linux-kernel - Re: [PATCH v10 19/24] x86/resctrl: Report "Unassigned" for MBM events in mbm_cntr

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Message-ID: <6f42b6c8-bf7d-4c20-b6a4-113b3d9ac817@amd.com>
Date: Sat, 21 Dec 2024 08:04:48 -0600
From: "Moger, Babu" <bmoger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>,
 Babu Moger <babu.moger@....com>, corbet@....net, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
 tony.luck@...el.com, peternewman@...gle.com
Cc: fenghua.yu@...el.com, x86@...nel.org, hpa@...or.com, paulmck@...nel.org,
 akpm@...ux-foundation.org, thuth@...hat.com, rostedt@...dmis.org,
 xiongwei.song@...driver.com, pawan.kumar.gupta@...ux.intel.com,
 daniel.sneddon@...ux.intel.com, jpoimboe@...nel.org, perry.yuan@....com,
 andipan.das@....com, kai.huang@...el.com, xiaoyao.li@...el.com,
 seanjc@...gle.com, xin3.li@...el.com, andrew.cooper3@...rix.com,
 ebiggers@...gle.com, mario.limonciello@....com, james.morse@....com,
 tan.shaopeng@...itsu.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, maciej.wieczor-retman@...el.com,
 eranian@...gle.com
Subject: Re: [PATCH v10 19/24] x86/resctrl: Report "Unassigned" for MBM events
 in mbm_cntr_assign mode

Hi Reinette,

On 12/19/2024 5:59 PM, Reinette Chatre wrote:
> Hi Babu,
> 
> On 12/12/24 12:15 PM, Babu Moger wrote:
>> In mbm_cntr_assign mode, the hardware counter should be assigned to read
>> the MBM events.
>>
>> Report 'Unassigned' in case the user attempts to read the events without
>> assigning the counter.
>>
>> Signed-off-by: Babu Moger <babu.moger@....com>
>> ---
> 
> ..
> 
>> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
>> index c075fcee96b7..3ec14c314606 100644
>> --- a/Documentation/arch/x86/resctrl.rst
>> +++ b/Documentation/arch/x86/resctrl.rst
>> @@ -430,6 +430,16 @@ When monitoring is enabled all MON groups will also contain:
>>   	for the L3 cache they occupy). These are named "mon_sub_L3_YY"
>>   	where "YY" is the node number.
>>   
>> +	When supported the mbm_cntr_assign mode allows users to assign a
> 
> "When supported" -> "When enabled"? Or perhaps just drop that and start with
> "mbm_cntr_assign mode allows users ..."

ok.

> 
> 
>> +	counter to mon_hw_id, event pair enabling bandwidth monitoring for
>> +	as long as the counter remains assigned. The hardware will continue
>> +	tracking the assigned mon_hw_id until the user manually unassigns
>> +	it, ensuring that counters are not reset during this period. With
>> +	a limited number of counters, the system may run out of assignable
>> +	counters. In that case, MBM event counters will return 'Unassigned'
>> +	when the event is read. Users must manually assign a counter to read
>> +	the events.
>> +
>>   "mon_hw_id":
>>   	Available only with debug option. The identifier used by hardware
>>   	for the monitor group. On x86 this is the RMID.
>> diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>> index 200d89a64027..8e265a86e524 100644
>> --- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>> +++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>> @@ -527,6 +527,12 @@ void mon_event_read(struct rmid_read *rr, struct rdt_resource *r,
>>   	/* When picking a CPU from cpu_mask, ensure it can't race with cpuhp */
>>   	lockdep_assert_cpus_held();
>>   
>> +	if (resctrl_arch_mbm_cntr_assign_enabled(r) && is_mbm_event(evtid) &&
>> +	    !mbm_cntr_assigned(r, d, rdtgrp, evtid)) {
>> +		rr->err = -ENOENT;
>> +		return;
>> +	}
>> +
> 
> hmmm ... d can be NULL here after the SNC support. Since the file that needs a
> sum is essentially software backed I do not think assigning counters would
> apply to it (but it may theoretically apply to the domains it consists of).
> I think it may be safer to just move this check into rdtgroup_mondata_show()
> where it reads data for a single domain.

Sure.

> 
> I am not sure if we need to change the documentation because of this. One option
> could be a rewording to "MBM event counters may return 'Unassigned' or
> 'Unavailable' when the event is read".

ok.
Thanks
Babu