linux-kernel - Re: [PATCH v11 20/23] x86/resctrl: Configure mbm_cntr

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Message-ID: <c866ffa5-0a4e-49a2-bd11-86c94360b7bf@amd.com>
Date: Mon, 24 Feb 2025 15:18:55 -0600
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>,
 James Morse <james.morse@....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: 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,
 sandipan.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, 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 v11 20/23] x86/resctrl: Configure mbm_cntr_assign mode if
 supported

Hi Reinette,


On 2/24/25 11:01, Reinette Chatre wrote:
> Hi James and Babu,
> 
> On 2/24/25 7:49 AM, Moger, Babu wrote:
>> Hi James,
>>
>> On 2/21/25 12:06, James Morse wrote:
>>> Hi Babu,
>>>
>>> On 22/01/2025 20:20, Babu Moger wrote:
> 
>>> This sequence has me confused:
>>>
>>>> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
>>>> index 3d748fdbcb5f..a9a5dc626a1e 100644
>>>> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
>>>> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
>>>> @@ -1233,6 +1233,7 @@ 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;
>>>
>>>> +			hw_res->mbm_cntr_assign_enabled = true;
>>>
>>> Here the arch code sets ABMC to be enabled by default at boot.
>>>
>>>
>>>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>> index 6922173c4f8f..515969c5f64f 100644
>>>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>> @@ -4302,9 +4302,13 @@ int resctrl_online_mon_domain(struct rdt_resource *r, struct rdt_mon_domain *d)
>>>>  
>>>>  void resctrl_online_cpu(unsigned int cpu)
>>>>  {
>>>> +	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
>>>> +
>>>>  	mutex_lock(&rdtgroup_mutex);
>>>>  	/* The CPU is set in default rdtgroup after online. */
>>>>  	cpumask_set_cpu(cpu, &rdtgroup_default.cpu_mask);
>>>> +	if (r->mon_capable && r->mon.mbm_cntr_assignable)
>>>> +		resctrl_arch_mbm_cntr_assign_set_one(r);
>>>>  	mutex_unlock(&rdtgroup_mutex);
>>>>  }
>>>
>>> But here, resctrl has to call back to the arch code to make sure the hardware is in the
>>> same state as hw_res->mbm_cntr_assign_enabled.
> 
> Another scenario needing to be supported by this flow is when CPUs come online later ...
> after resctrl is mounted and potentially after the user modified the assignable counter
> mode.

If the user modifies the assignable counter mode. It is recorded in
mbm_cntr_assign_enabled already. When the new CPU comes online, the
hotplug handler(resctrl_arch_online_cpu) is will update the CPU to the new
mode after checking mbm_cntr_assign_enabled.

Are you talking about different case here? Please elaborate.

> 
>>>
>>> Could this be done in resctrl_arch_online_cpu() instead? That way resctrl doesn't get CPUs
>>> in an inconsistent state that it has to fix up...
> 
> Could you please elaborate the inconsistent state that would need to be fixed up?
> 
>>>
>>
>> Sure. Here is the diff.
>>
>> diff --git a/arch/x86/kernel/cpu/resctrl/core.c
>> b/arch/x86/kernel/cpu/resctrl/core.c
>> index 22399f19810f..f48b298413bc 100644
>> --- a/arch/x86/kernel/cpu/resctrl/core.c
>> +++ b/arch/x86/kernel/cpu/resctrl/core.c
>> @@ -771,6 +771,12 @@ static int resctrl_arch_online_cpu(unsigned int cpu)
>>                 domain_add_cpu(cpu, r);
>>         mutex_unlock(&domain_list_lock);
>>
>> +       r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
>> +       mutex_lock(&rdtgroup_mutex);
>> +       if (r->mon_capable && r->mon.mbm_cntr_assignable)
>> +               resctrl_arch_mbm_cntr_assign_set_one(r);
>> +       mutex_unlock(&rdtgroup_mutex);
>> +
>>         clear_closid_rmid(cpu);
>>         resctrl_online_cpu(cpu);
> 
> This would require every architecture to duplicate the above, no?
> 
> Also, please note there is more appropriate domain_add_cpu_mon().

Yes. This may be better place to add this code. Will wait once James
clarifies on "inconsistent state".
-- 
Thanks
Babu Moger