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

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Message-ID: <ec3c179b-87b1-4afc-b688-618f980584ab@intel.com>
Date: Thu, 19 Dec 2024 15:59:40 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: 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 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 ..."


> +	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.

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".

Reinette