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

[Moger, Babu]

Hi Reinette,

On 8/10/2026 10:18 PM, Reinette Chatre wrote:
> 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)

Sure.

> > 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.

Sure. Will do.

> > +	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?

Yes.

> > +		 */
> > +		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.

if (WARN_ON(!rdtgrp)) {
	rdt_last_cmd_puts("Invalid kernel mode group\n");
	ret = -ENOENT;
	goto out_unlock;
}

Thanks
Babu