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Subject[v2 079/115] sysctl: single subheader path: optimisation for paths used only once
Date
This is an optimisation for registering paths that you know will be
used to register a single table. Because such directories will be used
only once, sysctl will always create an entry for it when it sees it.

When sysctl registers a table, for each directory that may be used
while registering other tables we do a linear search to see if it's
already added, and, if not, add it ourselves.

For example: each netdevice will register a single table under
/proc/sys/net/ipv4/conf/DEVNAME/.

The 'DEVNAME' component of the path is not used to register other
headers, and we can optimise adding that directory: we don't have to
check if it's already registered.

This will have a positive performance impact when registering many
such directories because we're doing a O(nr of sibling directories)
search. With @has_just_one_subheader=1 set we skip that search and add
the directory directly because we know no other sibling directory with
the same name was registered.

NOTE: in this example setting @has_just_one_subheader=1 for the 'conf'
ctl_path would be wrong because it's used when registering other
subheaders too (e.g. subheaders for other netdevices).

Signed-off-by: Lucian Adrian Grijincu <lucian.grijincu@gmail.com>
---
include/linux/sysctl.h | 31 +++++++++++++++++++++++++++++++
kernel/sysctl.c | 12 +++++++-----
2 files changed, 38 insertions(+), 5 deletions(-)

diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 22b6eb8..bdc8c97 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -1083,6 +1083,37 @@ struct ctl_table_header {
/* struct ctl_path describes where in the hierarchy a table is added */
struct ctl_path {
const char *procname;
+
+
+ /* This is an optimisation for registering paths that you know
+ * will be used to register a single table. Because such
+ * directories will be used only once, sysctl will always
+ * create an entry for it when it sees it.
+ *
+ * When sysctl registers a table, for each directory that may
+ * be used while registering other tables we do a linear
+ * search to see if it's already added, and, if not, add it
+ * ourselves.
+ *
+ * For example: each netdevice will register a single table
+ * under /proc/sys/net/ipv4/conf/DEVNAME/.
+ *
+ * The 'DEVNAME' component of the path is not used to register
+ * other headers, and we can optimise adding that directory:
+ * we don't have to check if it's already registered.
+ *
+ * This will have a positive performance impact when
+ * registering many such directories because we're doing a
+ * O(nr of sibling directories) search. With
+ * @has_just_one_subheader=1 set we skip that search and add
+ * the directory directly because we know no other sibling
+ * directory with the same name was registered.
+ *
+ * NOTE: in this example setting @has_just_one_subheader=1 for
+ * the 'conf' ctl_path would be wrong because it's used when
+ * registering other subheaders too (e.g. subheaders for other
+ * netdevices). */
+ int has_just_one_subheader;
};

extern struct ctl_table_header *__register_sysctl_paths(struct ctl_table_group *g,
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index c207c19..9b2c05a 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1906,11 +1906,13 @@ static struct ctl_table_header *sysctl_mkdirs(struct ctl_table_header *parent,
retry:
sysctl_write_lock_head(parent);

- h = mkdir_existing_dir(parent, dirs[i]->ctl_dirname);
- if (h != NULL) {
- sysctl_write_unlock_head(parent);
- parent = h;
- continue;
+ if (!path[i].has_just_one_subheader) {
+ h = mkdir_existing_dir(parent, dirs[i]->ctl_dirname);
+ if (h != NULL) {
+ sysctl_write_unlock_head(parent);
+ parent = h;
+ continue;
+ }
}

if (likely(!create_first_netns_corresp)) {
--
1.7.5.134.g1c08b


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