From 3821fd35b58dba449bd894014fbf4e1c43c9e951 Mon Sep 17 00:00:00 2001 From: Jason Baron Date: Fri, 3 Feb 2017 15:42:24 -0500 Subject: jump_label: Reduce the size of struct static_key The static_key->next field goes mostly unused. The field is used for associating module uses with a static key. Most uses of struct static_key define a static key in the core kernel and make use of it entirely within the core kernel, or define the static key in a module and make use of it only from within that module. In fact, of the ~3,000 static keys defined, I found only about 5 or so that did not fit this pattern. Thus, we can remove the static_key->next field entirely and overload the static_key->entries field. That is, when all the static_key uses are contained within the same module, static_key->entries continues to point to those uses. However, if the static_key uses are not contained within the module where the static_key is defined, then we allocate a struct static_key_mod, store a pointer to the uses within that struct static_key_mod, and have the static key point at the static_key_mod. This does incur some extra memory usage when a static_key is used in a module that does not define it, but since there are only a handful of such cases there is a net savings. In order to identify if the static_key->entries pointer contains a struct static_key_mod or a struct jump_entry pointer, bit 1 of static_key->entries is set to 1 if it points to a struct static_key_mod and is 0 if it points to a struct jump_entry. We were already using bit 0 in a similar way to store the initial value of the static_key. This does mean that allocations of struct static_key_mod and that the struct jump_entry tables need to be at least 4-byte aligned in memory. As far as I can tell all arches meet this criteria. For my .config, the patch increased the text by 778 bytes, but reduced the data + bss size by 14912, for a net savings of 14,134 bytes. text data bss dec hex filename 8092427 5016512 790528 13899467 d416cb vmlinux.pre 8093205 5001600 790528 13885333 d3df95 vmlinux.post Link: http://lkml.kernel.org/r/1486154544-4321-1-git-send-email-jbaron@akamai.com Cc: Peter Zijlstra Cc: Ingo Molnar Cc: Joe Perches Signed-off-by: Jason Baron Signed-off-by: Steven Rostedt (VMware) --- include/linux/jump_label.h | 23 +++++++++++++++-------- 1 file changed, 15 insertions(+), 8 deletions(-) (limited to 'include/linux/jump_label.h') diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h index a0547c571800..680c98b2f41c 100644 --- a/include/linux/jump_label.h +++ b/include/linux/jump_label.h @@ -89,11 +89,17 @@ extern bool static_key_initialized; struct static_key { atomic_t enabled; -/* Set lsb bit to 1 if branch is default true, 0 ot */ - struct jump_entry *entries; -#ifdef CONFIG_MODULES - struct static_key_mod *next; -#endif +/* + * bit 0 => 1 if key is initially true + * 0 if initially false + * bit 1 => 1 if points to struct static_key_mod + * 0 if points to struct jump_entry + */ + union { + unsigned long type; + struct jump_entry *entries; + struct static_key_mod *next; + }; }; #else @@ -118,9 +124,10 @@ struct module; #ifdef HAVE_JUMP_LABEL -#define JUMP_TYPE_FALSE 0UL -#define JUMP_TYPE_TRUE 1UL -#define JUMP_TYPE_MASK 1UL +#define JUMP_TYPE_FALSE 0UL +#define JUMP_TYPE_TRUE 1UL +#define JUMP_TYPE_LINKED 2UL +#define JUMP_TYPE_MASK 3UL static __always_inline bool static_key_false(struct static_key *key) { -- cgit v1.2.3-59-g8ed1b