When creating new binder references, the driver assigns a descriptor id
that is shared with userspace. Regrettably, the driver needs to keep the
descriptors small enough to accommodate userspace potentially using them
as Vector indexes. Currently, the driver performs a linear search on the
rb-tree of references to find the smallest available descriptor id. This
approach, however, scales poorly as the number of references grows.

This patch introduces the usage of bitmaps to boost the performance of
descriptor assignments. This optimization results in notable performance
gains, particularly in processes with a large number of references. The
following benchmark with 100,000 references showcases the difference in
latency between the dbitmap implementation and the legacy approach:

  [  587.145098] get_ref_desc_olocked: 15us (dbitmap on)
  [  602.788623] get_ref_desc_olocked: 47343us (dbitmap off)

Note the bitmap size is dynamically adjusted in line with the number of
references, ensuring efficient memory usage. In cases where growing the
bitmap is not possible, the driver falls back to the slow legacy method.

A previous attempt to solve this issue was proposed in [1]. However,
such method involved adding new ioctls which isn't great, plus older
userspace code would not have benefited from the optimizations either.

Link: https://lore.kernel.org/all/20240417191418.1341988-1-cmllamas@google.com/ [1]
Cc: Tim Murray <timmurray@google.com>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Martijn Coenen <maco@android.com>
Cc: Todd Kjos <tkjos@android.com>
Cc: John Stultz <jstultz@google.com>
Cc: Steven Moreland <smoreland@google.com>
Suggested-by: Nick Chen <chenjia3@oppo.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20240612042535.1556708-1-cmllamas@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>