On Mon, 26 Sep 2022 17:02:50 -0500, Nathan Lynch wrote:

At boot time, it is not necessary to delay between polls of
cpu_callin_map when waiting for a kicked CPU to come up. Remove the
delay intervals, but preserve the overall deadline (five seconds).

At run time, the first poll result is usually negative and we incur a
sleeping wait. If we spin on the callin word for a short time first,
we can reduce __cpu_up() from dozens of milliseconds to under 1ms in
the common case on a P9 LPAR:

[...]

Applied to powerpc/next.

[1/1] powerpc/smp: poll cpu_callin_map more aggressively in __cpu_up()
      https://git.kernel.org/powerpc/c/b37ac1894ac3c014863986d6b8ed880195213e78

cheers