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

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Message-ID: <b3dc2fc5-4ee6-414d-90bb-97baa0d67dc5@arm.com>
Date: Fri, 21 Feb 2025 18:06:58 +0000
From: James Morse <james.morse@....com>
To: Babu Moger <babu.moger@....com>, corbet@....net,
 reinette.chatre@...el.com, 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,
 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 Babu,

On 22/01/2025 20:20, Babu Moger wrote:
> Configure mbm_cntr_assign mode on AMD platforms. On AMD platforms, it
> is recommended to use mbm_cntr_assign mode if supported, because
> reading "mbm_total_bytes" or "mbm_local_bytes" will report 'Unavailable'
> if there is no counter associated with that event.

(If you agree with my comment on patch 7, it would be good to update this
wording to match.)


> The mbm_cntr_assign mode, referred to as ABMC (Assignable Bandwidth
> Monitoring Counters) on AMD, is enabled by default when supported by the
> system.
> 
> Update ABMC across all logical processors within the resctrl domain to
> ensure proper functionality.
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index c006c4d8d6ff..2480698b643d 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -734,4 +734,5 @@ int resctrl_unassign_cntr_event(struct rdt_resource *r, struct rdt_mon_domain *d
>  void mbm_cntr_reset(struct rdt_resource *r);
>  int mbm_cntr_get(struct rdt_resource *r, struct rdt_mon_domain *d,
>  		 struct rdtgroup *rdtgrp, enum resctrl_event_id evtid);
> +void resctrl_arch_mbm_cntr_assign_set_one(struct rdt_resource *r);
>  #endif /* _ASM_X86_RESCTRL_INTERNAL_H */

Could this be put in include/linux/resctrl.h, its where it needs to end up eventually.



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.

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...


Thanks,

James