Re: [PATCH v3 12/19] x86/resctrl: Make resctrl_mounted checks explicit

[Reinette Chatre]

Hi James,

On 3/20/2023 10:26 AM, James Morse wrote:
> The rdt_enable_key is switched when resctrl is mounted, and used to
> prevent a second mount of the filesystem. It also enables the
> architecture's context switch code.
> 
> This requires another architecture to have the same set of static-keys,
> as resctrl depends on them too.
> 
> Make the resctrl_mounted checks explicit: resctrl can keep track of
> whether it has been mounted once. This doesn't need to be combined with
> whether the arch code is context switching the CLOSID.
> Tests against the rdt_mon_enable_key become a test that resctrl is
> mounted and that monitoring is enabled.

The last sentence above makes the code change hard to follow ...
(see below)

> 
> This will allow the static-key changing to be moved behind resctrl_arch_
> calls.
> 
> Tested-by: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxx>
> Signed-off-by: James Morse <james.morse@xxxxxxx>
> ---
>  arch/x86/kernel/cpu/resctrl/internal.h |  1 +
>  arch/x86/kernel/cpu/resctrl/monitor.c  |  5 +++--
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c | 17 +++++++++++------
>  3 files changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index 7262b355e128..7d5188e8bec3 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -142,6 +142,7 @@ extern bool rdt_alloc_capable;
>  extern bool rdt_mon_capable;
>  extern unsigned int rdt_mon_features;
>  extern struct list_head resctrl_schema_all;
> +extern bool resctrl_mounted;
>  
>  enum rdt_group_type {
>  	RDTCTRL_GROUP = 0,
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index f38cd2f12285..6279f5c98b39 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -834,7 +834,7 @@ void mbm_handle_overflow(struct work_struct *work)
>  
>  	mutex_lock(&rdtgroup_mutex);
>  
> -	if (!static_branch_likely(&rdt_mon_enable_key))
> +	if (!resctrl_mounted || !static_branch_likely(&rdt_mon_enable_key))

... considering the text in the changelog the "resctrl_mounted" check seems
unnecessary. Looking ahead I wonder if this check would not be more
appropriate in patch 15?

>  		goto out_unlock;
>  
>  	r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
> @@ -867,8 +867,9 @@ void mbm_setup_overflow_handler(struct rdt_domain *dom, unsigned long delay_ms)
>  	unsigned long delay = msecs_to_jiffies(delay_ms);
>  	int cpu;
>  
> -	if (!static_branch_likely(&rdt_mon_enable_key))
> +	if (!resctrl_mounted || !static_branch_likely(&rdt_mon_enable_key))
>  		return;

same here

> +

This seems unnecessary.

>  	cpu = cpumask_any_housekeeping(&dom->cpu_mask);
>  	dom->mbm_work_cpu = cpu;
>  	schedule_delayed_work_on(cpu, &dom->mbm_over, delay);
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 2306fbc9a9bb..5176a85f281c 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -42,6 +42,9 @@ LIST_HEAD(rdt_all_groups);
>  /* list of entries for the schemata file */
>  LIST_HEAD(resctrl_schema_all);
>  
> +/* the filesystem can only be mounted once */

Please start sentences with capital letters and end with period.

> +bool resctrl_mounted;
> +
>  /* Kernel fs node for "info" directory under root */
>  static struct kernfs_node *kn_info;
>  
> @@ -796,7 +799,7 @@ int proc_resctrl_show(struct seq_file *s, struct pid_namespace *ns,
>  	mutex_lock(&rdtgroup_mutex);
>  
>  	/* Return empty if resctrl has not been mounted. */
> -	if (!static_branch_unlikely(&rdt_enable_key)) {
> +	if (!resctrl_mounted) {
>  		seq_puts(s, "res:\nmon:\n");
>  		goto unlock;
>  	}
> @@ -2463,7 +2466,7 @@ static int rdt_get_tree(struct fs_context *fc)
>  	/*
>  	 * resctrl file system can only be mounted once.
>  	 */
> -	if (static_branch_unlikely(&rdt_enable_key)) {
> +	if (resctrl_mounted) {
>  		ret = -EBUSY;
>  		goto out;
>  	}
> @@ -2511,8 +2514,10 @@ static int rdt_get_tree(struct fs_context *fc)
>  	if (rdt_mon_capable)
>  		static_branch_enable_cpuslocked(&rdt_mon_enable_key);
>  
> -	if (rdt_alloc_capable || rdt_mon_capable)
> +	if (rdt_alloc_capable || rdt_mon_capable) {
>  		static_branch_enable_cpuslocked(&rdt_enable_key);
> +		resctrl_mounted = true;
> +	}
>  
>  	if (is_mbm_enabled()) {
>  		r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
> @@ -2783,6 +2788,7 @@ static void rdt_kill_sb(struct super_block *sb)
>  	static_branch_disable_cpuslocked(&rdt_alloc_enable_key);
>  	static_branch_disable_cpuslocked(&rdt_mon_enable_key);
>  	static_branch_disable_cpuslocked(&rdt_enable_key);
> +	resctrl_mounted = false;
>  	kernfs_kill_sb(sb);
>  	mutex_unlock(&rdtgroup_mutex);
>  	cpus_read_unlock();
> @@ -3610,7 +3616,7 @@ void resctrl_offline_domain(struct rdt_resource *r, struct rdt_domain *d)
>  	 * If resctrl is mounted, remove all the
>  	 * per domain monitor data directories.
>  	 */
> -	if (static_branch_unlikely(&rdt_mon_enable_key))
> +	if (resctrl_mounted && static_branch_unlikely(&rdt_mon_enable_key))
>  		rmdir_mondata_subdir_allrdtgrp(r, d->id);
>  
>  	if (is_mbm_enabled())
> @@ -3687,8 +3693,7 @@ int resctrl_online_domain(struct rdt_resource *r, struct rdt_domain *d)
>  	if (is_llc_occupancy_enabled())
>  		INIT_DELAYED_WORK(&d->cqm_limbo, cqm_handle_limbo);
>  
> -	/* If resctrl is mounted, add per domain monitor data directories. */
> -	if (static_branch_unlikely(&rdt_mon_enable_key))
> +	if (resctrl_mounted && static_branch_unlikely(&rdt_mon_enable_key))
>  		mkdir_mondata_subdir_allrdtgrp(r, d);
>  
>  	return 0;

Above also, the resctrl_mounted check does not seem to be needed.

Reinette