT_ID))
>> +		resctrl_assign_cntr_event(r, NULL, rdtgrp,
>> +					  &mon_event_all[QOS_L3_MBM_TOTAL_EVENT_ID]);
> 
> Switching the namespace like this is confusing to me. rdtgroup_assign_cntrs()
> has prefix rdtgroup_ to indicate it operates on a resource group. It is confusing
> when it switches namespace to call resctrl_assign_cntr_event() that actually assigns
> a specific event to a resource group. I think this will be easier to follow if:
> resctrl_assign_cntr_event() -> rdtgroup_assign_cntr_event()

Sure.

> 
>> +
>> +	if (resctrl_is_mon_event_enabled(QOS_L3_MBM_LOCAL_EVENT_ID))
>> +		resctrl_assign_cntr_event(r, NULL, rdtgrp,
>> +					  &mon_event_all[QOS_L3_MBM_LOCAL_EVENT_ID]);
>> +}
>> +
>> +/*
>> + * rdtgroup_unassign_cntrs() - Unassign the counters associated with MBM events.
>> + *			       Called when a group is deleted.
>> + */
>> +static void rdtgroup_unassign_cntrs(struct rdtgroup *rdtgrp)
>> +{
>> +	struct rdt_resource *r = resctrl_arch_get_resource(RDT_RESOURCE_L3);
>> +
>> +	if (!r->mon_capable || !resctrl_arch_mbm_cntr_assign_enabled(r))
>> +		return;
>> +
>> +	if (resctrl_is_mon_event_enabled(QOS_L3_MBM_TOTAL_EVENT_ID))
>> +		resctrl_unassign_cntr_event(r, NULL, rdtgrp,
>> +					    &mon_event_all[QOS_L3_MBM_TOTAL_EVENT_ID]);
> 
> same here, I think this will be easier to follow when namespace is
> consistent:
> resctrl_unassign_cntr_event() -> rdtgroup_unassign_cntr_event()
> 

Sure.

> 
> Also, the struct rdt_resource parameter should not be needed when
> struct mon_evt is provided and resource can be obtained from mon_evt::rid.
> 
>> +
>> +	if (resctrl_is_mon_event_enabled(QOS_L3_MBM_LOCAL_EVENT_ID))
>> +		resctrl_unassign_cntr_event(r, NULL, rdtgrp,
>> +					    &mon_event_all[QOS_L3_MBM_LOCAL_EVENT_ID]);
>> +}
>> +
>>  static int rdt_get_tree(struct fs_context *fc)
>>  {
>>  	struct rdt_fs_context *ctx = rdt_fc2context(fc);
> 
> 
> Reinette
> 

-- 
Thanks
Babu Moger