linux-kernel - Re: [RFC PATCH v3 05/17] x86/resctrl: Introduce the interface to display the assignment state

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Message-ID: <9fd1c700-fa24-4290-aa77-a6eda9758ace@intel.com>
Date: Fri, 3 May 2024 16:28:24 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Babu Moger <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: [RFC PATCH v3 05/17] x86/resctrl: Introduce the interface to
 display the assignment state

Hi Babu,

On 3/28/2024 6:06 PM, Babu Moger wrote:
> The ABMC feature provides an option to the user to assign an RMID
> to the hardware counter and monitor the bandwidth for a longer duration.
> System can be in only one mode at a time (Legacy Monitor mode or ABMC
> mode). By default, ABMC mode is disabled.

"By default, ABMC mode is disabled." seems to contradict later work.

> 
> Provide an interface to display the monitor mode on the system.
>     $cat /sys/fs/resctrl/info/L3_MON/mbm_assign
>     abmc

This example seems to contradict earlier statements in two ways:
(a) it only shows one mode vs. there are two modes (legacy or ABMC)
(b) there is no active mode vs. one mode is always active.

> 
> When the feature is enabled
>     $cat /sys/fs/resctrl/info/L3_MON/mbm_assign
>     [abmc]
> 
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---
> v3: New patch to display ABMC capability.
> ---
>  Documentation/arch/x86/resctrl.rst     |  5 +++++
>  arch/x86/kernel/cpu/resctrl/monitor.c  |  4 +++-
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c | 17 +++++++++++++++++
>  3 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index 68df7751d1f5..cd973a013525 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -257,6 +257,11 @@ with the following files:
>  	    # cat /sys/fs/resctrl/info/L3_MON/mbm_local_bytes_config
>  	    0=0x30;1=0x30;3=0x15;4=0x15
>  
> +"mbm_assign":
> +	Available when assignable monitoring features are supported.
> +	Reports the list of assignable features supported and the enclosed brackets
> +	indicate the feature is enabled.

"indicate the feature is enabled" -> "indicate which feature is enabled" or
"indicates the currently enabled feature" or ...?

> +
>  "max_threshold_occupancy":
>  		Read/write file provides the largest value (in
>  		bytes) at which a previously used LLC_occupancy
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index 735b449039c1..48d1957ea5a3 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -1058,8 +1058,10 @@ int __init rdt_get_mon_l3_config(struct rdt_resource *r)
>  						 RFTYPE_MON_INFO | RFTYPE_RES_CACHE);
>  		}
>  
> -		if (resctrl_arch_has_abmc(r))
> +		if (resctrl_arch_has_abmc(r)) {
>  			r->mbm_assign_capable = ABMC_ASSIGN;
> +			resctrl_file_fflags_init("mbm_assign", RFTYPE_MON_INFO);

I think this will need some more thought when considering the fs/arch split.
The architecture can be expected to set r->mbm_assign_capable as above but
having the architecture meddle with the fs flags does not seem like the right
thing to do. I think that RFTYPE_MON_INFO may not be accessible to arch code
anyway.

> +		}
>  	}
>  
>  	l3_mon_evt_init(r);
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index dda71fb6c10e..5ec807e8dd38 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -846,6 +846,17 @@ static int rdtgroup_rmid_show(struct kernfs_open_file *of,
>  	return ret;
>  }
>  
> +static int rdtgroup_mbm_assign_show(struct kernfs_open_file *of,
> +				    struct seq_file *s, void *v)
> +{
> +	struct rdt_resource *r = of->kn->parent->priv;
> +
> +	if (r->mbm_assign_capable)
> +		seq_puts(s, "abmc\n");
> +
> +	return 0;
> +}

Should it print "legacy" if not mbm_assign_capable? Or actually, I think
the expectation is that this file will only be accessible if 
r->mbm_assign_capable is true ... so having that if (r->mbm_assign_capable)
check is not clear to me ... if that is false then it would be a kernel
bug, no?

> +
>  #ifdef CONFIG_PROC_CPU_RESCTRL
>  
>  /*
> @@ -1903,6 +1914,12 @@ static struct rftype res_common_files[] = {
>  		.seq_show	= mbm_local_bytes_config_show,
>  		.write		= mbm_local_bytes_config_write,
>  	},
> +	{
> +		.name		= "mbm_assign",
> +		.mode		= 0444,
> +		.kf_ops		= &rdtgroup_kf_single_ops,
> +		.seq_show	= rdtgroup_mbm_assign_show,
> +	},
>  	{
>  		.name		= "cpus",
>  		.mode		= 0644,

Reinette