Re: [PATCH v5 01/18] cgroup/misc: Add per resource callbacks for CSS events

["Huang, Kai" <kai.huang@intel.com>]

On Fri, 2023-09-22 at 20:06 -0700, Haitao Huang wrote:
> From: Kristen Carlson Accardi <kristen@linux.intel.com>
> 
> The misc cgroup controller (subsystem) currently does not perform
> resource type specific action for Cgroups Subsystem State (CSS) events:
> the 'css_alloc' event when a cgroup is created and the 'css_free' event
> when a cgroup is destroyed, or in event of user writing the max value to
> the misc.max file to set the usage limit of a specific resource
> [admin-guide/cgroup-v2.rst, 5-9. Misc].
> 
> Define callbacks for those events and allow resource providers to
> register the callbacks per resource type as needed. This will be
> utilized later by the EPC misc cgroup support implemented in the SGX
> driver:
> - On css_alloc, allocate and initialize necessary structures for EPC
> reclaiming, e.g., LRU list, work queue, etc.
> - On css_free, cleanup and free those structures created in alloc.
> - On max_write, trigger EPC reclaiming if the new limit is at or below
> current usage.

Nit:

Wondering why we should trigger EPC reclaiming if the new limit is *at* current
usage?

I actually don't quite care about why here, but writing these details in the
changelog may bring unnecessary confusion.  I guess you can just remove all the
details about what SGX driver needs to do on these callbacks.

> 
> Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
> Signed-off-by: Haitao Huang <haitao.huang@linux.intel.com>
> ---
> V5:
> - Remove prefixes from the callback names (tj)
> - Update commit message (Jarkko)
> 
> V4:
> - Moved this to the front of the series.
> - Applies on cgroup/for-6.6 with the overflow fix for misc.
> 
> V3:
> - Removed the released() callback
> ---
>  include/linux/misc_cgroup.h |  5 +++++
>  kernel/cgroup/misc.c        | 32 +++++++++++++++++++++++++++++---
>  2 files changed, 34 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/misc_cgroup.h b/include/linux/misc_cgroup.h
> index e799b1f8d05b..96a88822815a 100644
> --- a/include/linux/misc_cgroup.h
> +++ b/include/linux/misc_cgroup.h
> @@ -37,6 +37,11 @@ struct misc_res {
>  	u64 max;
>  	atomic64_t usage;
>  	atomic64_t events;
> +
> +	/* per resource callback ops */

Nit:

This comment isn't quite useful IMHO.  And it seems you should just expand the
existing comment for the 'struct misc_res', which already covers the existing
members.

Or as Jarkko suggested, maybe you can introduce another structure 'misc_res_ops'
and comment more details for all these callbacks just like 'struct misc_res'.

Anyway it's cgroup maintainer's call.

> +	int (*alloc)(struct misc_cg *cg);
> +	void (*free)(struct misc_cg *cg);
> +	void (*max_write)(struct misc_cg *cg);
>  };
>  
>  /**
> diff --git a/kernel/cgroup/misc.c b/kernel/cgroup/misc.c
> index 79a3717a5803..62c9198dee21 100644
> --- a/kernel/cgroup/misc.c
> +++ b/kernel/cgroup/misc.c
> @@ -276,10 +276,13 @@ static ssize_t misc_cg_max_write(struct kernfs_open_file *of, char *buf,
>  
>  	cg = css_misc(of_css(of));
>  
> -	if (READ_ONCE(misc_res_capacity[type]))
> +	if (READ_ONCE(misc_res_capacity[type])) {
>  		WRITE_ONCE(cg->res[type].max, max);
> -	else
> +		if (cg->res[type].max_write)
> +			cg->res[type].max_write(cg);
> +	} else {
>  		ret = -EINVAL;
> +	}
>  
>  	return ret ? ret : nbytes;
>  }
> @@ -383,23 +386,39 @@ static struct cftype misc_cg_files[] = {
>  static struct cgroup_subsys_state *
>  misc_cg_alloc(struct cgroup_subsys_state *parent_css)
>  {
> +	struct misc_cg *parent_cg;

Nit: 

The below variable '*cg' can be moved here together with 'parent_cg'.
 
>  	enum misc_res_type i;
>  	struct misc_cg *cg;
> +	int ret;
>  
>  	if (!parent_css) {
>  		cg = &root_cg;
> +		parent_cg = &root_cg;

Nit:
		parent_cg = cg = &root_cg;
?

>  	} else {
>  		cg = kzalloc(sizeof(*cg), GFP_KERNEL);
>  		if (!cg)
>  			return ERR_PTR(-ENOMEM);
> +		parent_cg = css_misc(parent_css);
>  	}
>  
>  	for (i = 0; i < MISC_CG_RES_TYPES; i++) {
>  		WRITE_ONCE(cg->res[i].max, MAX_NUM);
>  		atomic64_set(&cg->res[i].usage, 0);
> +		if (parent_cg->res[i].alloc) {
> +			ret = parent_cg->res[i].alloc(cg);
> +			if (ret)
> +				goto alloc_err;
> +		}
>  	}
>  
>  	return &cg->css;
> +
> +alloc_err:
> +	for (i = 0; i < MISC_CG_RES_TYPES; i++)
> +		if (parent_cg->res[i].free)
> +			cg->res[i].free(cg);
> +	kfree(cg);
> +	return ERR_PTR(ret);
>  }
>  
>  /**
> @@ -410,7 +429,14 @@ misc_cg_alloc(struct cgroup_subsys_state *parent_css)
>   */
>  static void misc_cg_free(struct cgroup_subsys_state *css)
>  {
> -	kfree(css_misc(css));
> +	struct misc_cg *cg = css_misc(css);
> +	enum misc_res_type i;
> +
> +	for (i = 0; i < MISC_CG_RES_TYPES; i++)
> +		if (cg->res[i].free)
> +			cg->res[i].free(cg);
> +
> +	kfree(cg);
>  }
>  
>  /* Cgroup controller callbacks */