path: root/Documentation
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authorJason Baron <>2017-02-03 15:42:24 -0500
committerSteven Rostedt (VMware) <>2017-02-15 09:02:26 -0500
commit3821fd35b58dba449bd894014fbf4e1c43c9e951 (patch)
tree1cda103a9fd67edb26a225ce1679ac3c83595903 /Documentation
parent7257634135c247de235f3cdfdaa22f9eb5f054e4 (diff)
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 Link: Cc: Peter Zijlstra <> Cc: Ingo Molnar <> Cc: Joe Perches <> Signed-off-by: Jason Baron <> Signed-off-by: Steven Rostedt (VMware) <>
Diffstat (limited to 'Documentation')
1 files changed, 3 insertions, 1 deletions
diff --git a/Documentation/static-keys.txt b/Documentation/static-keys.txt
index ea8d7b4..32a25fa 100644
--- a/Documentation/static-keys.txt
+++ b/Documentation/static-keys.txt
@@ -155,7 +155,9 @@ or:
There are a few functions and macros that architectures must implement in order
to take advantage of this optimization. If there is no architecture support, we
-simply fall back to a traditional, load, test, and jump sequence.
+simply fall back to a traditional, load, test, and jump sequence. Also, the
+struct jump_entry table must be at least 4-byte aligned because the
+static_key->entry field makes use of the two least significant bits.
* select HAVE_ARCH_JUMP_LABEL, see: arch/x86/Kconfig