• Yury Norov's avatar
    lib/bitmap.c: make bitmap_parselist() thread-safe and much faster · 0a5ce083
    Yury Norov authored
    Current implementation of bitmap_parselist() uses a static variable to
    save local state while setting bits in the bitmap.  It is obviously wrong
    if we assume execution in multiprocessor environment.  Fortunately, it's
    possible to rewrite this portion of code to avoid using the static
    variable.
    
    It is also possible to set bits in the mask per-range with bitmap_set(),
    not per-bit, as it is implemented now, with set_bit(); which is way
    faster.
    
    The important side effect of this change is that setting bits in this
    function from now is not per-bit atomic and less memory-ordered.  This is
    because set_bit() guarantees the order of memory accesses, while
    bitmap_set() does not.  I think that it is the advantage of the new
    approach, because the bitmap_parselist() is intended to initialise bit
    arrays, and user should protect the whole bitmap during initialisation if
    needed.  So protecting individual bits looks expensive and useless.  Also,
    other range-oriented functions in lib/bitmap.c don't worry much about
    atomicity.
    
    With all that, setting 2k bits in map with the pattern like 0-2047:128/256
    becomes ~50 times faster after applying the patch in my testing
    environment (arm64 hosted on qemu).
    
    The second patch of the series adds the test for bitmap_parselist().  It's
    not intended to cover all tricky cases, just to make sure that I didn't
    screw up during rework.
    
    Link: http://lkml.kernel.org/r/20170807225438.16161-1-ynorov@caviumnetworks.comSigned-off-by: default avatarYury Norov <ynorov@caviumnetworks.com>
    Cc: Noam Camus <noamca@mellanox.com>
    Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
    Cc: Matthew Wilcox <mawilcox@microsoft.com>
    Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    0a5ce083
bitmap.c 36.4 KB