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

Reinette Chatre
2024-07-12T22:14:18Z

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@amd.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