• Jason Baron's avatar
    jump_label: Reduce the size of struct static_key · 3821fd35
    Jason Baron authored
    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 <peterz@infradead.org>
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: Joe Perches <joe@perches.com>
    Signed-off-by: default avatarJason Baron <jbaron@akamai.com>
    Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
static-keys.txt 11.8 KB