Nest and core IMC (In-Memory Collection counters) assigns a particular
cpu as the designated target for counter data collection. During
system boot, the first online cpu in a chip gets assigned as the
designated cpu for that chip(for nest-imc) and the first online cpu in
a core gets assigned as the designated cpu for that core(for
core-imc).

If the designated cpu goes offline, the next online cpu from the same
chip(for nest-imc)/core(for core-imc) is assigned as the next target,
and the event context is migrated to the target cpu. Currently,
cpumask_any_but() function is used to find the target cpu. Though this
function is expected to return a `random` cpu, this always returns the
next online cpu.

If all cpus in a chip/core is offlined in a sequential manner,
starting from the first cpu, the event migration has to happen for all
the cpus which goes offline. Since the migration process involves a
grace period, the total time taken to offline all the cpus will be
significantly high.

Example:
  In a system which has 2 sockets, with
  NUMA node0 CPU(s):     0-87
  NUMA node8 CPU(s):     88-175

  Time taken to offline cpu 88-175:
  real    2m56.099s
  user    0m0.191s
  sys     0m0.000s

Use cpumask_last() to choose the target cpu, when the designated cpu
goes online, so the migration will happen only when the last_cpu in
the mask goes offline. This way the time taken to offline all cpus in
a chip/core can be reduced.

With the patch:

  Time taken  to offline cpu 88-175:
  real    0m12.207s
  user    0m0.171s
  sys     0m0.000s

Offlining all cpus in reverse order is also taken care because,
cpumask_any_but() is used to find the designated cpu if the last cpu
in the mask goes offline. Since cpumask_any_but() always return the
first cpu in the mask, that becomes the designated cpu and migration
will happen only when the first_cpu in the mask goes offline.

Example: With the patch,

  Time taken to offline cpu from 175-88:
  real    0m9.330s
  user    0m0.110s
  sys     0m0.000s
Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Reviewed-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>