linux-kernel - Re: [PATCH v6 20/22] x86/resctrl: Enable AMD ABMC feature by default when supported

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Message-ID: <6b1ad4b2-d99f-44fb-afcc-b9f48e51df6e@intel.com>
Date: Tue, 20 Aug 2024 11:12:21 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: <babu.moger@....com>, <corbet@....net>, <fenghua.yu@...el.com>,
	<tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>
CC: <x86@...nel.org>, <hpa@...or.com>, <paulmck@...nel.org>,
	<rdunlap@...radead.org>, <tj@...nel.org>, <peterz@...radead.org>,
	<yanjiewtw@...il.com>, <kim.phillips@....com>, <lukas.bulwahn@...il.com>,
	<seanjc@...gle.com>, <jmattson@...gle.com>, <leitao@...ian.org>,
	<jpoimboe@...nel.org>, <rick.p.edgecombe@...el.com>,
	<kirill.shutemov@...ux.intel.com>, <jithu.joseph@...el.com>,
	<kai.huang@...el.com>, <kan.liang@...ux.intel.com>,
	<daniel.sneddon@...ux.intel.com>, <pbonzini@...hat.com>,
	<sandipan.das@....com>, <ilpo.jarvinen@...ux.intel.com>,
	<peternewman@...gle.com>, <maciej.wieczor-retman@...el.com>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<eranian@...gle.com>, <james.morse@....com>
Subject: Re: [PATCH v6 20/22] x86/resctrl: Enable AMD ABMC feature by default
 when supported

Hi Babu,

On 8/19/24 11:18 AM, Moger, Babu wrote:
> On 8/16/24 17:33, Reinette Chatre wrote:
>> On 8/6/24 3:00 PM, Babu Moger wrote:

>>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> index 66febff2a3d3..d15fd1bde5f4 100644
>>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> @@ -2756,6 +2756,23 @@ void resctrl_arch_mbm_cntr_assign_disable(void)
>>>        }
>>>    }
>>>    +void resctrl_arch_mbm_cntr_assign_configure(void)
>>> +{
>>> +    struct rdt_resource *r =
>>> &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
>>> +    struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
>>> +    bool enable = true;
>>> +
>>> +    mutex_lock(&rdtgroup_mutex);
>>> +
>>> +    if (r->mon.mbm_cntr_assignable) {
>>> +        if (!hw_res->mbm_cntr_assign_enabled)
>>> +            hw_res->mbm_cntr_assign_enabled = true;
>>> +        resctrl_abmc_set_one_amd(&enable);
>>
>> Earlier changelogs mentioned that counters are reset when ABMC is enabled.
>> How does that behave here when one CPU comes online? Consider the scenario
>> where
>> a system is booted without all CPUs online. ABMC is initially enabled on
>> all online
>> CPUs with this flow ... user space could start using resctrl fs and create
>> monitor groups that start accumulating architectural state. If the remaining
>> CPUs come online at this point and this snippet enables ABMC, would it reset
>> all counters? Should the architectural state be cleared?
> 
> When new cpu comes online, it should inherit the abmc state which is set
> already. it should not force it either way. In that case, it is not
> required to reset the architectural state.
> 
> Responded to your earlier comment.
> https://lore.kernel.org/lkml/0256b457-175d-4923-aa49-00e8e52b865b@amd.com/
> 
> 
>>
>> Also, it still does not look right that the architecture decides the policy.
>> Could this enabling be moved to resctrl_online_cpu() for resctrl fs to
>> request architecture to enable assignable counters if it is supported?
> 
> Sure. Will move the resctrl_arch_mbm_cntr_assign_configure() here with
> changes just to update the abmc state which is set during the init.
> 

I do not think we are seeing it the same way. In your earlier comment you mention:

> We need to set abmc state to "enabled" during the init when abmc is
> detected.  resctrl_late_init -> .. -> rdt_get_mon_l3_config
> 
> This only happens once during the init.


I do not think that the ABMC state can be set during init since that runs
before the fs code and thus the arch code cannot be aware of the fs policy
that "mbm_assign_mode" is the default. This may become clear when you move
resctrl_arch_mbm_cntr_assign_configure() to resctrl_online_cpu() though
since I expect that the r->mon.mbm_cntr_assignable check will move
into the fs resctrl_online_cpu() that will call the arch helper to
set the state to enabled.

Reinette