sched/numa: Scale scan period with tasks in group and shared/private

Running 80 tasks in the same group, or as threads of the same process,
results in the memory getting scanned 80x as fast as it would be if a
single task was using the memory.

This really hurts some workloads.

Scale the scan period by the number of tasks in the numa group, and
the shared / private ratio, so the average rate at which memory in
the group is scanned corresponds roughly to the rate at which a single
task would scan its memory.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: jhladky@redhat.com
Cc: lvenanci@redhat.com
Link: http://lkml.kernel.org/r/20170731192847.23050-3-riel@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>