Re: [RESEND PATCH v4 08/15] fs/resctrl: Add interface to display supported and active kernel-mode policy

[Reinette Chatre]

Hi Babu,

On 7/7/26 2:50 PM, Babu Moger wrote:
> Kernel-mode resctrl policies define how kernel work is associated with
> resource allocation and monitoring relative to the user task. Generic
> resctrl tracks the supported policies, the active policy, and the resctrl
> group backing any global assignment policy. However, this information is
> not exposed to user space.
> 
> Introduce a new resctrl file, info/kernel_mode, to expose the global
> kernel-mode policy and its associated resource group (when applicable).
> This read-only sysfs file lists all supported policies, one per line, and
> highlights the active policy using square brackets.

(please rewrite with consistent terms)

> 
> Signed-off-by: Babu Moger <babu.moger@xxxxxxx>



> +/**
> + * resctrl_kernel_mode_show() - Display supported and active kernel-mode policies
> + * @of: kernfs open file
> + * @seq: output seq_file
> + * @v: unused
> + *
> + * Displays one line per mode set in resctrl_kcfg.kmode. Bracket the active
> + * policy (resctrl_kcfg.kmode_cur).
> + *
> + * INHERIT_CTRL_AND_MON is displayed as "[inherit_ctrl_and_mon]" when active
> + * or "inherit_ctrl_and_mon" when supported but inactive, with no :group=
> + * suffix in either case.
> + *
> + * Global-assign modes append :group=. An inactive mode is emitted as
> + * "<mode>:group=uninitialized". An active mode with a bound group is emitted
> + * as "[<mode>:group=<ctrl>/<mon>/]", where <ctrl>/<mon>/ is derived from
> + * resctrl_kcfg.k_rdtgrp.
> + *
> + * Return: 0 on success, or -ENOENT on error.
> + */
> +static int resctrl_kernel_mode_show(struct kernfs_open_file *of,
> +				    struct seq_file *seq, void *v)
> +{
> +	enum resctrl_kernel_mode mode;
> +	struct rdtgroup *rdtgrp;
> +	const char *ctrl, *mon;
> +	bool active;
> +	int ret = 0;
> +
> +	mutex_lock(&rdtgroup_mutex);

The changes to make resctrl more robust have since been merged. Please consider:
2d77f9768850 ("fs/resctrl: Prevent deadlock and use-after-free in info file handlers")

When you rebase, please use the new info_kn_lock()/info_kn_unlock() helpers.

> +	for (mode = 0; mode < RESCTRL_NUM_KERNEL_MODES; mode++) {
> +		if (!test_bit(mode, &resctrl_kcfg.kmode))
> +			continue;
> +
> +		active = (resctrl_kcfg.kmode_cur == mode);
> +
> +		if (mode == INHERIT_CTRL_AND_MON) {
> +			seq_printf(seq, active ? "[%s]\n" : "%s\n",
> +				   resctrl_mode_str[mode]);
> +			continue;
> +		}
> +
> +		if (!active) {
> +			seq_printf(seq, "%s:group=uninitialized\n",
> +				   resctrl_mode_str[mode]);
> +			continue;
> +		}
> +
> +		/*
> +		 * There should be a valid group when any of the global
> +		 * assign mode is active; otherwise, report an error.

This would indicate a resctrl bug, no?

> +		 */
> +		rdtgrp = resctrl_kcfg.k_rdtgrp;
> +		if (!rdtgrp) {
> +			ret = -ENOENT;
> +			goto out_unlock;

If this fails there needs to be content in last_cmd_status. Otherwise this
fails and then last_cmd_status reads "ok" or worse .. an old failure message.

> +		}
> +
> +		ctrl = "";
> +		mon = "";
> +		if (rdtgrp->type == RDTMON_GROUP) {
> +			ctrl = rdt_kn_name(rdtgrp->mon.parent->kn);
> +			mon = rdt_kn_name(rdtgrp->kn);
> +		} else {
> +			ctrl = rdt_kn_name(rdtgrp->kn);
> +		}
> +		seq_printf(seq, "[%s:group=%s/%s/]\n",
> +			   resctrl_mode_str[mode], ctrl, mon);
> +	}
> +
> +out_unlock:
> +	mutex_unlock(&rdtgroup_mutex);
> +	return ret;
> +}
> +
>  void *rdt_kn_parent_priv(struct kernfs_node *kn)
>  {
>  	/*
> @@ -1915,6 +1999,13 @@ static struct rftype res_common_files[] = {
>  		.seq_show	= rdt_last_cmd_status_show,
>  		.fflags		= RFTYPE_TOP_INFO,
>  	},
> +	{
> +		.name		= "kernel_mode",
> +		.mode		= 0444,
> +		.kf_ops		= &rdtgroup_kf_single_ops,
> +		.seq_show	= resctrl_kernel_mode_show,
> +		.fflags		= RFTYPE_TOP_INFO,
> +	},
>  	{
>  		.name		= "mbm_assign_on_mkdir",
>  		.mode		= 0644,

Reinette