On Sat, Apr 15, 2017 at 07:01:27PM +0200, Thomas Gleixner wrote: > There are no more (known) nested calls to get_online_cpus() so it's > possible to remove the nested call magic and convert the mutex to a > percpu-rwsem, which speeds up get/put_online_cpus() significantly for the > uncontended case. > > The contended case (write locked for hotplug operations) is slow anyway, so > the slightly more expensive down_write of the percpu rwsem does not matter.
So the previous thing was reader biassed and thus prone to writer starvation. So a slightly more expensive write path doesn't matter; esp. as its now fair and provides a guarantee it will happen, unlike the previous one.
