mm: only re-generate demotion targets when a numa node changes its N_CPU state

Abhishek reported that after patch [1], hotplug operations are taking
roughly double the expected time.  [2]

The reason behind is that the CPU callbacks that
migrate_on_reclaim_init() sets always call set_migration_target_nodes()
whenever a CPU is brought up/down.

But we only care about numa nodes going from having cpus to become
cpuless, and vice versa, as that influences the demotion_target order.

We do already have two CPU callbacks (vmstat_cpu_online() and
vmstat_cpu_dead()) that check exactly that, so get rid of the CPU
callbacks in migrate_on_reclaim_init() and only call
set_migration_target_nodes() from vmstat_cpu_{dead,online}() whenever a
numa node change its N_CPU state.

[1] https://lore.kernel.org/linux-mm/[email protected]/
[2] https://lore.kernel.org/linux-mm/[email protected]/

[[email protected]: add feedback from Huang Ying]
  Link: https://lkml.kernel.org/r/[email protected]

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 884a6e5d1f93b ("mm/migrate: update node demotion order on hotplug events")
Signed-off-by: Oscar Salvador <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Tested-by: Baolin Wang <[email protected]>
Reported-by: Abhishek Goel <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Cc: Abhishek Goel <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions