linux-kernel - Re: [PATCH v4 17/19] x86/resctrl: Introduce the interface switch between ABMC and mbm

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Date: Thu, 13 Jun 2024 18:51:47 -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: [PATCH v4 17/19] x86/resctrl: Introduce the interface switch
 between ABMC and mbm_legacy

Hi Babu,

On 5/24/24 5:23 AM, Babu Moger wrote:
> Introduce interface to switch between ABMC and legacy_mbm modes.

shortlog and first sentence of changelog do not match: mbm_legacy vs legacy_mbm?

> 
> By default ABMC is enabled on resctrl mount if the feature is available.
> However, user will have the option to go back to legacy_mbm if required.
> 
> $ cat /sys/fs/resctrl/info/L3_MON/mbm_assign
> [abmc]
> mbm_legacy
> 
> To enable the legacy monitoring feature:
> $ echo  "mbm_legacy" > /sys/fs/resctrl/info/L3_MON/mbm_assign

Missing information about user visible impact to counters/events
and any mitigations needed in implementation.

> 
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---
> v4: Minor commit text changes. Keep the default to ABMC when supported.
> 
> v3: New patch to address the review comments from upstream.
> ---
>   Documentation/arch/x86/resctrl.rst     | 10 +++++++
>   arch/x86/kernel/cpu/resctrl/rdtgroup.c | 40 +++++++++++++++++++++++++-
>   2 files changed, 49 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index ab3cde61a124..fd050d4d22cd 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -271,6 +271,16 @@ with the following files:
>   	  [abmc]
>   	  mbm_legacy
>   
> +	* To enable ABMC feature:
> +	  ::
> +
> +	    # echo  "abmc" > /sys/fs/resctrl/info/L3_MON/mbm_assign
> +
> +	* To enable the legacy monitoring feature:
> +	  ::
> +
> +	    # echo  "mbm_legacy" > /sys/fs/resctrl/info/L3_MON/mbm_assign
> +

No information about what the features are or what will happen on such a switch.

>   "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/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index f452b6d9bb99..d77ff059269a 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -908,6 +908,43 @@ static int rdtgroup_mbm_assign_show(struct kernfs_open_file *of,
>   	return 0;
>   }
>   
> +/*
> + * rdtgroup_mode_write - Modify the resource group's mode

Comment does not match function name. Seems like "mode" is the generic term
to use instead of "assign".

> + */
> +static ssize_t rdtgroup_mbm_assign_write(struct kernfs_open_file *of,
> +					 char *buf, size_t nbytes,
> +					 loff_t off)
> +{
> +	struct rdt_resource *r = of->kn->parent->priv;
> +	int ret = 0;
> +
> +	if (!r->abmc_capable)
> +		return -EINVAL;
> +
> +	/* Valid input requires a trailing newline */
> +	if (nbytes == 0 || buf[nbytes - 1] != '\n')
> +		return -EINVAL;
> +
> +	buf[nbytes - 1] = '\0';
> +
> +	cpus_read_lock();
> +	mutex_lock(&rdtgroup_mutex);
> +
> +	rdt_last_cmd_clear();
> +
> +	if (!strcmp(buf, "mbm_legacy"))
> +		resctrl_abmc_disable(RDT_RESOURCE_L3);
> +	else if (!strcmp(buf, "abmc"))
> +		ret = resctrl_abmc_enable(RDT_RESOURCE_L3);
> +	else
> +		ret = -EINVAL;
> +
> +	mutex_unlock(&rdtgroup_mutex);
> +	cpus_read_unlock();
> +
> +	return ret ?: nbytes;
> +}
> +
>   #ifdef CONFIG_PROC_CPU_RESCTRL
>   
>   /*
> @@ -2087,9 +2124,10 @@ static struct rftype res_common_files[] = {
>   	},
>   	{
>   		.name		= "mbm_assign",
> -		.mode		= 0444,
> +		.mode		= 0644,
>   		.kf_ops		= &rdtgroup_kf_single_ops,
>   		.seq_show	= rdtgroup_mbm_assign_show,
> +		.write		= rdtgroup_mbm_assign_write,
>   	},
>   	{
>   		.name		= "cpus",


Reinette