linux-kernel - Re: [PATCH v5 17/20] x86/resctrl: Introduce the interface switch between monitor modes

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Message-ID: <0815b30c-8bff-484d-895c-4f40e9b2d1db@intel.com>
Date: Fri, 12 Jul 2024 15:14:09 -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 v5 17/20] x86/resctrl: Introduce the interface switch
 between monitor modes

Hi Babu,

On 7/3/24 2:48 PM, Babu Moger wrote:
> Introduce interface to switch between ABMC and legacy modes.
> 
> By default ABMC is enabled on boot if the feature is available.
> Provide the interface to go back to legacy mode if required.
> 
> $ cat /sys/fs/resctrl/info/L3_MON/mbm_mode
> [abmc]
> legacy
> 
> To enable the legacy monitoring feature:
> $ echo "legacy" > /sys/fs/resctrl/info/L3_MON/mbm_mode
> 
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---
> v4: Minor commit text changes. Keep the default to ABMC when supported.
>      Fixed comments to reflect changed interface "mbm_mode".
> 
> v3: New patch to address the review comments from upstream.
> ---
>   Documentation/arch/x86/resctrl.rst     | 10 +++++++
>   arch/x86/kernel/cpu/resctrl/rdtgroup.c | 37 +++++++++++++++++++++++++-
>   2 files changed, 46 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index 11b7a5f26b40..4c41c5622627 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -291,6 +291,16 @@ with the following files:
>   	Without ABMC enabled, monitoring will work in "legacy" mode
>   	without assignment option.
>   
> +	* To enable ABMC feature:
> +	  ::
> +
> +	    # echo  "abmc" > /sys/fs/resctrl/info/L3_MON/mbm_mode
> +
> +	* To enable the legacy monitoring feature:
> +	  ::
> +
> +	    # echo  "legacy" > /sys/fs/resctrl/info/L3_MON/mbm_mode
> +

Needs details on what user can expect to happen to counters/data when
switching between modes.

>   "num_mbm_cntrs":
>   	The number of monitoring counters available for assignment.
>   
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 475a0c7b2a25..531233779f8d 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -910,6 +910,40 @@ static int rdtgroup_num_mbm_cntrs_show(struct kernfs_open_file *of,
>   	return 0;
>   }
>   
> +static ssize_t rdtgroup_mbm_mode_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->mon.abmc_capable)
> +		return -EINVAL;
> +

Why should a user not be able to write "legacy" into this
file if "legacy" is the only mode supported?

> +	/* 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, "legacy"))
> +		resctrl_arch_abmc_disable();
> +	else if (!strcmp(buf, "abmc"))
> +		ret = resctrl_arch_abmc_enable();
> +	else
> +		ret = -EINVAL;
> +
> +	mutex_unlock(&rdtgroup_mutex);
> +	cpus_read_unlock();
> +
> +	return ret ?: nbytes;
> +}
> +
>   #ifdef CONFIG_PROC_CPU_RESCTRL
>   
>   /*
> @@ -2103,9 +2137,10 @@ static struct rftype res_common_files[] = {
>   	},
>   	{
>   		.name		= "mbm_mode",
> -		.mode		= 0444,
> +		.mode		= 0644,
>   		.kf_ops		= &rdtgroup_kf_single_ops,
>   		.seq_show	= rdtgroup_mbm_mode_show,
> +		.write		= rdtgroup_mbm_mode_write,
>   	},
>   	{
>   		.name		= "cpus",

Reinette