Re: [PATCH v2 10/59] dyndbg: replace classmap list with a vector

[Louis Chauvet <louis.chauvet@bootlin.com>]

Le 20/03/2025 à 19:51, Jim Cromie a écrit :
> Classmaps are stored in an elf section/array, but currently are
> individually list-linked onto dyndbg's per-module ddebug_table for
> operation. This is unnecessary.
> 
> Just like dyndbg's descriptors, classes are packed in compile order;
> so even with many builtin modules employing multiple classmaps, each
> modules' maps are packed contiguously, and can be treated as a
> array-start-address & array-length.
> 
> So this drops the whole list building operation done in
> ddebug_attach_module_classes(), and removes the list-head members.
> The "select-by-modname" condition is reused to find the start,end of
> the subrange.
> 
> NOTE: This "filter-by-modname" on classmaps should really be done in
> ddebug_add_module(1); ie at least one step closer to ddebug_init(2),
> which already splits up pr-debug descriptors into subranges by
> modname, then calls (1) on each.  (2) knows nothing of classmaps
> currently, and doesn't need to.  For now, just add comment.
> 
> Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
> ---
>   include/linux/dynamic_debug.h |  1 -
>   lib/dynamic_debug.c           | 61 ++++++++++++++++++-----------------
>   2 files changed, 32 insertions(+), 30 deletions(-)
> 
> diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
> index ac199293d203..e458d4b838ac 100644
> --- a/include/linux/dynamic_debug.h
> +++ b/include/linux/dynamic_debug.h
> @@ -83,7 +83,6 @@ enum class_map_type {
>   };
>   
>   struct ddebug_class_map {
> -	struct list_head link;
>   	struct module *mod;
>   	const char *mod_name;	/* needed for builtins */
>   	const char **class_names;
> diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
> index dd7320b9616d..d5572712ce55 100644
> --- a/lib/dynamic_debug.c
> +++ b/lib/dynamic_debug.c
> @@ -45,10 +45,11 @@ extern struct ddebug_class_map __start___dyndbg_classes[];
>   extern struct ddebug_class_map __stop___dyndbg_classes[];
>   
>   struct ddebug_table {
> -	struct list_head link, maps;
> +	struct list_head link;
>   	const char *mod_name;
> -	unsigned int num_ddebugs;
>   	struct _ddebug *ddebugs;
> +	struct ddebug_class_map *classes;
> +	unsigned int num_ddebugs, num_classes;
>   };
>   
>   struct ddebug_query {
> @@ -147,13 +148,15 @@ static void vpr_info_dq(const struct ddebug_query *query, const char *msg)
>   		  query->first_lineno, query->last_lineno, query->class_string);
>   }
>   
> +#define __outvar /* filled by callee */
>   static struct ddebug_class_map *ddebug_find_valid_class(struct ddebug_table const *dt,
> -							  const char *class_string, int *class_id)
> +							const char *class_string,
> +							__outvar int *class_id)

Thanks for your explanation of __outvar! It makes sense. I never seen 
this pattern anywhere in the kernel, maybe a simple doc comment is 
enough to carry the information:

	/**
	 * ddebug_find_valid_class - Find a valid class for a
	 * given string
	 * @dt: debug table to inspect
	 * @class_string: string to match on
	 * @class_id: output pointer for the class_id value
	 *
	 * Returns: Pointer to the ddebug_class_map instance, if found.
	 * @class_id will be set to the id of this class. If no class
	 * matching @class_string is found, returns NULL and class_id is
	 * set to -ENOENT.
	 */

Or at maybe change the documentation of __outvar to be a bit more 
explicit about how it works (I had difficulties to understand because a 
lot of "magic macro" exist in the kernel and sometimes carry information 
to the compiler, but this one is always empty):

	/**
	 * __outvar - Denotes that an argument is used as an output
	 * value
	 * This macro does nothing apart clarify  for the reader that
	 * the parameter will be filled by the callee
	 */

With any of the modifications (feel free to reword them as you want):

Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>

>   {
>   	struct ddebug_class_map *map;
> -	int idx;
> +	int i, idx;
>   
> -	list_for_each_entry(map, &dt->maps, link) {
> +	for (map = dt->classes, i = 0; i < dt->num_classes; i++, map++) {
>   		idx = match_string(map->class_names, map->length, class_string);
>   		if (idx >= 0) {
>   			*class_id = idx + map->base;
> @@ -164,7 +167,6 @@ static struct ddebug_class_map *ddebug_find_valid_class(struct ddebug_table cons
>   	return NULL;
>   }
>   
> -#define __outvar /* filled by callee */
>   /*
>    * Search the tables for _ddebug's which match the given `query' and
>    * apply the `flags' and `mask' to them.  Returns number of matching
> @@ -1122,9 +1124,10 @@ static void *ddebug_proc_next(struct seq_file *m, void *p, loff_t *pos)
>   
>   static const char *ddebug_class_name(struct ddebug_iter *iter, struct _ddebug *dp)
>   {
> -	struct ddebug_class_map *map;
> +	struct ddebug_class_map *map = iter->table->classes;
> +	int i, nc = iter->table->num_classes;
>   
> -	list_for_each_entry(map, &iter->table->maps, link)
> +	for (i = 0; i < nc; i++, map++)
>   		if (class_in_range(dp->class_id, map))
>   			return map->class_names[dp->class_id - map->base];
>   
> @@ -1208,30 +1211,31 @@ static const struct proc_ops proc_fops = {
>   	.proc_write = ddebug_proc_write
>   };
>   
> -static void ddebug_attach_module_classes(struct ddebug_table *dt,
> -					 struct ddebug_class_map *classes,
> -					 int num_classes)
> +static void ddebug_attach_module_classes(struct ddebug_table *dt, struct _ddebug_info *di)
>   {
>   	struct ddebug_class_map *cm;
> -	int i, j, ct = 0;
> +	int i, nc = 0;
>   
> -	for (cm = classes, i = 0; i < num_classes; i++, cm++) {
> +	/*
> +	 * Find this module's classmaps in a subrange/wholerange of
> +	 * the builtin/modular classmap vector/section.  Save the start
> +	 * and length of the subrange at its edges.
> +	 */
> +	for (cm = di->classes, i = 0; i < di->num_classes; i++, cm++) {
>   
>   		if (!strcmp(cm->mod_name, dt->mod_name)) {
> -
> -			v2pr_info("class[%d]: module:%s base:%d len:%d ty:%d\n", i,
> -				  cm->mod_name, cm->base, cm->length, cm->map_type);
> -
> -			for (j = 0; j < cm->length; j++)
> -				v3pr_info(" %d: %d %s\n", j + cm->base, j,
> -					  cm->class_names[j]);
> -
> -			list_add(&cm->link, &dt->maps);
> -			ct++;
> +			if (!nc) {
> +				v2pr_info("start subrange, class[%d]: module:%s base:%d len:%d ty:%d\n",
> +					  i, cm->mod_name, cm->base, cm->length, cm->map_type);
> +				dt->classes = cm;
> +			}
> +			nc++;
>   		}
>   	}
> -	if (ct)
> -		vpr_info("module:%s attached %d classes\n", dt->mod_name, ct);
> +	if (nc) {
> +		dt->num_classes = nc;
> +		vpr_info("module:%s attached %d classes\n", dt->mod_name, nc);
> +	}
>   }
>   
>   /*
> @@ -1263,10 +1267,9 @@ static int ddebug_add_module(struct _ddebug_info *di, const char *modname)
>   	dt->num_ddebugs = di->num_descs;
>   
>   	INIT_LIST_HEAD(&dt->link);
> -	INIT_LIST_HEAD(&dt->maps);
>   
>   	if (di->classes && di->num_classes)
> -		ddebug_attach_module_classes(dt, di->classes, di->num_classes);
> +		ddebug_attach_module_classes(dt, di);
>   
>   	mutex_lock(&ddebug_lock);
>   	list_add_tail(&dt->link, &ddebug_tables);
> @@ -1379,8 +1382,8 @@ static void ddebug_remove_all_tables(void)
>   	mutex_lock(&ddebug_lock);
>   	while (!list_empty(&ddebug_tables)) {
>   		struct ddebug_table *dt = list_entry(ddebug_tables.next,
> -						      struct ddebug_table,
> -						      link);
> +						     struct ddebug_table,
> +						     link);
>   		ddebug_table_free(dt);
>   	}
>   	mutex_unlock(&ddebug_lock);

-- 
Louis Chauvet, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com